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Cooling System

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The sectioned MGB at the British Motor Museum, Gaydon

Coolant

Never ever mix traditional glycol-based anti-freeze with modern OAT fluids or a green crystalline sludge forms blocking coolant passages and causing overheating, which can take a lot of chemical and water flushing to get rid of (Image from John Maguire in Oz). Also see this.

Something other than plain water is highly desirable if not essential, to prevent corrosion as well as freezing, and the usual product is a glycol-based fluid that has to be diluted with water. A 25% concentration protects down to -12C, and a 33% solution down to -18C, which is probably good enough under most circumstances. However if yours is a daily driver, then although it is unusual to go below -10C in central England, it did get down to -15C in December 2010, and has been as low as -27C (Newport, Shropshire, 1982).

If you are putting anti-freeze into a system that has previously contained plain water - or even if replacing old anti-freeze with fresh, there is a right way and a wrong way of doing it. DO NOT drain the system, and refill it with an already diluted mix, as many web sources advise. There will be a significant amount of water/old coolant left behind which will dilute your mix even further. Calculate how much anti-freeze is required to give the required dilution, add the anti-freeze neat, then top up with clean water, and you will get the correct mix. The total capacity of the MGB system according to Haynes is:

  • Early models - 9.5 Imperial pints (5.4 Litres, 11.4 US pints)
  • 18V engines, GHN5 and GHD5 from model No. 410002 - 11.5 Imperial pints (6.6 Litres, 13.8 US pints)
  • 18V engines 1976 to 78 - 10 Imperial pints (5.6 Litres, 12 US pints)
  • All models 1978 onwards - 12 Imperial pints (6.8 Litres, 14.4 US pints)
  • All plus 0.5 Imperial pints (0.28 Litres, 0.6 US pints) where a heater is fitted.
But these change points don't make sense. For the first change point - '18V engines, GHN5 and GHD5 from model No. 410002' - 18V engines began in August 1971, GHN5/D5 dates from 1969 to 1974, and model number '410002' is the second (not the first) roadster built for the 1977 model year. I can understand a significant increase for the 1977 model year as the radiator moved forwards so the top and bottom hoses were longer and there was a remote expansion tank. Also according to Clausager and the Parts Catalogue the radiator did change for 1976 and again for 1978. So it makes more sense if 'Early models' is taken as 'prior to 1976' at 9.5 Imperial pints, 1976 is 10 Imperial pints, 1977 is 11.5 Imperial pints, and 1978 on is 12 Imperial pints. See also Drain and Refill.

Looking around on the internet you will see replacement intervals of 30-60k or 12 years or more quoted, however that will be for modern cars. The Workshop Manual recommends replacement at 2-yearly intervals, and checking the specific gravity at each service. There are testers available from the usual sources for a couple of pounds upwards.

There are alternatives such as 4-Life and Evans Waterless, which on the face of it seem to offer benefits. In the case of 4-Life it is said to indicate a head gasket leak by changing colour, but on a pals car with a cracked head and compression leaking into the coolant (detected by other means) there was no colour change. Evans Waterless amongst other things is said to reduce system pressures and can't boil, but you need to go through a multi-stage flushing process before using it to replace glycol-based fluids. Both can only be topped-up with the same stuff, which you have to carry around with you, and neither guarantee you won't lose fluid e.g. from hose failure, which given the quality of rubber these days is bound to happen at some time. Both are expensive compared to glycol based concentrates, and personally I can't see any good reason to use them, millions of cars have been running for decades with glycol-based perfectly happily. Neither the 4-cylinder even when run in desert states nor the V8 suffers from cooling system problems, unlike some other marques which are prone to problems even in the UK unless everything is perfect.

Coolant Temperatures:

Temperature Gradients
Boiling points

    Temperature Gradients: Added September 2010
I took the following set of comparative temperatures on the roadster, warming up at a fast idle on a day when the ambient temperature was a cool 9.5C (all temperatures in degrees C):

Time
(mins)
HeadThermoHeaderMiddleFooter
19.59.59.59.59.5
234129.59.59.5
441169.59.59.5
56025111211
77233121312
88043131513
108551141514
119055141615
139280551915
149283653021
169589753929
'Inlet' and 'Outlet' are on the header and footer tanks directly opposite the inlet and outlet ports. Note particularly the differential between these two. A large differential here, if your running temps are higher than they should be, can indicate slow flow through the rad. This will allow a lot of heat to be taken out of the coolant that is in the radiator, but not enough heat transfer from the engine to the radiator. This can be seen in the table where the thermostat has just started to open - the head temp only goes up from 90 to 92, the rad inlet jumps from 14 to 55, but the outlet takes longer to start rising at all, and then more gradually anyway. With low circulation the outlet will stay at a relatively low temperature. Low circulation can come from any restriction in the cooling circuit e.g. sludge or thermostat not fully opening, but can also be caused by a problem with the water pump i.e. heavily corroded vanes or even the incorrect pump. A low differential can indicate good flow, but either the engine is generating too much heat (e.g. timing or fuel issues) or the radiator isn't getting rid of it. This latter can be checked by scanning the surface of the rad with an infra-red thermometer looking for spots that are cooler than the surroundings, particularly where cool spots are above hotter areas, but it can also be caused by problems with the fan either of the wrong type, the wrong way round (still flows air through the rad but not as efficiently, or the wrong spacing to the radiator, see the sections on electric and mechanical fans.

Update July 2013 (phew, what a scorcher):
Quite a few people complain about 'overheating' and ask about installing uprated radiators, and electric fans to cars with mechanical fans. This is very rarely required, especially in the UK. As I say elsewhere these cars run in desert states in standard form without problems and have done so for many years. If your car is running hotter than it should then you need to be considering why, not throwing money and bits at the problem. Firstly, if a car isn't losing coolant, then it isn't overheating. Secondly, anywhere between the upper part of the C zone and the lower part of the H zone (on CNH gauges) is considered 'normal', depending on climate and usage. Consider a car in a temperate climate, pootling along a flat open road, and the temp gauge should be about the middle of it's travel i.e. near the N of CNH gauges.

  • In very cold ambients with the same car the surface cooling from the sump and block, the oil cooler and use of the heater can all mean the coolant temperature never rises enough to open the thermostat, and the temp gauge will read lower than normal. A radiator blind can help here, but it doesn't prevent the radiator overcooling the coolant as many think because the thermostat is closed anyway. What it does is significantly reduce the icy blast around the engine, and blanking off the oil cooler may have an even bigger effect.
  • Conversely in very hot ambients, if you are slogging up a mountain, you are generating a lot of heat, but the slower passage through the air and the high ambients means the radiator isn't dissipating as much heat as at other times. The thermostat is already wide open, so the coolant continues to get hotter and hotter, and the temp gauge rises inexorably towards the H zone. In this case an additional or even replacement electric fan may give more cooling, if the volume of air moved by the electric fan is going to be greater than that moved by the mechanical fan. But you need to consider motor revs against engine revs, and comparative efficiencies of the blades. Another aspect of electric fans often not considered is the blocking effect of the blades when the fan isn't running. Whilst it's true that the fan will almost certainly be 'windmilling' when the car is under way this takes energy out of the air-flow, slowing it down, and a significant amount of air will be flung off the tips of the blades. If the fan is in front of the radiator i.e. a pusher both these aspects are going to reduce the amount of air passing through the radiator. If behind i.e. a puller then at least the air will have passed through the rad before it is flung off, even if there is still some back-pressure reducing the air-flow through the rad slightly. Factory fans were always pushers, and the left-hand fan (when looking into the engine-bay from the front) on twin-fan installations is particularly inefficient as part of the arc described by the fan isn't pointing at the radiator anyway. But even with a V8 that should be enough to cope with anything likely to be encountered in the UK, and six or so LHD V8s were shipped over to America and put through Federal testing with a view to marketing them there as well, so the factory must have expected the cooling to be satisfactory there as well.

    In the current high temperatures (for the UK) I've been taking some measurements, which may be useful as a comparison if you think you are having problems. On one of several days with an afternoon ambient of 26C, the roadster engine compartment got up to 50C in town traffic after a local run, using a digital thermometer with its probe through one of the holes in the bulkhead shelf. Temp gauge was about an Ns width above its normal position. Rad inlet was 87C, outlet 75C, measured with an infra-red thermometer placed right on the fins. The V8 got up to 58C in town traffic after a motorway run, the rad inlet was 92C, and the outlet was 85C. The temp gauge was in the normal range for when the cooling fans are cutting in and out i.e. between N and about 1/3rd of the way between N and H.

      Boiling points by Bob Muenchausen

    Boiling Points of Ethylene Glycol solutions
    in water at various pressurizations
    System Pressure >>>
    0 PSI
    4 PSI
    8 PSI
    12 PSI
    16 PSI
    20 PSI
    24 PSI
    Plain Water
    212 F
    225 F
    233 F
    242 F
    252 F
    260 F
    265 F
    33% Solution
    220 F
    230 F
    240 F
    253 F
    260 F
    268 F
    273 F
    44% Solution
    224 F
    234 F
    245 F
    257 F
    265 F
    272 F
    279 F
    50% Solution
    226 F
    236 F
    248 F
    259 F
    267 F
    275 F
    280 F
    60% Solution
    231 F
    241 F
    253 F
    264 F
    273 F
    280 F
    285 F

    Pints of Anti-Freeze to add to MGB Cooling System (12 pts) to produce various solutions vs. freezing points of Coolant.
    Pints to add >
    1
    2
    3
    4
    5
    6
    7
    New Freezing Point >
    +26 F
    (8.3%)
    +19 F
    (17%)
    +10 F
    (25%)
    0 F
    (33%)
    -15 F
    (41%)
    -34 F
    (50%)
    -57 F
    (58%)

    This article is copyright 2000 by Bob Muenchausen and may be reproduced for personal use as long as the copyright and authorship is acknowledged. Please direct any questions to: bobmunch64@yahoo.com.

  • Draining and Refilling May 2014

    Originally rads had a dinky brass tap, which probably dates from when people used to drain the rad each evening in winter and refill with water next morning, although as there is still a significant amount of water remaining in the system it wasn't always proof against block damage. Later rads had a threaded plug, but the problem with those is that unless the threads are lubricated they can seize and you can turn the whole fitting out of the bottom of the rad. Subsequently even that was dropped, presumably as cost-reduction, which means the only way of draining the system is by removing the bottom hose.

    Cars with mechanical fan: I can't remember ever removing the bottom hose on Bee over the 25 years I've had her until I needed to replace the thermostat, and hadn't appreciated just how awkward it is to get it off the rad. You do not want to completely remove it while under the car as coolant goes everywhere, but you will have to undo the clamp and get the hose moving on the rad port at the very least. Then from above you will need to pull the hose upwards, which means folding it to some extent, and it will probably come off all of a sudden. It's easier to remove it from the pump first, then pull it up, but that means coolant going everywhere twice. With old hoses that have hardened you may have to do it this way anyway. However the bottom hose that came with Bee 25 years ago is still supple and crack-free, which is more than can be said for the other hoses that I've had to replace on both cars over the years.

    Cars with electric fans including V8: Much easier as there is more space to get at the hose, and direct it into a container, it's also a straight pull backwards so no folding of the hose required.

    When refitting bottom-hoses on the mechanical fan cars and the V8, great care needs to be taken with positioning, as described here.

      Refilling has its own considerations and pit-falls. When using glycol-based antifreeze that comes as a concentrate, to get the correct ratio you should add the required quantity of neat glycol to the drained system, then add water to fill the system. Unless the engine and heater are completely dry, as after a strip and rebuild, there will always be some old coolant left behind. If you simply refill with dilute anti-freeze, then whatever is remaining in the engine and heater will dilute it still further. The total capacity seems to have changed several times over the years, see here. How you get on with replacing glycol with ForLife I don't know, but I do know that with Evans waterless you have to go through several flushing cycles first.

    For the first start after refilling you can get very different results depending on what thermostat you have, which unless you have changed it will be an unknown quantity. Unless you have a thermostat with one of the self-bleeding functions, the thermostat will trap a large volume of air under it, until it opens. When this happens the level in the radiator will drop like a stone, so you will have to leave the cap off watching the level, and have a kettle-full of hot water to hand to top-up. When I refilled with a non-bleeding thermostat I only got the required amount of concentrate in, i.e. just under 2 litres, and that brought the level up above the tubes. When the stat started to open and the level dropped I had to add over a litre of hot water to get it above the tubes again. If you have a self-bleeding stat you will be able to put at least 1 litre of cold water in as well as the concentrate to half fill the header tank before starting the engine.

    Once the thermostat fully opens it may then drop more slowly as it purges itself, so you can trickle some more water it to keep the level visible. The heater valve should be open, and ideally the nose of the car higher than the rear to aid purging. Check the level again when cold and top-up with plain water, it may need a couple of heat/cool cycles to fully purge itself. After that, any topping-up required should be done with the required dilution, I keep a 2 litre container of ready mixed for this.

    I have the mid-era top-fill rad so it is easy to see the coolant level, and top up while running. On the earlier rear-fill rads you can't see the tubes so will have to maintain the coolant to just above the bottom of the filler elbow. If done cold this can chuck some out, not so if checked warm, but be careful removing the rad cap if hot. Herb Adler added a catch bottle to his as described here. The remote expansion tank on later systems can't be used to fill the system. 4-cylinder cars have a fill port on top of the thermostat housing, and V8s have one on top of the radiator. But both of these only fill the radiator, there will still be a large amount of air trapped below the stat if that isn't self-bleeding. The V8 fill port can be used to observe the level, and the sudden drop and consequent topping-up with hot water, but I suspect the 4-cylinder thermostat housing fill port could well overflow with hot coolant from pump pressure and flow as the thermostat opens.

    Electric Fans

    4-cylinder and V8 cooling fan schematics
    Take the fan load off the battery when cranking.

    My V8 cooling was always a bit marginal in that the temp gauge was higher in hot weather than I would have expected. And although I have a high-efficiency rad (replaced the old one when it had successive leaks, looks the same but has 25% more tubes) which improved matters slightly I was on the lookout for a bit more.

    My 'Otter' switch (so-called because it switches on the fans when the water gets 'otter) was getting a bit 'late' (the normal switch-on point is about mid-way between 'N' and the red, or 90C/194F degrees F) and erratic so I bought a replacement (BHA5252) from SU Burlen. This is a copy that actually switches on slightly earlier than the original which is no bad thing in itself, and also has the side-effect that hot oil pressure after idling for a while with the fans cycling on and off is noticeably higher. However it has a much higher hysteresis than the original, which means that once switched on the temp has to fall further before they switch off again, i.e. the fans run for much longer than they used to. At least, most of the time it does. Very occasionally, when I am stuck in traffic, it will cycle on and off much more frequently such that the temp gauge only varies slightly at a point between N and H. Why, I have no idea, but there it is, and I wish it were like this all the time. At the time of writing (February 2011) I see Brown & Gammons also have the switch, it's quite a bit more expensive than Burlen, but if it is an original Otter I would opt for that one. There may also be other sources.

    I also decided to investigate the voltages in the fan circuit as they draw a high current and any resistance in wiring and connectors causes a significant volt-drop. I was quite surprised to find I was losing 0.3 volts in various connectors in the brown circuit, 0.8 volts in the relay, and 0.7 volts in the earths - nearly two volts altogether. A PO had added some crimped connectors in the brown circuit and omitted to solder them so I rectified that, the relay was also getting very hot in use so I replaced it, and I added some earths from the fan connectors to terminals bolted between the fan brackets and the bonnet slam panel. Got rid of most of the volt drops (some in the wiring is inevitable) and the fans are now audibly faster and cool quicker.

    I have also been looking at the possibilities of fitting some additional puller fans - they would have to be very slim and of a small diameter to clear the water pump. However, after having seen some fans of this type used in pusher mode I am very unimpressed. Despite having a shroud I don't think they are as effective as the factory fans, which really is saying something. The most noticeable difference is just how long the fans continue to spin after the power is switched off, which says to me they aren't pumping very much air, backed up by feeling how much (little) air is pumped through the rads when they are running. They have 'spiral' blades, which one fan manufacturer claims they tried, and promptly dumped them. At a minimum of 60 each I think I'll give them a miss and look further at fitting shrouds to the existing fans.

    The latest enhancement was to run a heavy-gauge brown wire from the unused output spade on my alternator direct to the fan relay, which happened to have a second spade on that terminal. This has made another notable improvement in fan speed and rate of cooling, subsequently I shut it in the garage (exhaust piped outside) on a 30 degree day and left it fast-idling to see what happened. The air going into the grill was being recirculated from the engine bay rather than being 'fresh' air at ambient and actually got up to 41 degrees, but still the temp gauge never got more than two-thirds the way from N to H. Travelling through France to LeMans and back in 2002, which was very warm, I was able to keep the needle on Normal at all times even in the hottest conditions. Auxiliary fans no longer required.

    Added October 2008:

    Unlike mechanical fans which can only turn one way with the engine (clockwise when viewed from the front of the car), electric fans will usually change direction according to how the motor is connected, these are motors that have a permanent magnet stator. If you should have a motor with a wound stator reversing the polarity to the motor as a whole will have no effect. Starter motors are like this, and early heater fan motors were the same, but all motors you are likely to find for cooling fan applications will almost certainly have the potential (ho ho) to run backwards. So the first thing to do with a new fan motor, or if you have cooling problems, is to check which way the air is blowing when the fan is running. The fan should rotate anti-clockwise (i.e. opposite to the mechanical fans) when viewed from the front of the car to push air through the radiator. Note that turning the blades over on the motor spindle will not affect air direction, only changing the motor polarity will. However as with mechanical fans the blades are 'handed' which means they move air more effectively one way round than the other, and this is where turning the blades over may be required. An old fan from my V8 has an arrow on one side of the boss of the blades showing the direction is should spin. This is on the side facing the motor so you have to squint down the gap between motor and fan boss to see it. If you can't see an arrow, or other marking indicating direction (one person has reported that his fan has 'B' on one side for 'Blow' and 'S' on the other for 'Suck' but that may just be BS ...) you will have to look at the tips of the blades. These should resemble a wing profile, and have be relatively blunt on one edge and tapered on the other. Just like an aircraft wing the blunt edge is the leading edge, and the tapered the trailing. See the accompanying photos (click thumbnail) for examples of the direction arrow and profile.

     Update October 2010

    Haydn Davies has written to me saying he understands the theory, but several sources show the fans being the other way round relative to the motor. Looking at the drawings in the Parts Catalogue the orientation of the motor and fan is incorrect according to my V8, on which the slot for the grub-screw is on the side of the blades facing away from the motor. If you look carefully, the blades at top, bottom and right in the drawing have the concave side facing the motor which is incorrect for a pusher fan, the thicker edge will also be trailing for a motor that spins anti-clockwise when looking from the back of the motor through the blades at the radiator which is also incorrect, and the blade on the left has the concave side facing the other way to the others which is obviously incorrect! With the exception of this blade everything else would be correct if the fan were facing the other way. My Leyland and Haynes workshop manuals don't show the fan itself, but various online parts catalogues with exploded drawings show the same motor and fan to the Parts Catalogue (identical at Moss, mirror image at MGOC), even to the extent of showing the three blades with the concave side facing the motor and the odd one away! These are obviously cribs from the Leyland Parts Catalogue, and have consequently repeated the error.

    Update May 2016

    After the MOT I noticed how much the cooling fans seemed to affect the cranking speed on a hot start, and thought they would be a far more worthy candidate for being controlled by the 'accessories' circuit which is cut off during cranking, than the washers, wipers and heater fan. So gazing at the fan relay and fuse and working out what would be required, I noticed the rubber cap had been pushed off the blade-type fuse holder. I'd added this some time ago as the car came to me with unfused fans, at the time Halfords only had 6 amp glass-type in-line fuse holders, the fans take a lot more than that, and I happened to have this blade-type which should be good for more than that. However being 'recycled', one of the socket connectors was slightly corroded, which gave a slightly bad connection (not enough to notice from the fan speed), which had heated up and melted the fuse body, which had pushed the cap off. By this time I had bought a bunch of 20 amp glass-type holders, so fitted one of those.

    The accessories fuse is literally right next to the fan relay, but no convenient connectors so I had to cut the output wire from the fuse (green/pink) and solder on two bullets, and fit a 4-way connector between them. Then about 4" of wire was all that was needed to go from there to the relay, in place of the green (fused ignition) wire. This wire only operates the 4-terminal relay that I have, it doesn't power the fans, so the current is negligible. Turned on the ignition to the accessories position, and I was surprised to hear the relay click and get a burst of spinning from the fans, and the tell-tale in the switch was glowing slightly. I realised that the tell-tale also needs a voltage supply, which I had taken from a fused ignition source. So effectively what was happening now was that in the accessories position 12v was being fed through the relay back to the override switch, through the tell-tale, onto the green circuit i.e. the fused ignition supply. The standing loads on that such as ignition and fuel pump allowed enough current to flow to briefly operate the relay and cause the lamp to glow, but not enough to power the ignition circuits. It all worked correctly with the ignition on, but I didn't want to leave it like that. I could have fitted a diode at the relay to block the reverse current, but opted to change the wiring at the override switch to pick up accessories power from the heater switch which is right next to it, instead of fused ignition. The irony was that when I removed the (POs) wire that had been feeding fused ignition to the override switch to power the tell-tale, I found it went all the way to the fusebox, and was long enough to have reached the fan relay. So I could have reused that to feed accessories power from the switch to the relay, instead of cutting in to the accessories wire by the fuse! Oh well, such is life.

    Subsequently I realised I could have done it without any cutting, which I normally try to do. If you get one of these fuses with a loop of wire between the ends and not the pre-cut type, then you can fit it between the two ends of the existing fuse-holder, i.e. effectively have two fuse-holders in parallel. You can have a fuse in both, or put a metal bar the same size as a fuse in the new holder. The power comes through the old fuse-holder first on white/green, so that should still have a fuse. Then all you need to do is tee another wire (soldered and heat-shrunk please, not Scotchlok) into the new wire between the fuse-holders, and that is your fused accessories supply to the fan relay once you have removed the original green supply from it (insulate it to stop it shorting on anything). To reverse, simply remove the new fuse and its tee, connect the two halves of the original fuse-holder together, and reconnect the green supply to the relay.

    Fan Belt Added December 2009

    Adjustment
    Size Considerations Added September 2010

     Adjustment: At the moment I'm going to restrict this to the main water pump and alternator drive belt, and not get into the additional belts used on North American spec cars with air-pumps let alone air-conditioning.

    For years I've checked my belt tension by seeing how far the middle of the longest run can be deflected from its 'at rest' position to or from the other side of the loop with light-ish finger pressure, and set it to about 1/4"-1/2". I've never had belt squealing in 40 years, and only had to replace a water pump once each on two cars quite soon after coming to me after many miles with other owners. Some manufacturers are a little more precise and state a deflection with a particular force, and others require two marks to be made on the belt a certain distance with no tension, and the alternator adjusted to cause those marks to move apart to another certain distance, i.e. the belt is stretched in use. The Leyland Workshop Manual in the Cooling section states for both dynamo and alternator systems it should be possible to move the belt laterally in the middle of its longest run by no less than one inch! Is that in one direction which seems a helluva lot? Or a total in both directions i.e. 1/2" each way which seems more reasonable? The Electrical section just says to "remove any undue slackness". Haynes in its cooling section says there should be a sideways deflection in the middle of the longest run of 1/2" on 'early' models, and 1/4" under an applied load of 7.5 to 8.2 lbf. Both 'lateral' and 'sideways' imply to me to be pushing or pulling it towards the front or rear of the car, and not towards or away from the other side of the loop which is always the way I have done it. Also is it in one direction or both? It seems likely that when using a spring-balance to apply the stated force for later models it would be in one direction only. The distance for that is half that for 'early' models, so do the early ones have twice the play? Or is theirs in terms of total deflection in both directions? And for that matter what is the definition of 'early' and 'late'? (Dynamo vs Alternator?). FWIW my 1967 Mini Workshop Manual also quotes an inch in the longest run, but an 80s Metro Manual quotes a much smaller deflection of 3/16" and in the shortest run i.e. between alternator and water pump pulleys. It also shows it being pressed down towards the crank pulley i.e. not sideways, and only in one direction from rest. Interestingly it also quotes a torque wrench value of 11 to 11.5 lbf when applied to the alternator pulley nut to produce slip, which is perhaps the most useful and relevant approach.

    The Lucas Fault Diagnosis Manual has this drawing, which is repeated with the same deflection distance for dynamos and alternators, and shows 1/2" to 3/4" (13-19mm) in one direction, and towards the opposite side of the loop. The V8 Workshop Manual Supplement specifies "1/2" (13mm) total deflection on longest run".
     
     

     Size Considerations Added September 2010: I've been asked a question about fanbelts widths for different alternator types, in particular how those for the higher output Lucas 20ACR and 25ACR compare to the 18ACR, so did a little research.

    It will depend on the pulley. As the same alts definitely have different diameter pulleys for different applications in theory you should be able to swap them round i.e. between different 18ACRs from different cars. That implies a common size of spindle, but whether this extends to different models of alternator I don't know. Different makes of alternator do seem do have different spindle diameters, one site claims 15mm for Lucas and 18mm for Bosch. My roadster has an alternator with an alloy end plate and not plastic like the originals, I don't actually know what model it is, but the standard belt fits fine. I have a Lucas A115-45 from a wrecked 80s Metro which I have trial fitted to the roadster. The pulley is a different diameter (which gives a lower output at idle but is fine when driving) but the same belt fits just fine, so I keep it as an emergency spare. With a V-belt and pulley as long as the belt isn't sitting on the bottom of the vee (and hence no pressure on the sides) but is being wedged into the vee of the pulley, it shouldn't really matter unless the belt is so wide that the top of the belt is above the top of the vee and hence only part of the vee of the belt is engaging with the vee of the pulley.

    The clue may be in the belt number. I know many indicate the length, and Goodyear for example say the first two digits of their belt numbers indicates the width as well as the last four indicating the length.

    This site gives codes for widths, and the examples in the image could well contain width as well as length info printed on them like the Goodyear.

    This Triumph (I know, "wash my mouth out") site says 15ACR and 18ACR were used with no A/C, or 20ACR and 25ACR with, you would have to compare pulley and belt numbers. But one thing occurs to me concerning A/C and higher-rated alternators, and that is that the more current any alternator delivers, the harder it is to turn, which puts more load on the fan-belt. It could be that higher current alternators do have a bigger (wider? deeper?) belt because of this. It would only be an issue if you were going to use the extra current, but then a bigger belt would impact on the water pump and crank pulleys as well!

    So what size belt is required for standard engines (i.e. standard crank and water pump pulleys and standard dynamo/alternator) on 4-cylinder engines? The Parts Catalogue quotes:

    EngineWidthLength
    GFB103 for 18G and 18GA (3-bearing) engines, and 18GB 4-bearing engines10mm900mm
    GFB176 for 18GD to GK engines, 18V engines (except as below)
    and those for Germany and Switzerland from 1975 on
    10mm900mm
    GFB205 for 18V 797/798 and 18V 846/84710mm950mm
    GFB255 for all engines and all markets from 77 on10mm950mm

    Modern equivalents are the GCB10900 and GCB10950 which are direct replacements for those in the above table, the first two digits signifying the width (10mm) and the last three the length (900mm and 950mm respectively). You may well get slightly longer ones to fit, or maybe even slightly shorter ones, it depends what pulleys have been used, which may not be original. A tip on changing is to get the belt over the crank and alt pulleys first as they wrap furthest round these, and the water pump last - having got it over the mechanical fan blades first of course! If the belt is still slightly too short then try removing the adjuster bolt altogether (while fitting the belt!) which may get the alternator that bit closer to the block.

    For the V8 the original was specified as GFB 148, recoded as GCB 11125 i.e. 10mm wide (although shouldn't that be GCB 101125?) and 1125mm long. However some have reported problems with that belt too long on both conversions and factory cars, and mine is right at the end of the adjuster but could still do with a little more tension. A shorter belt would be preferable, GCB 11088 has been mentioned i.e. 1088mm, but then the problem is then getting the alternator down far enough to get the belt on as it hits the rocker cover way before the inner end of the adjuster is reached. Maybe something between the two would be better, e.g. GCB 11100 (1100mm), although with that someone has reported the adjuster bracket fouling the alternator fan! To repeat the tip above, try fitting a short belt over the crank and alternator pulleys first, and the water pump last.

    In any event any belt you buy as a spare should be trial fitted when you get it, it should be kept in the car and not the garage(!), along with the tools to change it, and you always should be carrying a spare, even if only by the Law of Sod to guarantee you will never need it!

    Hoses was 'V8 Bottom Hose Guard', amended April 2009

    V8 Bottom Hose
    V8 Top Hose

    Both Bee and Vee came to me with old hoses of course. Bee's top hose showed slight signs of surface cracking, so I replaced it as a precaution, only for the new one to look the same after a couple of years. Replaced that with a Kevlar, and that does seem to be lasting better, although contrary to what I originally thought it isn't a Kevlar 'sleeve' embedded in the rubber like the earlier fabric reinforced hoses have, but apparently just chips of Kevlar embedded in the rubber. They may resist some abrasion in the short-term, but it is the quality of the rubber that will determine longevity. By contrast Bee's bottom hose remains unchanged (until May 2014 as part of a cooling system problem) and still pliable and crack-free, after 20 years and 50k miles just in my ownership - "They don't make them like they used to".

      When refitting hoses on the mechanical fan cars and the V8, great care needs to be taken with positioning. On the 4-cylinder there is only about 1/4" of clearance either side of the hose between the timing cover and the fan blades. After pushing the hose on make absolutely sure the blades are well clear, and allow room for hose expansion under pressure. However putting it too close to the timing cover can result in rubbing from that. Electric fan 4-cylinder cars should have no problems, but the V8 has this and additional issues as below.

      V8 Bottom Hose: Travelling at speed on the motorway in Vee one day I became aware of a slight misfire when accelerating. It was raining so could have been road spray affecting the HT leads, but it had never happened before. The thought immediately occurred to me that it could be a coolant leak spraying onto the HT leads. If so I'd be in illustrious company - I had recently read about an MG record attempt where exactly that happened, although in that case it was very high cylinder pressures lifting the head and causing coolant to spurt out past the gasket! All the gauges were normal, and as I had just passed a service area I carried on keeping a close eye on things. At the next service area I pulled off and as I slowed to parking speeds could hear Vee hissing like a steam engine! Stopped and opened the bonnet to see steam issuing from what looked like a pin-hole in the bottom hose right by the fan-belt, together with what looked like a series of cuts in the same area. Wasn't carrying a spare hose (then, I do now!), so it was a job for the AA. They came in due course, no replacement hose of course, so it was a repair jobby with a special kit (I now carry one of those as well!). Very little coolant came out when the hose was removed, yet the temp and oil gauges had stayed on normal all the time. The repair got me up to Liverpool and back via Clive Wheatley for a replacement hose, but was beginning to weep the next day, when I replaced the hose.

    With the hose removed and the pressure released I could see where the fan-belt flapping had worn right through the hose in the one spot as well as reducing the thickness over a wider area. This hose is very soft and pliable even when cold so it's quite possible the pressure in the cooling system caused it to balloon out and press against the fan belt, whereas they usually harden and crack with age. Conversely there are those that claim that the action of the pump sucks the bottom hose flat, but they are usually Americans and I have no idea what happens on American cars. On the Rover V8 another cause of this damage is caused by the fan belt 'flapping' and hitting the hose on rapid deceleration from a high rpm.

    Added August 2009:

    Allan Reeling has reminded me of something I remember hearing about a long time ago, and that is some pumps have the pipe for the bottom hose pointing directly across the car, whereas it should be angled back very slightly, viewed against the line of the fan-belt, as in this picture. Pumps that have the port parallel to the fan-belt will put the hose very close to the belt further down.

    Because of both factors I decided to fabricate a guard to fit between the hose and fan-belt. There were convenient mounting points on the bottom alternator mounting bolt and a stud and nut on the engine front cover, and by careful shaping I could get it to fit the curve of the hose so as to take up minimum space. Whilst the fan belt may now be flapping against the guard on deceleration, it has a smooth surface and so should cause minimal wear to the belt, but in any case it is easy to monitor the condition of the belt, and if the worst happens and it breaks it is much easier to replace at the roadside (I carry a spare one of those too!) than a hose.

    Having had that hose fail I've always carried spares since in both cars. It was the fan belt that had cut through the bottom hose on Vee, apart from that it and the top hose were as pliable and crack-free as Bee's bottom hose. But after doing the top-end rebuild of the V8 in 2002 and having to disconnect all the hoses I decided to replace them all rather than refit unknown quantities. At the time my supplier Clive Wheatley warned me that the quality was poor and they were likely to only last a couple of years, but they were the best he could get at the time, and to keep an eye on them. I kept the old top hose as a spare as that was also still very pliable and crack-free. When checking the bearings last month I noticed that Vee's bottom hose was quite badly cracked, but typically only the bottom half, it looked fine from above! On the face of it seven years is quite a long time, but it has only done about 15-20k which is an average 2 years-worth so Clive was right! So I fitted my replacement, which has a Kevlar label on it, so hopefully better than the previous one. At the Stoneleigh spares show this February Clive said he had at last been able to source some quality hoses again, and they do feel much better - shiny black rather than dull grey like the poor quality ones. He also has silicone ones available, even blacker and shinier, but at several times the price!

     

    Fitting the new bottom hose this time I noticed exactly the same thing as with the previous replacement. Even when the hose is pushed onto the radiator outlet as far as it will go, part of it is pressed up against the flange on the front oil hose coming off the pump, which is not a good thing as the two will be moving relative to one another and the flange will cut into the hose. It had only made a small dent in the old hose, but admittedly with only 15k miles. By contrast there is at least half an inch clearance between the bottom curve of the hose and the anti-roll bar, even though I have an uprated bar. So both times I have cut (despite the Kevlar in this case) 1/4" off the bottom end of the hose, which puts it closer to the anti-roll bar (but still 1/4" clear) and now only just touches the oil pipe. I'd move it in a bit more but the bottom bend is so near the end I'm concerned it wouldn't fit and seal properly to the straight outlet on the radiator. You also have to be careful not to move it so far forward that it comes too close to the fan-belt, unless you have a guard as I have.

      July 2014: Noticed a spot of coolant on the floor where Vee had been parked - bottom hose dripping. It was coming from by the flange on the oil cooler hose, which I mention above a couple of times. It was touching the flange, which had worn a groove, despite me positioning it with a small clearance when I last replaced it, which was only 5 years and 12k ago. Also signs of surface cracking, again at the bottom, and when freeing it from the water pump inlet it split very easily. All of which confirms an opinion that the rubber in these 'Kevlar' hoses is still crap. So I fitted the hose bought in 2009, which again needed some cut off the rad end.

    This time I've managed to get a definite clearance to the oil cooler hose flange, while still maintaining a clearance to the anti-roll bar. One thing I noticed though is that with the engine running the bottom of the rad, and the body flange it is bolted to, flaps back and fore a bit, i.e. towards the cooler hose flange. The body flanges are moving relative to the chassis rails, and it looks like there were originally welds between the flange and the top of the chassis rails, which have failed. It would be difficult enough cleaning the left-hand side up and rewelding, even more so on the other side with the oil pipes in the way. On the back of the flanges there is a rad mounting bolt head about an inch above the chassis rail, and a plastic plug on top of the chassis rail about 4" back, so it should be possible to make up a strut with a right-angle under the bolt head, a longer piece lying on top of the chassis rail, and some kind of pin or fastening in the chassis rail hole once the plastic plug has been removed. OTOH maybe a run of weld across the chassis rail behind the flange will be enough to stop it coming forwards, even if it doesn't securely weld the two together.

    In the meantime I taken up the suggestion of a pal to put a section of the old hose - split lengthwise - around the new one adjacent to the flange as a different type of guard. Not a very good fit originally due to the different radii of the ID of the old piece and the OD of the new hose, but a cable tie should hold it in place. Cutting this section out of the old hose revealed just how much the thickness of the wall varies around its circumference, something I found on the burst top hose last year as well

      V8 Top Hose June 2013

    Burst while in the Lake District. I knew nothing about it until coming away at the end of the weekend, when the coolant level warning stayed red. We had a pretty hot run over the Honister Pass to get to our B&B on Buttermere, I think it burst due to heat-soak after I parked the car. I was carrying a spare, and coolant, so only took a few minutes and a couple of litres to resolve and we were on our way again. Needless to say I got a replacement within a few days.

    It had a 4" split, along what look like a moulding seam along the length of the hose. The thickness at the split is probably half that of the ends. I was warned about these hoses when I bought them, that they would only last a couple of years but they were the only ones he could get at the time. It never looked 'right', being grey with a matt texture, not black and shiny like much older hoses, and the spare I fitted and the replacement for that. It had also developed fine surface cracking over the years, I had looked at it a couple of weeks earlier and felt that the cracking was a bit more pronounced, wondered if I should change it anyway but didn't.

    Mechanical Fans Added August 2007

    For details of which fans were used when see below.

    Fans are usually 'handed' i.e. they have one side that should face the engine and a different side facing the radiator. Reversing it does not change the direction of air flow (only changing the direction of rotation can do that), but it does have an effect on efficiency i.e. how much air it moves at a given speed. Both metal and plastic types seem to have sculpted blades where one side is concave (i.e. slightly cup-shaped) and the other convex. The concave side should face the engine as it is more effective at pushing air towards the engine i.e. pulling it through the radiator. Metal types also seem to have one gently rounded corner and one tightly rounded on each blade. The gently rounded corner should be on the leading edge, probably for safety reasons if one gets one's fingers in the way. The individual blades of metal fans are attached to the stubs of the central boss on the engine side, with rounded rivets, both of which reduce turbulence and improve efficiency. Plastic fans often have the blades with an aircraft wing profile, with the blunter edge leading and the more finely tapered trailing.

    That's the theory. However, after someone asking which way round the fan should go, and more than one of us saying basically the above, the next time I looked at my roadster it seemed to be the wrong way round. So I took it off and turned it round, and had only just started attaching it when I noticed that the blades were going to be very close to the crank pulley and alternator spindle, and significantly further away from the radiator than they were before. So I took it off again, laid it on a flat surface first one way up then the other, and measured how far away from the flat surface the centre of the boss was in each case (see pics). It was about 6mm one way round, and about 2.6mm the other! Mounted the 'wrong' way round the blades have a clearance of about 7/8" from the crank pulley and alternator spindle, and about 1 3/4" from the radiator core. The right way gives virtually no clearance to the alternator and crank pulley and 2 3/4" to the radiator.

    Now because the fan isn't shrouded, the further away it is from the radiator the more air will be drawn past the rear face of the radiator instead of through it, reducing cooling efficiency at slow road speeds or at a standstill. OTOH it wouldn't do to go too close to the radiator with a mechanical fan, as being mounted on the engine if the engine moves on its mounts i.e. under heavy braking of an impact it will chew through the radiator. But it then became apparent that the 3-bladed fan, common to both 18G and 18V engines at various times, needs a spacer (12H 3910) when fitted to the 18V, which is what mine is missing. Steve Loft was kind enough to respond to my questions and send me a comparative picture of his 74 engine with short-nosed pump, which clearly shows the spacer mine is missing, and the leading edge of his blades about 1 1/4" from the core. David Bolton did the same thing, and with his long-nosed pump (so no spacer) and 6-blade metal fan, he also has about 1 1/4" clearance to the core.

    From the photographic evidence, the offset of the fan boss, and the relative distance of the fan blades from the alternator pulley on mine and Steve's engines, I reckon the spacer is about 1". These don't seem to be available from the bigger suppliers (and in any case need longer bolts) so I may make one in due course, but as the roadster has never given me a moments concern about overheating, even in very warm weather and with a rally plaque just in front of the radiator grille, I'm not in any rush. Update September 2007: I've located and obtained a 2nd-hand spacer from Andy Jennings, but unfortunately he couldn't supply the bolts (HZS 415) which need to be about 1 3/4" minimum length to go through the 7/8" spacer and the 1/2" (or so) distance piece inside the fan grommets. I'm wary about ordering them online or by phone as none of the online parts lists seem to differentiate between the two lengths of bolt, so I'll probably visit Leacy's sometime as I want some other heavy parts as well and it's cheaper to collect than pay postage.

    Spacer cleaned up and fitted, fan repainted, and fitted the right way round. But it's still 1 3/4" from the core, i.e. 1/2" more than even Steve's with apparently exactly the same components. Our two fan belts are the same distance from the core - about 4", but looking at my fan the blades seem to be angled backwards. Whether that is 'original', or whether whoever fitted it the wrong way round bent them to get them closer to the radiator, will never be known. It's not worth removing it to bend them forwards, I've never had the slightest qualms about Bee's cooling, even stuck in a traffic jam on a hot day. Despite the marks on the spacer on the concave side which appear to have been made by the distance tubes going through the fan grommets, the logical way is the concave side facing the pulley and pump (as per the Parts Catalogue drawing) as it clears the pump nose, and the smaller diameter is closer to the pulley diameter than the larger. With an extra inch on the front of the pump I was wondering if the reduced space to the radiator would cause problems with the bolts and lock-strips. But I removed the two bottom bolts securing the radiator to the shroud and backed the top two right off, which allowed me to wedge the radiator itself an inch or so forward of the shroud. Despite the mini 'fan guard' at the top of the shroud still being in its original position I had plenty of room to access the bolts and lock-strips. Tip: Offer the spacer, fan, washers, lock-strips and bolts up to the pulley as a pre-assembled unit as you almost certainly won't be able to insert the bolts with the fan in position. Also decide which of the two 'ears' you are going to use on each end of each lock-strip (depending on where the blades are one is usually easier to get at than the other) and give that a little bend away from the washer underneath it to leave space to get a drift in and flatten it the rest of the way onto the nut flat. It won't interfere with getting a spanner on the nut, and without doing it you will probably need a sharp blade like a chisel to start it moving.

    Update October 2007

    David Bolton writes with an interesting idea from Newcastle-upon-Tyne for hot-country touring. He was planning to take a recently restored 72 roadster to Portugal and was a little concerned about engine cooling, even though it kept its cool in the UK 2006 temps of 35C. One source advised fitting an additional electric fan, which is quite a common change, whilst another suggested fitting the 6-blade cooling fan that was used on exports to hot countries. The less-intrusive 6-blade fan was more appealing and so one was sourced and fitted, as shown here. In the event in temps in the high 30s, road works, and the Picos de Europa the car still kept its cool. Now back in the UK David was wondering whether perhaps it was overcooling, as the temp gauge now doesn't get more than half-way from C to N. However the top hose is too hot to touch, so maybe it is a low gauge reading. As to keeping the 6-blade, on the face of it as long as the thermostat is able to maintain the correct engine temperature, the only down-side at the moment being be the greater noise and bhp losses. But if that means the thermostat is permanently restricting coolant flow through the engine, localised hot-spots could develop, which may cause other problems over time. And if the car is used in winter the much greater air-flow over the block and hoses etc. from the 6-blade fan could increase surface cooling beyond the point at which the thermostat can compensate, which wouldn't be good for the engine. So personally I felt that it would be better to revert to the 3-blade for UK use and keep the 6-blade for continental use, as by tilting the radiator forward instead of complete removal it should be possible to change it quite quickly and easily. There remains the question of what the coolant temperature actually is at that gauge reading, and whether the engine is currently overcooled (which shouldn't be possible if the thermostat is doing its job, except perhaps in very cold conditions with a high surface-cooling effect) or whether the gauge is reading low.

    Update January 2010

    The seven-bladed plastic is also handed. There is a flat side to the boss and a deeply dished side. It might seem logical that the dished side goes over the pulley, but it doesn't it faces forwards and the flat side goes up against the pulley. The blades also have one slightly curved edge and one straight edge, the curved edge is the leading edge and the fan rotates in a clock-wise direction when looking in from the front. With the fan the wrong way round the blades clearly foul several parts of the engine. In the accompanying photos the fan is fitted to a short-nosed pump without a spacer, I haven't checked the clearance to the radiator but the table in Pumps below does indicate one is required. This fan has its own metal insert for the bolt holes, so only the bolts and lock-tabs are required, not the spacer tubes or grommets.

    July 2014: Roger Parker covers the same topic in 'Enjoying MG' this month. However the measurements he gives are significantly different to the three above. He measures to the edge of the header tank, which is fair enough at that is closer to the blades than the core. He says that on several cars he measured it ranged from 3/8" to 1 1/4", but the average clearance on cars with the 'correct' spacing was 1/2"! Even subtracting 3/8" - which is the distance from the edge of the header tank to the face of the core - from our measurements that gives 7/8" for Steve's and David's, quite a bit more than Roger's 1/2", and 1 3/8" for mine which is nearly 3 times as much.

    November 2014:

    It was only when pal Terry and I were discussing the stay bars for his radiator that I realised there should be some packing strips between the radiator and the diaphragm it is mounted on (there are none on Bee). Two (AHH 6320) per side before chassis number 146506, one per side from then until the 1977 model year. These are about 1/8" thick, so two per side will give an extra 1/4" spacing from the fan. They will also close up the gap between the footer tank on the rad and the apron, giving a bit more forced air through the rad, although you wouldn't want vibration to wear through the rad. Note these do not apply to the V8 or the 77 and later 4-cylinder - the V8 at least mounting panels have three holes but both rads only have two welded nuts each side.

    Pumps (and fans) added October 2009

    Pump change June 2014

    Water pumps are one of those things one changes when they go wrong, and generally what goes wrong is the seals wear and they leak, or the bearings wear which causes noise, wobble and also leaks. I have also heard of the impeller coming off the shaft, which will cause overheating of course. Bee's pump started leaking shortly after I bought her. At that time I wasn't aware of the variations between engines, but the proprietor of the MG parts supplier had a look and got the one that I needed, with a new gasket - or so I thought! When I came to fit it I found it was alloy instead of steel (probably a copy as I know now), one of the bolt holes was about half a hole out even though the gasket was correct (but being alloy was easy enough to file into an oval), and another had a thick boss which meant the bolt wouldn't engage with the threads. Fortunately also easily correctable. To add to all that shortly after fitting I noticed it had started weeping slightly during warm up! So I bought another, steel this time, but as it happens in all the miles and years since then the leak hasn't got any worse and water loss is negligible and I still have the steel one in the boot ... (until June 2014, that is).

    I've heard it said that there are at least two pumps with different impellers, both fit at least one of the engines, but the incorrect combination results in a low coolant flow rate and overheating. If that's true it can only be the pumps for the 3-bearing and 5-bearing engines that are incompatible, as all the other pump changes occurred during productions runs of the various five-bearing pumps and so must be interchangeable - with a caveat.

    The caveat is that the pumps changed from having a long nose to a short nose in August 71 for the 18V engines. Whilst the pumps themselves are interchangeable it has a knock-on effect on the pulleys and fan blades that can be used. Long-nosed pumps use a deeply dished pulley, so of course a short-nosed pump will move the V-groove much closer to the engine and way out of line with the crank and dynamo/alternator pulleys. The fan bolts up to the pulley so that is also moved closer to the engine and in fact hits it. Short-nosed pumps used a shallower pulley to keep the correct V-groove alignment. The fan blades would still hit the engine of course, so a 7/8" spacer was fitted between the pulley and the fan. You can get away without the spacer if you have a 3-blade metal fan by turning the fan round the wrong way, because the blades are offset to the centre this actually puts the blades at almost exactly the right position between engine and radiator. They are turning the wrong way of course, but the difference in effectiveness is marginal, I ran Bee like this for many years before discovering it, and obtaining and fitting the spacer. In theory you can also use a deep pulley with the short-nosed pump, by putting the spacer between pump and pulley instead of pulley and fan, but I've not tried it. You need longer bolts when using the spacer in any configuration, of course.

    Culled from various sources this is what I have managed to work out for the many combinations of engine, market, period, pump, fan, and the various bits and bobs to put them all together:

    EngineMarketDatesPumpGasketBoltsLong/
    Short
    Pump NotesPulleySpacer?FanBoltsNotes
    18G, GAAllMay 62 - Oct 64GWP 11512H8142xHBZ514, 2xHZS510Long 12B174N12H1058HZS4083-blade metal
    18GBAllOct 64 - Aug 6712H 226788G4302xHBZ514, 2xHZS510LongReplaced by and compatible with GWP 11412B174N12H1058HZS4083-blade metal
    18GBAllAug 67 - Nov 67GWP 11488G4302xHBZ514, 2xHZS510LongClausager indicates a further change in Jun 66 but this doesn't appear in the Parts Catalogue12B174N12H1058HZS4083-blade metal
    18GD, GGnot NANov 67 - Aug 71GWP 11488G4302xHBZ514, 2xHZS510Long 12B174N12H1058HZS4083-blade metal
    18GD Rc (Auto)Not NANov 67 - Oct - 68GWP 11488G4302xHBZ514, 2xHZS510Long 12B174NBHH1604HZS4086-blade metal
    18GF, GH, GJNANov 67 - Aug 70GWP 114
    with 2 spacers
    88G4302xHBZ514, 2xHZS510Long 12H2452NBHH1604HZS4086-blade metal
    18GG Rc (Auto)not NAOct 68 - Aug 71GWP11488G4302xHBZ514, 2xHZS510Long 12B174NBHH1604HZS4086-blade metal
    18GKNAAug 70 - Aug 71GWP 114
    with 2 spacers
    88G4302xHBZ514, 2xHZS510LongGWP114 is interchangeable with GWP117 (short) given the shallower pulley and fan spacer12H2452N12H4230HZS4077-blade plastic
    18V 581/582/583not NAAug 71 - Nov 73GWP 11788G4302xHZS511, 2xHBZ515ShortGWP117 is compatible with GWP 114 (short) given the deeper pulley and discarding the fan spacer12H3696Y12H1058HZS4153-blade metal
    18V 584/585
    18V 672 101-27269
    18V 673 101-3644
    NAAug 71 - Jan 74GWP 117
    with 2 spacers
    62H3502xHZS511, 2xHBZ515ShortGWP 117 and GWP 123 must also be interchangeable12H3700Y12H4230HZS4077-blade plastic
    18V 779/780not NANov 73 - Feb 74GWP 12362H3503xHZS508, 1xHBZ514Short 12H3696Y12H1058HZS4153-blade metal
    18V 779/780not NAMar 74 - Sep 74GWP 12362H3503xHZS508, 1xHBZ514Short 12H3696Y12H4744HZS4157-blade plastic, metal inserts
    18V 672 27270 on
    18V 673 3645 on
    NAJan 74 - Feb 74GWP 123
    with 1 long and
    1 short spacer
    62H3503xHZS508, 1xHBZ514ShortGWP 117 and GWP 123 must also be interchangeable12H3700Y12H4230HZS4077-blade plastic
    18V 672 27270 on
    18V 673 3645 on
    NAMar 74 - Sep 74GWP 123
    with 1 long and
    1 short spacer
    62H3503xHZS508, 1xHBZ514ShortGWP 117 and GWP 123 must also be interchangeable12H3700Y12H4744HZS4167-blade plastic, metal inserts
    18V 846, 847Not NASep 74 - Aug 76GWP 13062H3503xHZS508, 1xHBZ514ShortClausager indicates a change in water pump in Dec 75, not shown in Parts Catalogue. Possibly the earlier GWP 123 was changed for the GWP130 which would make these two pumps interchangeable as wellCHM56Y12H4744HZS4167-blade plastic, metal inserts
    18V 797/798
    18V 801/802
    NADec 74 - Jun 76GWP 13062H3503xHZS508, 1xHBZ514Short BHH1864Y12H4744HZS4167-blade plastic, metal inserts
    18V 797/798
    18V 801/802
    Germany,
    Switzerland
    1975 to Aug 76GWP 13062H3503xHZS508, 1xHBZ514Short BHH1864Y12H1058HZS4153-blade metal
    18V 847Not NASep 76 - Oct 80GWP 13062H3501xBH605151, 3xSH605081Short CHM56NElectric cooling fan
    18V 883/884
    18V 890/891
    18V 892/893
    NA
    Japan
    Jun 76 - Oct 80GWP 13062H3501xBH605151, 1xSH605081, 2xBH605101Short 1xCHM56, 1xCAM1392NTwin electric cooling fans

    Moss Europe indicates there are steel and alloy versions of the GWP 130 fitted to the 18V672 and changing at engine number 27269 in 1974. MGOC states that GWP117 was superseded by GWP 130 and implies the change occurred with the change from chrome to rubber bumpers. This is probably the same change as from GWP 117 to GWP 123 listed in the table above, which also tends to confirm the GWP 123 was superseded by the GWP 130 subsequently. Both indicate that the fixing bolts changed at the same time.

    I don't know the part number but an alloy pump I bought many years ago was a very poor fit, I had to file out one of the mounting holes as it was about half a hole out although the gasket supplied with it was fine. I also remember having to grind down a boss on one corner so one of the original bolts would fit, however the bolt change indicated by Moss and MGOC indicates that the iron pump uses two longs bolts (and two short) whereas the alloy pump only uses one (and three short), which doesn't make sense with what I found. That also started losing a drop of water while warming up shortly after fitting so I bought a steel pump but didn't fit it immediately waiting for the alloy one to get worse - and it never has so the steel one has been sitting in the boot for a dozen years or more! 17 years in fact, as in June 2014 as part of a cooling system problem I have to fit it.

    Pump replacement: June 2014 Needed as part of investigations into a cooling system problem. First step is to drain the system and remove the hoses, which isn't as easy as on the V8! You can try undoing the block drain tap, but these are usually choked with casting sand and never work.

    Then remove the radiator.

    Next it's easiest to bend back the tabs on the cooling fan lock-tabs with the fan-belt still fitted as it holds the fan steady, and undo the bolts. Remove the fan, optional spacer if you have the short-nose pump, and the pulley.

    Slacken the alternator adjuster nut on the crankcase pin, remove the two upper bolts, and pull the alternator back clear of the mounting ear on the pump.

    Now you can remove the four bolts securing the pump to the crankcase. Note that more coolant will come out if you have been unable to get any coolant out of the block tap, which is usually the case.

    Scrape the block face, poke wire in and up the block to pump hole on the right-hand side, this should go up to almost the top of the head. There is another hole inside the water jacket above where the impeller would be, which goes up to the cavity under the thermostat, check this is clear as well. These are the bypass circuit passages which circulate coolant through the head and block to get an even warm-up, and also through the heater, before the thermostat opens.

    Check the gasket fits the holes, and especially that the bypass hole between pump and crankcase is clear, and that the gasket also fits the pump, i.e. the pump holes will be in-line with the crankcase holes!

    Then I'd advise a smear of gasket seal on both sides of the gasket, copper-grease on the bolts, and refit the pump. Pump bolt torque is 17 ft lb. Fan bolt torque is 7.3 to 9.3 ft lb. Loosely fit the fan screws first, then refit the alternator and fan belt. It helps to get the belt over crank pulley first, then fan blades but not in the slot yet, then into the alternator pulley slot, and finally ease it into the pump pulley slot while turning the fan blades. Now you can fully tighten the fan bolts, and tap down the lock-tabs with the fan held relatively firmly.

    Refit the radiator.

    Refit the hoses taking particular care that the bottom hose has clearance to the fan blades. For the top hose slide it onto the rad port as far as possible, then fit it onto the thermostat housing, then slide the hose towards the housing so that there is a relatively equal amount of hose on each port. If using new hoses Vaseline helps get them onto the ports, as they are a pretty snug fit when new.

    Then refill the system. Probably sensible to use plain water if you have been fixing a problem, to check it is fixed, before refilling with anti-freeze.

    Radiator June 2014

    Remove and refit: Remove the nuts and washers that attach the wing struts to the radiator upper mounting bolts, then slacken the nuts under the wing bolts, this will allow the struts to be swung out of the way later. Remove the six (only four on mine) bolts attaching the radiator to the diaphragm panel, noting the two top ones are longer for the stay bars and additional washers and nuts. Note there should be two packing strips (AHH 6320) each side prior to chassis number 146506, only one each side from 146506 (missing from the roadster) until the 1977 model year. Tilt the top forward - you should be able to clear the diaphragm foam seal - and lift out the radiator. Note there is still quite a bit of coolant in the rad because of the upturned return port - on centre-fill rads at least.

      To refit slot the rad into position with the packing strips, and get the upper bolts started. As they come through the nut on the rad flange, position the wing strut over the thread. If you wait until the bolt is fully tightened you may not be able to get it on the bolt without removing the strut from the wing bolt. Once the top bolts are started fit the other bolts and washers before fully tightening anything. Finally tighten the nuts on the struts under the wing bolts.

    Radiator Cap

    General description
    Is it holding the correct pressure? July 2013

     

    It is probably universally known that the radiator cap on the MGB allows the pressure in the cooling system to rise as the engine temperature rises, and this increase in pressure raises the boiling point of the coolant, so allowing the engine to run hotter without coolant loss which makes for greater efficiency, as well is delaying the point at which coolant loss might occur under extreme conditions. If the system pressure rises above that of the cap, the main seal in the cap will lift to release that pressure, which can lead to coolant loss and possibly boiling. But if this didn't happen and the pressure continued to rise, eventually something would give, possibly a hose bursting which is bad enough, but even worse if it should be the radiator which bursts. The main seal sits below the overflow pipe, which is unpressurised, and it is only when the main seal lifts and allows coolant past that coolant will run out of the overflow pipe.

    What might be less well known as that as well as the main pressure seal there are two other seals or valves in the MGB cap:

    The first is a spring-steel disc (the upper seal) that seals the top of the radiator filler orifice, i.e. above the overflow pipe. The main function of this is to allow coolant recovery from an overflow catch-bottle, if provided. Another function is a safety feature in that with sudden boiling and overflow very hot coolant is directed down the overflow pipe and not out from under the cap which could scald anyone working nearby at the time.

    The second is the low-pressure valve below and in the middle of the main rubber pressure seal. This is on the pressurised side of the main seal and has a light spring keeping it closed, although coolant pressure will close it even tighter. This comes into play when the system is cooling down. If the system heats up, pressure rises, but the pressure doesn't reach the cap pressure and there are no leaks or other sources of pressure loss in the system, then when it cools down again the pressure will gradually drop to atmospheric again and no lower. But if the cap has vented any pressure or coolant, or there are other leaks in the system, then when the system cools down it will develop a negative pressure, or partial vacuum. If left then the next time the system heats up it will be at a lower pressure than before for a given engine temperature, which could result in boiling. Under these partial vacuum conditions the low-pressure valve will open and allow air back into the system, so that the next time the engine runs the pressure will be as before.

    Under extreme conditions as mentioned earlier there can be coolant vented out of the overflow pipe, which is normally lost on the ground. Of course one could put a catch-bottle under the overflow pipe to catch it, then pour the expelled coolant back into the radiator when the system has cooled down again. But the two additional seals or valves in the radiator cap working together can cause this to happen automatically. If the catch-bottle is positioned such that the bottom of the overflow pipe reaches the bottom of the bottle, then as the system cools and draws air in through the low-pressure valve, the upper seal will ensure that this suction is applied to the overflow pipe, and hence will suck up any coolant the bottom of the overflow pipe is in contact with, and direct it back into the radiator automatically - a lost-coolant recovery system! This is an unpressurised catch-bottle, which must be open to atmospheric pressure, completely different to the remote pressurised reservoir (see below) used on MGCs, V8s, and 77 and later MGBs.

    The above seems to be the case on all UK caps I have come across. But if either the upper seal is missing or faulty, or the low-pressure valve isn't present, this coolant recovery won't work. People in the USA often mention caps available there can't always do this, indicating they are to a different design rather than being faulty. One source indicates these caps have the upper seal to prevent scalding, but not the low-pressure valve. Without this valve, not only can lost-coolant recovery not be employed, but the cooling system can experience negative pressure on cooling down, which could cause boiling at the next run. Another source indicates that the low pressure valve is present to prevent the partial vacuum in the cooling system, but the upper seal is missing so coolant recovery cannot take place. Note that only under extreme conditions should lost-coolant recovery be required, if it is happening at lower temperatures and lower loads as my V8 was doing, there is a fault which needs to be investigated and fixed.

    If you overfill any MGB cooling system when cold it will expel the excess when it warms up. The correct level is about 1" above the tops of the tubes when cold in the Mk2 radiator to 76. The V8, 77 and later MGB and MGC remote reservoirs should be kept about half full when cold. The Mk1 radiator with the right-angle filler is the most awkward as you cannot see the tops of the tubes. You must keep coolant just visible from the filler neck or it could have dropped way down, but if you keep it too high in the filler neck it will keep expelling the excess. Probably best to check this when still warm (but not hot!) and if you can see coolant it should be fine, even if it drops out of sight when fully cold.

     Remote reservoirs were necessary to allow the installation of a lower-profile radiator mounted further forward, without it the V8 and later MGBs would have needed a bonnet bulge, and the MGC would have needed a much bigger one. These remote reservoirs are pressurised, but the cap, overflow and potential for lost-coolant recovery are exactly the same as for the earlier MGBs. The pipe between the radiator and the remote reservoir is at the top of the radiator but the bottom of the reservoir, which ensures that any air in the radiator is pushed through into the reservoir as the system heats up, to be replaced by coolant as it cools down again. So even though the reservoir and radiator cap are below the top of the engine and radiator, the system is self-purging, getting rid of any air e.g. after a coolant drain and refill, over no more than a couple of heat-cool cycles.

    Mk1 MGBs used a 7lb cap and radiator with rear-mounted right-angle filler.
    Mk2 cars up to and including the 75 model year used a 10lb cap and radiator with top-mounted straight filler.
    The 76 model year used a revised radiator (visually similar and exchangeable with the previous) and 13lb cap.
    For the 77 model year and onwards, and all V8s, the forward mounted, lower-profile radiator with remote reservoir was used with a 15lb cap. 4-cylinder cars have a brass or plastic filler plug on top of the thermostat housing, V8s have it on the radiator itself, note a plug-spanner fits this.
    The MGC was similar to this last MGB style but had two filler caps - one on the thermostat housing without a main pressure relief valve, and another on the remote reservoir rated at 10lb for the UK and 14lb for the USA. There was no filler plug on the radiator.

      July 2014: A couple of comments on the MGOC Bulletin Board about incorrect radiator caps being received, from MG parts suppliers. The caps were 22mm from seal to seal, but the filler neck needed a cap that was 26mm from seal face to seal face. I felt whilst one was a possible error by the supplier, two was too much of a coincidence (unless from the same supplier). So I measured mine and found them to be 20mm for both the cap and the filler neck. The inference being, that somehow these two people have incorrect filler necks on their radiators. You could say that mine might be incorrect at 20mm instead of 26mm, but having bought caps for both cars at different times from different MG parts suppliers, and they fit, and a pal recently having done the same, I doubt it. Then someone else posted that the Mk1 rear-fill radiators do have a deeper neck. But the Parts Catalogue states that the cap for those rads is GRC102, and any number of sites indicate that has the 'shorter' 3/4" or 19mm neck. Then someone else posted that he was getting the wrong caps from the MGOC, sent them a picture of his rad, and they immediately spotted it was a rear-fill rad in a MK2 car i.e. should have had a top-fill. But then someone else posted that they have a 67 Mk1 with the rear fill and that has a 20mm cap. Apparently the MGA did have the deeper 1" or 26mm neck, but whether this is compatible with the MGB I don't know. There also seems to be at least one after-market radiator that has a 7/8" neck!

     

    I said the caps were 20mm seal to seal, but in fact the new roadster cap was 18mm. Then I noticed a rubber seal against the spring-steel upper seal, and when that was removed that also gave 20mm. Looking at my others caps (fitted and spare) none of them have the rubber ring, but all the caps have exactly the same type of spring-steel sealing disc in the upper part of the cap, so the rubber ring is an 'extra', not an alternative. With the rubber ring fitted and my roadster cap having 18mm spacing, but a 20mm spacing on the filler neck, the implication is that it won't seal. But I know it does having had a pressure gauge on the cooling system recently. When fastening the cap with the sealing ring, it has to compress both the rubber ring and the spring-steel disk, and that cap has been extremely stiff to fasten and undo. But with the rubber ring removed it goes on and off easily. Almost too easily, it is almost loose. So I looked at the 'ears' on the cap, that go under the tapered flanges on the filler neck, and they seem to be angled away from the upper part of the cap, compared to the other caps. So I pinched them up very slightly, and now it feels more secure, but still much easier to remove and replace than before.

    Thinking about it, all the stiffness when trying to turn the cap on and off was down to the extra thickness of the upper seal. In theory the extra thickness will be trying to lift the cap higher, so reducing the pressure of the main seal on its seat, but in practice the ears on the cap and the flanges on the neck should be forcing the cap down into exactly the same position in both cases. However it was so stiff, that if anything the extra thickness of the upper seal might actually be tending to bend the ears on the cap down a bit, which will allow the upper part of the cap to be a bit higher, so reducing pressure on the main seal. That won't be happening now, and having pinched the ears a bit closer to the upper part of the cap, that will be tending to pull the body of the cap down a smidgen more, and hence applying a smidgen more pressure to the main seal. I'm not going to put the pressure gauge back on the system unless I have to, but I will see if any air is bubbling out on warm-up, which would indicate the main seal isn't sealing.

    What is surprising - to me at any rate, is that the distance between the seals on the cap (without the rubber ring) is the same as the distance between the sealing faces on the filler neck. Given the significant range of movement of the main seal on it's big spring, compared to the insignificant range of movement available to the spring-steel disk (whose job is to seal under conditions of cooling suction more so than pressure), I would have expected there to be a couple of mm at least more distance between the seals on the cap, compared to the distance between the sealing faces on the filler neck, i.e. to compress the big spring above the main seal by that amount.

      June 2013 Is it holding the correct pressure? For as long as I can remember the roadster has hissed when up to temperature, and the end of the overflow tube placed in a tub of water shows bubbling. This is when running, and for a few minutes after switch-off. The hissing and bubbling gradually slows and stops, then a few moments later it starts again but with a different sound, and now it is sucking air back in through the low pressure valve. In all that time it has never boiled or lost coolant, despite getting up to the edge of the H zone on a couple of occasions, and only needs topping-up once a year at most, and that is with a small weep from the water pump during warm up.

    I never really thought about it until I had a problem with the cooling system on the V8, where something was causing air bubbles to get into the radiator, but with the completely filled radiator and remote expansion tank on that car it was pushing coolant from the radiator into the expansion tank, and eventually out of that to be lost. That was probably the pump sucking in air, so could be happening on the roadster as well, but with the large air space above the coolant in the radiator there was no chance of any coolant being ejected, short of getting hot enough to boil. I thought something must have been producing the air that is continuously coming out, but what? As there were no other symptoms or problems, like a head gasket leaking, I had decided to leave well alone. The other oddity is why is it pulling in air when it is cooling down? Ordinarily I'd have expected the system to pressurise to 'normal' i.e. below cap level as the coolant expands with running, then as it cools back down the pressure should drop back to zero, and not need to pull any air back in. Only if you remove the cap when warm, and release some pressure, then replace the cap, will it pull air back in as the system cools further.

    It was only when a pal mentioned his had started hissing, so he was going to get a new rad cap, that I started thinking more about mine, and whether the cap was holding pressure. I could simply have bought a new cap, but it seemed far more fun to do a bit of digging i.e. measure the pressure. It's easy with the remote expansion tank system as you can put a gauge tee in the small-bore hose between rad and expansion tank - I'd done that on the V8 and a pals 78 when he found it was vomiting during heat-soak. However the earlier system is not so easy. Ordinarily one would have to beg or borrow a testing kit which consists of an adapter that fits between the neck of the rad and the cap, but precludes running like that as the bonnet won't shut, and that was what had really stopped me before. However I suddenly realised that the heater hoses are relatively small bore, and hunting through my stock of bits ("If you haven't found a use for something yet you haven't kept it long enough") found the wherewithal: The low-pressure gauge I used when monitoring the V8 system pressure; the fitting off one end of a gauge pipe I got from who knows where; a length of copper tubing which was part of the fuel line off a 1940s abandoned car I found in the 60s; a length of 2BA thread I had drilled and soldered a nut as a male to male adapter for some past project I know not what or when; a length of 1/2" steel tubing left over from the Mk2 wind-stop I made in 2007; and a length of clear 1/2" hose that had been left in the garden shed by the previous owners of the house we bought 28 years ago! Sundry other purpose-built and home-made Tees and adapters found in my stock were considered and put back for another day for various reasons as not suitable.

    The bit of copper pipe was swaged to the correct size at each end to fit the gauge fitting at one end and the 2BA male-to-male adapter at the other, and soldered. A 2" length of the steel pipe was drilled and tapped for 2BA, and the adapter screwed in and soldered. The heater hose from the control valve seemed the best place to insert it, but as the hose is quite short a 2" length between the adapter and the valve is going to distort the hose quite a bit, so I would need a loop. I tried a length about 12" long first, but it had to be pulled round in an unnaturally tight loop, which would still pull on the heater hose quite a bit even when held in place with a couple of cable ties, so in the end I settled for a much longer length which allowed a lazy loop and no stress. A couple of hose clamps I just happened to have (of course) completed the installation.

    Originally I had hoped to use the same plastic gauge line I had used on the V8 so I could have the gauge in the cabin and monitor the pressure while driving, but I just couldn't come up with the right fittings, so settled for mounting the gauge on the Tee in the engine compartment. Subsequently, when investigating a cooling system problem, I needed to be able monitor the pressure while driving, so kept fiddling around until I did come up with the right fittings.

    August 2016: Subsequent a pal was going through the same process and wondered if it was possible to butcher a spare rad cap as a pressure point, but keep its profile low enough to be able to shut the bonnet. In fact all you need to do is drill through the main seal and backing plate, or remove the low-pressure valve. That will pressurise the space above the main seal, so you can attach the gauge to the overflow port on the filler neck. It will need a good seal between the upper part of the cap and the top of the filler neck though. There is a spring-steel disc seal there, but that is only to prevent a sudden pressure release from spurting out from under the cap (it should escape via the overflow pipe instead) or to allow a coolant recovery system from an open catch bottle with the overflow pipe in it. That may not be enough to hold the main cooling system pressure, but more recently caps have had an additional rubber ring below the spring-steel disc, and that should be enough. See here. However you can also get radiator blanking caps without a pressure seal, which are used on some cars with remote expansion tanks, where the pressure cap is on that tank. Note that the cap portrayed in that link does have '15lb' stamped into it, one assumes that is an example from the range of blanking and pressure caps they have available.

    Went for a brief run, and on my return the pressure showed about 3psi. This didn't surprise me as the V8 was only showing 3psi when running in free air after the cooling system problem was resolved. I then blanked off the radiator to force the temperature higher, and about mid-way between N and H the pressure was still only 4psi, which did seem a bit low. Switched off, hissing and bubbling as expected. Over the next few minutes the pressure dropped, and at 0.5psi the hissing and bubbling stopped. Now that I wasn't expecting as the temp gauge showed nearly at the H zone with heat-soak. I removed the radiator cap, and immediately it started boiling slightly - fortunately not enough to overflow but with some gurgling.

    After a bit of thought all (well most) became clear - the radiator cap is leaking, in fact only holding about 0.5psi. But the rate of leakage is slow enough for enough pressure to develop from micro-boiling at the hot-spots close to the combustion chambers, to prevent boiling over during running including in temperatures of 30C and with the temp gauge on the edge of the H zone, over the last 20 years or more. However as soon as you switch off, and the micro boiling stops producing more water vapour and hence pressure, the pressure that is already there leaks away past the cap. So when the engine starts to cool and the coolant contract, it has to suck air in through the low-pressure valve in the cap which is there to prevent negative pressure collapsing the hoses. The remaining oddity is, if the system is continually venting air when running, which is presumably water vapour from micro-boiling, why aren't I losing coolant? As I said it probably takes less than a pint over the course of a year, and there is a small weep from the water pump during warm-up. I did some Googling and discovered that water vapour at 107C (i.e. about the upper end of our temp gauges) has a density of 0.0361 lb per cubic foot, whereas liquid water (temperature not specified) weighs 62lb per cubic foot, or 1700 times more. Estimating the bubbles that come out of the overflow at about 3/16" diameter, or .000767 cubic foot, and one bubble per second, is an escape rate of 2.76 cubic feet per hour. Say 3000 miles per year travelled, at an average speed of 35mph, is 86 hours, or an escape rate of 236 cubic feet per year. That's water vapour, so converting that back to liquid water at 1,700 times the density, is 0.14 cubic feet or 1 gallon - E&OE! I kid you not, working through those conversions and estimations got me to 1.047 gallons, I did not start with 1 gallon and work back. The oddity is, I'm putting nowhere near a gallon a year back in, so where is all that water vapour, air, gas or whatever coming from?

    I don't have a spare 10lb cap but I do have a 20lb which will do as a test, to make sure it isn't an issue with the cap seat on the radiator itself. I repeat the run with that and if anything the pressure is slightly lower, but blanking off the radiator gets up to 7psi which is closer to what I would expect. Also no hissing or bubbling. New cap ordered, so time will tell if there are any other differences when running.

    Update May/June 2014: New 10lb cap received and checked against the old - spring definitely stronger. But after a 25 mile run it is hissing and blowing air from the overflow tube just like the old one! No change in the temp gauge or anything else either, so it will just have to hiss. Update 2: On a subsequent trip there was no hissing! At which point it will just have to hiss or not as the case may be. Then in late June I discovered the head gasket was leaking combustion gases into the coolant, and replacing that seems to have resolved all the issues.

    Radiator Diaphragm Seals

    The roadster has always had the rubber strip across the top of the radiator diaphragm (mounting panel) which seals to the bonnet sound-deadening pads, but for many years never had the foam strips that seal the gap between the radiator and the diaphragm. As I mention above I've never had cooling problems with the roadster, but nevertheless after many years got round to buying and fitting one. It consists of two square-section lengths of foam glued side-by side slightly offset. You glue one face to the diaphragm across the top and down the sides a little way, and that is that.

    Or so I thought. Shortly after fitting I had occasion to lift the bonnet again, to find the front strip that seals to the roadster had come away and was just lying there. Good job I spotted it, I wouldn't have been best pleased if I had lost it. There was virtually no adhesive between the two pieces, just one thin wavy line and that for only part of the length. Made a better job of gluing it than the manufacturer and since then it has been fine.

    Also since then we have had some pretty warm weather, and I'm sure the readings on the gauge have been lower since fitting it. Unsealed there is a pretty large gap between the top of the radiator and the diaphragm allowing ram-air to bypass the radiator, whereas when sealed more will be forced through the radiator to cool the engine, enough to make a visible difference it seems.

    Rubber Bumpers and cooling issues October 2016

    A cousin had two rubber bumper GTs one after the other, but found he could not live with the temp gauge rising in warm weather, and gave up on them. More recently a chap with a V8 like mine said his gauge always rose significantly when travelling at speed on the motorway, with the fans on all the time. I expressed surprise as mine has tended to do the opposite - rising when stuck in traffic, but then very rapidly coming back down again when back underway, even in the warmest weather. Then he said "That's because you have the after-market spoiler!".

    Whilst I knew that 4-cylinder cars had the apron that curved down and under, I hadn't realised (even after 22 years ...) that the V8 did as well, and a PO had fitted the STR 0189 air-dam to Vee. That 'scoops' air into the slot that feeds the oil-cooler and lower section of the radiator, rather than tending to push it down and under, and is obviously very effective. By the same token it reduces lift and increases stability at speed. But you don't have to go to the trouble and expense of finding and fitting an air-dam to improve matters.

    On all MGBs the bumper is mounted below the bumper, which places it in front of the slot in the fairing. It's in front of the slot in are air-dam as well, but the scoop is below that and obviously adds far more air-flow than the number plate takes away. On rubber bumper cars there are two brackets which are bolted to the back of the bumper armature, which come down and forwards under the bumper, then down again to position the backing plate below the bumper and slightly back from the front face. If you get longer strips of metal of the same gauge, bend them forwards as the originals, but keep coming forwards then bend them up in front of the bumper to take the backing plate such that the number-plate is between the inset indicators, it 'opens up' the slot and makes a significant difference. Looking for images of this I found no less than three V8s on the V8 Register 'cars for sale' pages, so it may have been a technical tip there in the past. It may not be quite as 'pedestrian friendly' as the factory position, but the chrome-bumper plate also protrudes in front of the bumper. You could perhaps use a stick-on 'plate' direct to the front of the bumper, but there is some discussion as to whether they are legal or not. In fact Vee came to me with a stick-on number plate, but on the air-dam rather than the bumper. It was pretty scruffy so I soon replaced it with the standard arrangement. The number plate cracked and the backing plate bent as a result of a 'Vee 1, pheasant 0' situation some years ago. I straightened the plate, MOT'ers have ignored the crack ever since, but the plate is now pretty corroded at the edges so time to replace both as part of Vee's restoration. But I digress. There is the possibility of stones entering the slot and damaging the cooler, but it doesn't seem to be a big problem with the air-dam (maybe the number-plate in front of the slot acts as protection), although quite soon after getting Vee I noticed that one corner of the cooler was mucky with oil and dirt - not dripping - so possibly a very fine crack, and replaced it. If in doubt, fit an expanded mesh screen immediately behind the fairing slot, as in the main intake above the apron.

    A change that I made was to the fibre-board 'trunking' that sits behind the slot. Although the slot and the cooler are distinctly rectangular, the board had a very pronounced curve which was allowing a lot of air to leak past the cooler underneath. I folder sharper corners in the board to match the slot and the cooler, then put a metal strut underneath and up the sides to keep it in that shape, clamped by the strips that hold the board in place, and again it seems to have made an improvement to how quickly the temperature comes down when getting underway after being stuck in traffic.

    Temp Gauge Added September 2007

    Temp gauge oscillation
    Electric temperature gauge

    Mechanical gauge

    What lies inside
    Questions often come up about various aspects of the temperature gauge, occasionally where it would be useful to know what actual temperature is indicated by the later 'C-N-H' gauges. Clausager doesn't have any clear enough pictures, but I happened to take a photo of the speedo of the car that appears on the front of Lyndsay Porter's 'Guide to Purchase and DIY Restoration of the MGB' when I came across it recently (it lives quite near me) to show the odometer reading of (then, it has done over 400,000 now) 399,419 miles. The picture included the temperature gauge, and it was always a fiddle trying to locate the original picture in my files, so I have cropped it down to the temp gauge only and included it here hopefully easier to find. This is a UK Fahrenheit gauges, Centigrade versions were used in some other markets.

      Dual gauge replacement: Failed for the second time (first time not long after coming to me in 1990) on the way back from the Rose of the Shires run in May 2012. Checked round the usual suspects and MGBHive were a few quid cheaper than anyone else and I've dealt with them before so they got my order. Outright purchase which surprised me, previously they (had to replace Vees in the past as well) have been exchange i.e. you have to return the old one. Specials like Jaeger and magnolia are still exchange, so one could suppose it's something to do with volume of the 'standard' gauges Caerbont are producing, but inside surely they are all the same, it's only the dial that is different?

    First thing was to check the new gauge read with the probe in boiling water - showed just above N so that will have to be good enough, comparing with C and F gauges it looks like boiling point should be more or less the start of the H zone. Next was to check I could undo the nut holding the sender in the head and that was OK too. Nut turned independently of the sender which would be essential if you were removing a good gauge for other reasons, but technically doesn't matter in the case of a failed gauge. Next got the centre air vents out - little screwdriver lifting the tabs, and prised out with a finger-nail. Left hand in the resultant space I could just get my fingers on the knurled nuts, and just undo them - one earth wire under each. The far one was a fiddle as my fingers were round the capillary, so I had to work the nut past the pipe without dropping it (a bit like those rings beloved of magicians that join together and come apart with no visible join). Gauge pulled forwards I could undo the oil pipe and remove the illumination bulb. Now time to completely remove the sender from the head, and to limit coolant loss I got an under-sized bolt and wrapped enough plastic tape round the end so it would 'screw' into the head. Undid the clip holding the capillary to the block, which I know now should be the bottom heater tap bolt rather than the unused tapped hole that the dynamo rear bracket bolts to on earlier cars. Unwound the capillary so it was straight, removed the next clip from under the oil pipe fitting on the shelf by the heater, then eased the whole lot out through the shelf and dash holes.

    Carefully unwound the capillary on the new gauge (don't just pull it straight which will twist the tube) so it could be fed through in the reverse direction. Once through the heater shelf, and with the gauge nearly all the way back into the dash (wait for it!) I wound the surplus capillary into three loops by the heater, then fed the sender under the heater tap to the front of the engine, then close to the block until a few inches from the port in the head where I made a graceful 'P'-shaped loop in the capillary away from the block and straight into the port. Removed my temporary plug and screwed the sender into the head. Back in the cabin I attached the oil pipe, and ran the engine to check the oil part worked, and there were no leaks from the joint. Back in the engine compartment I refitted the clip under the oil pipe fitting on the heater shelf, and this time connected the second clip to the bottom bolt of the heater tap. With the nut undone it leaked coolant slightly, but done up again has been leak free (I use Hermetite Red non-setting sealant). In the cabin refitted the illumination bulb, fiddled the mounting clip onto the studs, then the earth wires, and then the nuts onto the studs. Pressing the gauge against the dash (make sure there is an O-ring on the gauge body) and tightening the nuts finger-tight makes them tight enough, the rubber O-ring supplies enough tension to stop them coming undone, it seems. Turned on the lights to make sure the all gauge lighting worked, and refitted the dash vents. Ran the engine for long enough to check the temp gauge moved, then subsequently went for a decent run. Disappointingly confirmed the saucepan test and reads quite a bit lower than the previous gauge, so it will have to be a case of getting used to what is 'normal' for this gauge, then watching out for departures from that as before. Following other problems with this supplier I shan't be using them again.

    Temp Gauge Oscillation

    Three different situations: Slight oscillation during warm-up
    Wild oscillations at any time
    V8 conversions

    Slight oscillation during warm-up: My V8 had always done this before gradually settling on the 'N'. As the roadster does it too I took no notice. But after doing a top-end overhaul on the V8 I noticed that it no longer does it, but just rises slowly and gradually to 'N' then stops there. During the overhaul I did find the thermostat bypass pipe that is inside the inlet manifold (not the heater return pipe bolted to the bottom) choked with scale which I cleared maybe this was the cause. But then again, why did it always do it, even in the winter when the heater valve was open? Or maybe the thermostat has chosen the same time to start sticking open, we shall see (Update summer 2005: Apart from settling at a slightly lower point by N in winter compared to summer the temp gauge indicates the thermostat is working just fine, and the oscillations haven't returned). May 2014: I changed the roadster thermostat when it apparently started sticking and sending the temp gauge into the H zone during warm-up. On test in the garage immediately after changing it seemed fine and had stopped oscillating altogether, but when driving off from cold it oscillates far more than before, and is chucking out some coolant. This has led to a more detailed investigation of the problem.

    Wild oscillations at any time:

    On a couple of occasions I have had wild oscillations, sometimes the temp needle getting up to 60psi on the oil gauge, during normal running as well as warming up. This was due to a blocked steam pipe that runs from the left-hand side of the inlet manifold immediately below where the carb adapter bolts to the thermostat housing on the radiator-side of the thermostat. The passages in the pipes on the thermostat cover and the inlet manifold are very small, they may need clearing out with a twist-drill as well as the hose replaced.

    May 2016: Someone has reported this effect on a 4-cylinder, and changing the capillary gauge seems to have cured it. Personally I can't see how, as the needle requires pressure in the tube to move it, which requires heat to expand it. It could be a mechanical fault in the gauge head i.e. with the gearing, but I'd expect it to be a lot more erratic than that - sticking high when cold and all sorts. But there is another possibility mentioned at the end of the thread which is a live wire shorting out on the tube. That would cause a significant current to flow in the tube, which will give an additional heating and expansion effect to that from the bulb in the cylinder head.

    Even before that I had wondered whether the common situation of the gauge reading lower in winter is something to do with the ambient temperature around the tube. This could be a good 20 to 30 degrees between winter and high summer, which is about 1/3rd the temperature difference between the needle just starting to move (30C) and normal (88C).

    V8 conversions. Some conversions don't seem to have the above steam pipe, and on one of these I have seen similar oscillations when the heater temp control is turned to 'off'. Turning the control to on stops it. The heater control being switched on acts similar to the steam pipe, allowing coolant to circulate albeit not through the radiator. Several other people have commented on the same thing and talked about putting a small bore bypass pipe between the heater supply and return pipes before the heater control valve.

    Thermostats Added January 2008

    Replacement May 2014
    Description:

    The vast majority of us will need a thermostat to achieve a normal running temperature in as quick a time as possible, only cars in very hot climates may not need one, and see below regarding this. An engine runs more efficiently at it's correct temperature, choke - or more correctly in the case of the SU and Zenith carbs - 'mixture enrichment' will be needed for the shortest time saving fuel, reducing pollution, and giving full performance sooner.

    The thermostat is a temperature-controlled valve between the head and the radiator. At less than its rated temperature it is closed, at more than its rated temperature it is open, and at around its rated temperature it can be open by varying amounts. In other words it is not like an electrical switch i.e. either fully on/open or fully off/closed, but variable like a water tap, taking typically 8 to 10 degrees to get from starting to open to fully open. Neither does the thermostat 'latch open' once it has reached its fully open temperature, and stay open until the engine has fully cooled down again.

    If conditions are right i.e. descending a hill in cool ambient conditions it can start to close again even though the engine is still running. Clausager in 'Original MGB' talks about a 'latch-open' thermostat being fitted from 1967. What this means I don't know, certainly the thermostats available these days only have a 2 or 3 degree difference between just reaching fully open when warming up, and starting to close again when cooling down.

    The type of thermostat fitted to MGBs has a metal cylinder containing wax, inside which is a rubber sleeve. Inside the rubber sleeve is a metal pin. When the wax is heated it expands, which presses on the rubber sleeve, which presses on the pin and forces it out of the rubber sleeve. In fact the pin is fixed and it is the metal cylinder that is forced down pulling the disc valve open with it (incidentally against pump pressure). As the coolant temperature is reduced the wax pellet contracts, and a large spring forces the metal cylinder back up and the disc valve closed. This construction and methodology would seem to be more likely to allow the thermostat to fail to open than to fail to close, but for some reason the reverse is true. The thermostat can actually be installed either way round in the head, but it must be installed such that the wax pellet is suspended in the head coolant. If it is installed upside down it won't open as the wax pellet will be on the cooler side of the valve.

     

    The thermostat would normally be closed when filling the system with coolant, and this would lead to an air-lock, so the thermostat usually has a bleed hole of some kind. Sometimes there is just a hole in the flange, but this will allow some coolant to circulate via the radiator even when the thermostat is closed, which will extend the warm-up time a little. Others thermostats have a 'jiggle-pin' in this hole, which when in air hangs down to open the hole, but when in coolant a 'float' on the bottom of the pin lifts it up to block the hole. This gives a quick filling time with zero circulation during warm-up. The thermostat in the V8 is on it's side in the inlet manifold, so I don't know how effective a jiggle-pin will be at closing off the bypass when the system is full. Yet other thermostats are 'cost reduced' without a hole in the flange and hence no jiggle-pin, but do have a very small notch in the edge of the valve. This still allow air the bleed through and so prevent an air-lock, albeit fairly slowly, but allows for minimal circulation via the radiator during warm-up.

    May 2014:

    Bee's thermostat (this one was in the car when it came to me 25 years ago) started sticking closed (or so I thought) so needed replacement. I noticed that not only did the new one have no jiggle-pin (the old one did), it didn't even have the notch in the valve. I dribbled some water onto the top of the valve, and eventually some trickled through. But whether this was just because of an imperfect seal, or a concealed bleed port I don't know. But on filling after draining the radiator filled very quickly i.e. not much went in, and on running the engine it got almost to the point of opening the thermostat when the level in the rad (I was watching it) dropped like a stone. I immediately switched off and it took half a kettle-full of hot water before the level got above the tubes again, so there had been a massive air-lock under the thermostat. Ran again, and with it warmed up some more I could see when the thermostat opened (coolant flowing across the top of the tubes now) it started to drop slowly. This time I trickled some cold water in - maybe half a litre - to bring it back up to half-way up the header tank. Overnight it had dropped a little more, but still above the tubes, so I added another half-litre. As this was only a test-fill with water after head gasket replacement I knew I was going to have to drain and refill and have the same problem again, so I discussed it with Roger Parker at the MGOC he said they had recently sourced thermostats with the correct jiggle-pin, but suggested I drill a hole in the flange rather than junking a new stat, which I did.

    The head has a bypass port below the valve which allows coolant to return from head to block without going via the radiator. This is important to get an even heating of head and block during warm-up to avoid hot-spots which can cause unequal expansion, warping and head gasket failure, and also to ensure heated coolant can circulate round the lower part of thermostat that contains the wax pellet. If this didn't happen the thermostat wouldn't open until sufficient heat had transferred through the metal to warm the static coolant round the thermostat. Incidentally the heater circuit performs the same 'even heating' function when the heater valve is open. The V8 has an additional bypass route, see the steam pipe below, which allows a small amount or coolant to bypass the thermostat and circulate via the radiator at all times.

    MGA stats had a cylindrical sleeve which moved as the stat opened to block this bypass port, and causes the vast majority of circulation to be via the radiator and virtually none via the bypass port. These are sometimes referred to as 'bellows' stats, however this refers to the technology used to open and close the stat, and not the presence or otherwise of this cylindrical sleeve. This type of stat is no longer available. Later wax-pellet stats as used in the MGB don't have this sleeve, meaning that some of the coolant still recirculates once the stat has opened. MGA engines are said to suffer from inadequate cooling in high ambient temperature conditions if a later wax-pellet stat is installed, but it doesn't appear to be a problem with MGBs.

     

    Competition engines don't usually have a thermostat fitted, but then they are used under very different conditions to road cars. They are generally carefully warmed up before being driven, so any stat will be open anyway, and it is unlikely to close again during use. As stats can fail closed occasionally (which can cause severe overheating and engine damage) not having one in a competition engine is one less thing to go wrong. Usually these engines will have a cylindrical blanking sleeve fitted in place of the thermostat, which greatly restricts the bypass port as the sleeve only has two very small holes in the side. This gets maximum coolant flow through engine and radiator and minimum recirculation, hence maximum cooling. But on some engines even this can be a problem. If you scan around the web you will find recommendations that too much circulation can cause as much of a problem as too little, due to turbulence and local recirculation in the head and block, again leading to localised overheating. In these case it is recommended that a restrictor is fitted instead, to reduce the aperture to the same size as an open thermostat. The easiest way to achieve this is to remove the wax pellet, spring and moving disc from a standard stat. In some cases you might want to fit a blanking sleeve and a restrictor to get both minimum recirculation and turbulence.

    The advice on what to do if not fitting a stat is very confused and confusing. Moss says "The thermostat can be removed to aid cooling but it is essential that a blanking sleeve is fitted in its place or the change in water flow may cause local overheating in the cylinder block" i.e. they are talking about the need for restriction. With a fully open stat and no blanking sleeve the bypass port is uncovered and so some coolant will be recirculating, and even an open stat results in a restriction to the main flow, both of which reduce the maximum cooling that can be achieved. But with a blanking sleeve and no stat the bypass-port is restricted so the whole of the coolant flow is via the radiator, the main coolant flow isn't restricted, both of which result in maximum cooling (assuming no turbulence and localised hot-spots due to the greater flow). Without either warm-up will be slightly quicker as there is some recirculation via the bypass which will reduce the likelihood of localised hot-spots during warm-up, but maximum cooling will be slightly reduced for the same reason. Therefore having neither puts you somewhere between having a just stat on one hand, and just a blanking sleeve on the other, but closer to having just a blanking sleeve. Moss's statement doesn't make sense, and with a blanking sleeve you would have to be careful warming the engine up to make sure it did so evenly. This Mini site also says to fit a blanking sleeve in place of the thermostat, then describes and shows a restrictor. And this one says to fit a blanking sleeve, says it is necessary to prevent overheating in certain parts of the engine i.e. they are talking about the need for a restrictor, they show a blanking sleeve, but say to cap-off the bypass ports as well which has already been achieved by the blanking sleeve!

    Bob Muenchausen has a comprehensive page on cooling and thermostats and reports that Neil Cotty in Australia fitted a standard wax-pellet stat and a blanking sleeve to a road car (although it's not clear which car this was done to) to get reduced circulation through the bypass port and slightly better cooling. Hopefully the reduced temperature difference between his ambient and normal running temperature eliminates the risk of hot-spots during warm-up. Neil professed himself happy with the result, but wondered if this arrangement in cooler countries would prevent the engine ever reaching its normal temperature. I can't see why that would be if a stat is fitted as seems to be the case. Bob in Idaho and its colder winters tried the same thing in his 68 MGB and reports that it does take slightly longer, but as I say I can't see how, unless the blanking sleeve reduces recirculation so much it prevents the hottest coolant reaching the gauge sensor, i.e. there is uneven heating. But surely the fitting of a blanking sleeve in cooler countries is unnecessary anyway, and with the bigger temperature difference between ambient and normal you run the risk of damage from localised hot-spots for no gain.

    Coolant Temperature too high:

    MGBs had hot weather testing during development, and many have been running in desert states in America and the Middle East for many years with no problems. If you are having cooling problems it is more likely that there is something wrong with your system - either the engine producing more heat than it should or the radiator failing to get rid of it, rather than any fundamental design problem requiring modifications or expenditure on after-market stuff. It may even be the temperature gauge over-reading, although this is more likely with electric gauges than capillary. Also don't forget that it is normal for the temperature gauge to show a higher reading when slogging up a hill in high temperatures than cruising on the flat in winter.

    Updated May 2008: Early in 2008 there was a heated discussion about thermostats where one person insisted that the stat controlled the maximum temperature, whereas everyone else myself included said it controlled the minimum. On consideration I changed my view, but to one that says the thermostat maintains a given temperature, rather than a maximum or a minimum. But like any thermostat it can only do this between certain upper and lower limits. If the ambient temperature is below a certain point surface cooling of the block, sump, oil cooler and use of the heater will prevent the coolant getting hot enough to open the stat at all but the engine will still be running below temperature. This is when people in cold areas talk about 'blanking off the radiator'. And if the ambient temperature is above a certain point the radiator won't be able to dissipate as much heat as normal, the stat will be fully open, but the engine will still be hotter than normal. The fact is that more of us experience the latter i.e. higher temp gauge readings in summer than low readings in winter, which is why most people say it controls the minimum temperature. Even giving this interpretation the 'maximum' man refused to acknowledge it and stuck to his guns, so there it was left.

    So under a wide range of ambient conditions the running temperature is set by the stat. Below the minimum is relatively easily dealt with by blanking off the radiator, but above the maximum will inevitably result in a higher temp gauge reading than normal. Even in a typical UK summer with temperatures in the high 70s and low 80s the temperature gauge will read higher than normal under some driving conditions. This isn't the end of the world, it just means that the coolant temperature will have to be higher than normal before the radiator can get rid of the usual amount of heat. Mid-way between N and H is no problem at all, and is actually about where the electric cooling fans cut in at 90C, whereas a standard stat is 82C. 'H' on the gauge represents about 110C/230F, which is above boiling point but you should be able to get right up to the H zone without any loss of coolant or steaming because of the pressurised system. Using a typical 33% anti-freeze solution a 12lb cap raise the boiling point to 123C/253F which is well above H on the gauge. I've seen my V8 inside the red zone without problems, other than for my nerves, although by that time you should be thinking about whether you are pushing it too hard for the conditions, or there might indeed be a problem. In either case turning on the heater full-blast can buy you a bit more time - at the expense of comfort!

    As mentioned before there are two main reasons why coolant temperature can rise more than it should - either the engine is pushing out more heat than it should or the radiator isn't getting rid of it. In the former case this can be head gasket blown, timing too advanced or retarded, drag on the engine from tight bearings e.g. just after a rebuild all of which can result in a significant increase in heat output, binding brakes (usually accompanied by a smell of them burning), underinflated tyres (slight effect) etc. In the latter case the radiator can be partially blocked either eternally by debris or internally by sludge, restricted airflow through the grille from additional lights, rally plaque, number plate etc. Flow can also be reduced by a thermostat not fully opening, water pump vanes corroded away, and I understand there may be an incorrect combination of engine and pump that results in reduced coolant flow. There is also another possible cause of engine overheating where the coolant temperature coming out of it isn't necessarily raised, and possibly not even an indication on the gauge, and that is when there is sludge in the block and heads restricting coolant flow across parts of the metal surfaces resulting in localised overheating and possibly gasket or head warping problems. Internal sludge when still soft can often be shifted by repeatedly forward and reverse flushing of the engine and radiator until the water runs clear, but hardened deposits may not respond to this and still remain even though the water is clear. There are some DIY radiator flush additives around but they are probably of marginal benefit, both from the safety of DIY use point of view and attacking rubber and alloy parts. Still worth a try before the next step, though. Hard deposits are bad enough in the radiator, but at least this can have the header or footer tank removed, the tubes rodded-out and the tank resoldered, or at worst the radiator replaced. But in an engine often complete dismantling and 'hot-tanking' will be required Hot-tanking is dunking in hot chemical solution to hopefully dissolve any deposits before it dissolves the engine! These chemicals can be pretty caustic and not only dangerous to handle but often destructive to alloy parts.

      Coolant Temperature too low:

    If the thermostat is stuck open, or not even fitted, the engine won't reach normal temperature in anything other than hot weather. This is bad for the engine, the environment and your wallet as well as causing low heat output from the heater. You can check the stat by warming the car up from cold and periodically feeling the radiator. As the temp gauge rises the header tank should stay relatively cool. As the temp gauge gets near N the header tank should suddenly get very hot (!) as the stat opens, and this indicates a normally functioning stat. If the header tank gradually warms up as the temp gauge rises with no sudden increase there is no stat or it is stuck open. You can see this on the temp gauge as a very slow rise, probably not getting anywhere near 'N' until you are stopped in traffic. Don't forget a low temperature gauge reading may also be due to problems with the gauge (capillary and electric) or sender (electric).

     

    However note that the factory V8 has a 'steam pipe' which connects the inlet manifold i.e. the engine side of the thermostat, with the radiator side of the thermostat, which effectively bypasses the thermostat for a noticeable amount of flow. This means that the inlet manifold and the header tank will start to warm up at the same rate, then the inlet manifold will heat up faster than the header tank, until it gets hot enough to open the stat, then the header tank will suddenly get very hot as with the 4-cylinder engine. Also thermostats with bleed holes - either from the manufacturer or owner drilled to prevent air-locks on refilling, will also cause a bleed of warm water into the radiator before the stat opens.

    In extremely cold conditions and even with a correctly functioning stat the temperature may never reach normal, due to the 'surface cooling' effect of freezing air passing over the sump, block, hoses, and use of the heater. As a point of interest it is this surface cooling that led to there being 'summer' and 'winter' thermostats. In winter a higher rated stat was fitted as more of the cooling would come from surface cooling of the block and sump. In summer a lower rated stat was fitted as the surface cooling effect would be lower. The effect of this surface cooling can be reduced by partially blocking the air-flow through the radiator and oil cooler. Contrary to popular belief this doesn't stop the coolant in the radiator from being cooled too much, as the thermostat closes anyway when the coolant temperature drops below the rated temperature, it simply reduces the amount of surface cooling. Blanking-off will however make the coolant too hot if you blank off too much or the weather doesn't warrant it.

    Michael Beswick has pointed out that the typical 13" x 6" (excluding the bump on the top) rally plaque frequently issued on organised runs in the UK tied on with string makes an excellent oil cooler blind for cold weather running, for those cars with the oil cooler above the apron at any rate.

    Thermostat Replacement: May 2014

    The day before a 300-mile run I went to fill up. Glancing at the temp gauge on the way to the petrol station I was surprised to see it on the edge of the H zone and coming down, normally it barely goes above N before coming down and oscillating a bit before settling on its 'normal' position. On my return home checked the coolant to see it barely above the top of the tubes, even though it was hot i.e. fully expanded. Topped up when cool and it took a litre - too late to do anything about whatever the problem might be now. Set off for Chepstow next morning with some trepidation, and the same thing happened again although as I was watching it this time I saw it go well into the H zone before settling down. Fine for the rest of the day, and on our return home checked the level and again it was well down, when cool it took another litre. Next morning took it round the block, same thing happened, level dropped by the same amount. Looked carefully round the engine, no sign of any leaks, oil is clean, but there is dampness below the overflow tube. As it seems to lose the same amount regardless of whether it is 3 miles or 300 miles from cold, I reckon the thermostat is sticking, it's getting hot enough to boil, and pressure is coming via the bottom hose and pushing the coolant out of the overflow. So thermostat replacement is an immediate task as the next run is in three weeks, and hope that fixes it.

    First job is to drain at least some of the coolant out, which entails removal of the bottom hose. This came to me with the car 25 years ago, although I have replaced the water pump, and I think I've also had it off the rad for flushing, both many years ago. Because the bottom rad pipe faces upwards it's a struggle to remove the bottom hose from that first, but if you remove the pump end first water will go everywhere, whereas at least removing the bottom end first you can catch most of it in a bucket. Before this happened I was planning to drain and flush the system anyway, and replace the coolant, as when hot the coolant in the rad is quite orange, but not when cold, implying that a lot of sediment is being stirred up when in use. I was going to do it by repeatedly filling with water, running up to temp, then draining hot or nearly so, but given the difficulties getting the hose off that's not really an option.

    This thermostat is original to me so the housing hasn't been off in over 25 years, and I know they can be problematic. The problem is that crud and corrosion builds up on the studs which makes them an interference fit in the holes in the housing, and with three studs there is not much scope for wiggling it, it has to be more or less a straight pull upwards. If I can get any studs out first it should be easier. I can get one out using double-nuts - the one at the front of the engine which is in a blind i.e. a dry hole, but the other two go down into the coolant space and can corrode - can't shift those. Dribble WD40 round the studs the night before. With the top hose and one stud removed I can get a pair of grips in the outlet and wiggle to see the housing is moving, so pull up while wiggling and gradually work it off.

    Very orange underneath, although the thermostat itself doesn't seem to be damaged in any way. With the housing off and more space I can get double-nuts on the other two studs and get those out as well, so I can clean up the head face. I'd delayed ordering a new stat until I got the housing off in case I damaged it - I hadn't, but it was heavily corroded including the sealing face by one of the stud holes which can lead to leaks up the studs, so ordered one anyway with stat and gasket. This is one of the rare occasions when I can't start and finish a job on one of my cars in the same session.

    I ran a file round the plain shank of the studs, and would have lightly filed out the holes in the old housing if I had been refitting it, and will use copper grease on them anyway. I was surprised to see the head end was coarse thread whereas the nut end is fine. Bolts into the head and block seem to be fine thread, but these and the manifold studs seem to be UNC/UNF.

    Next day as repeated filling and draining hot wasn't going to be an option, for the sake of four bolts I removed the rad so at least I could agitate that and rinse it out. The two struts that go from the top two bolts to the wing bolts have to be slackened at the wing end so they can be swung out of the way far enough to angle the rad forwards so the top connection clears the mounting panel (and my foam seal). When replacing these struts, remember to position them over the welded nuts on the rad flange as you are screwing the long bolts in, otherwise you will have to undo them again!

    Parts arrive, and I immediately realised I have ordered an 88C stat instead of an 82C - so another delay. After refitting the rad I reattached the bottom hose, but unfortunately 'stripped' the Jubilee clip, and don't have a spare of that size. And of course when ordering the correct stat a couple of hours later I forget to ask for one of those at the same time! The housing is an alloy copy rather than a steel original as they are quite a bit cheaper. Time will tell if that's a false economy.

    Replacement stat arrives. Decide to test it in a pan of water just in case. But that is not as straightforward as it might be as we have an induction hob which means using one of SWMBO's shiny pans. She's OK with it, but I find my digital oven thermometer is being confused by radiation or something from the hob, so I have to keep turning the hob off to read the correct temp. It seems to be opening a shade after 82C, but I had tested the old stat earlier using a blow-lamp on a tin (which took ages hence doing the new one - clean - differently) and that seemed to be about 90 or 92C. Not as high as I was expecting, the temp gauge was going higher than that - maybe to 105C on a Centigrade gauge, and my gauge reads a few degrees low.

    But I really have no choice but to go for it. I had left a rag stuffed in the hole in the head so I didn't drop anything down it. When I first removed the stat the space underneath was full of coolant, despite having removed the bottom hose from the rad. Now all the coolant has gone and I can see inside the head - not pretty! You can see why stripped engines are hot-tanked.

    So I reinstall the studs with copper grease, pop the stat in, smear a thin layer of non-setting flange seal on one face of the cork gasket, noting it is handed for both top/bottom and rotational position as it has a notch to clear the rocker cover. Put a smear of seal on the new housing and fit that, then the hoses. A spare Jubilee clip is a little large, but tightens down just before the 'threads' run out, but I've ordered a couple of the correct size and will fit one later.

    What to do about the bottom hose is a bit of a conundrum. As this one is still supple, shiny and smooth do I refit it? Or use the spare that I bought years ago? Given the quality of hoses these days a 'new' one could well fail before my 25+ year-old one. I've had my spare for nearly that long, so is that now perished? Or did I buy it long enough ago before the quality got so bad? I decide to put the original on first, fill it with water and run it up to temperature first to check that there is nothing else wrong and it was the stat before draining and refilling with anti-freeze. I'll fit the new hose and clip at that point.

    When it stops raining I get the cars out to do a test run. No leaks, but I'm slightly concerned to see the temp gauge goes to about mid-way between N and H before it comes back down again. No coolant loss, but then I've only just refilled an empty system so the level is probably low anyway. Switch off and it is well down, and takes about 1.5 litres from a just-boiled kettle. Let it cool down to about 70 degrees, and run it again. This time the temp only gradually creeps up from mid-way between C and N to it's normal position. Initially the rad was cooler than the thermostat housing which indicates the stat was closed, then it obviously opened. Switched off, checked the level which was as I had left it. So nothing more to do but put it back in the garage (it had started raining again anyway) until it has fully cooled down overnight and try again.

    Next day coolant level still good so run it again. This time I film the temp gauge while monitoring the temperature on the head just above the temp gauge sensor with my infra-red thermometer and the rad header tank with my hand. The rad only warms slightly to begin with, then rapidly got hot as it should. At that point the thermometer indicated about 78C. Replayed the video to see that the temp gauge simply rose to it's normal running position and stopped there, which is a relief. Coolant level still good, and nothing chucked out of the overflow. So the final job once it has cooled down is to drain the plain water which I have put in to start with, and refill with anti-freeze.

    The odd thing is that ever since I've had the roadster it's warm-up has been totally consistent in that the temp gauge would always go slightly above N, then come down quite a bit as the stat opened, then go up and down less and less until finally stabilising on its normal running position. When I got the V8 that was exactly the same so I took no notice. Then after many years and tens of thousands of miles it developed a cooling system problem that resulted in coolant loss, but after fixing that the temp gauge stopped oscillating during warm-up and now just rises to its normal running position and stays there.

    In the event it was a week or so later that I removed the bottom hose to drain some water out prior to adding antifreeze. This should be done by adding the required amount of neat glycol first - 1.9 litres for 33% with a heater for example - then top up with plain water as that takes account of any old coolant or plain water left in the system, which can be quite a bit. How you get on with replacing glycol with ForLife I don't know, but I do know that with Evans waterless you have to go through several flushing cycles first. I had completely removed the bottom hose as I had decided to be rash and fit my new spare. I smeared Vaseline round the first half inch or so of the inside of the new hose, as dry it was proving difficult to get the hose on any of the three pipes, and the heater return pipe was still quite hard, and tightened up the clamps. Put 2 litres of neat glycol in and that was enough to fill the radiator to above the tops of the tubes, which was a surprise, I had expected to be adding some water as well.

    Started it up to hear a load drumming noise so immediately switched off. As suspected the fan was hitting the new hose, hadn't even considered that when fitting it. There is only about 1/4" clearance each side of the hose to both the fan and the timing cover, so it has to be positioned quite carefully. I had noticed the old hose had been rubbing on the timing cover. Fortunately lost no coolant - which would have been neat coolant - when adjusting the hose.

    Restarted and let it warm up, looking round for any leaks and keeping an eye on the rad level with the cap off, feeling the thermostat housing, and looking at the gauge. Got maybe half-way from C to N (bearing in mind this gauge reads low) when the rad level suddenly dropped like a stone. Immediately switched off, boiled a kettle (neat water!) to top up with, and it took most of a full kettle. This must be due to the new thermostat having no jiggle valve, not even a notch in the main valve, so it traps air beneath it until it opens, even though it has coolant on top of it from filling the rad. This must be why when I first filled and ran it with the rad cap on, the temp gauge went up quite a way above N before it came back down, and I subsequently found the rad 'empty' needing quite a bit to bring it above the tubes again.

    Restarted, no movement in the header tank for a while, then suddenly started flowing across the tops of the tubes and turning blue. I hadn't noticed that it was clear to begin with, even though I had only put neat fluid in. I suppose it is 'heavier' and had displaced the water that was left in the bottom of the rad. At the same time the level started dropping again, but quite slowly, so I just trickled in some cold water to keep it above the tubes, perhaps half a litre. All this is best done with the front of the car slightly higher than the rear to aid bleeding, particularly of the heater, the tap for which should be open. Left it overnight and gave it another test in the garage, and all was well, no oscillation, and no coolant loss.

      However next day I took it round the block, to find the wild oscillations had come back. Got back home and found it had lost about half a litre. The same happened again next day, with a catch bottle on the overflow tube. This has to be more than simple purging of air left in the system. By now we were only a couple of days away from the Arden Run, so nothing I could do about it. But as it seems only to chuck some out if filled more than 1/4" above the tubes when cold, and only as the stat opens, it should be OK, and it was.

    I'm wondering if it's the pump sucking in air, as seemed to be the case with the V8. I changed the pump almost on getting the car in 1990 as the bearings on the original had gone. It was a pig to fit - an alloy after-market item, with one of the holes having to be ovalled to get all the bolts in, even though the gasket from the same source fitted as it should, and one of the bosses ground down as otherwise the bolt was too short. After a few years I noticed that started weeping on warm-up, so got a pukka cast-iron pump prior to a trip to France and put it in the boot waiting for the weep to get worse - that was 17 years ago and it's still in the boot! It could also be a problem with the bypass passages in the head and/or block not circulating coolant while the stat is closed, but the pump is easier to change as a first go.

    But before that I decide to do some pressure checks. I'd put a gauge Tee in the heater tap hose some time ago when I was investigating the radiator cap, albeit with the gauge under the bonnet as I couldn't come up with the necessary adapters to fit the gauge tubing. But this time I persevered and made a thick spacer washer to take up the slack that was left in one fitting after the tapered threads had tightened as far as was sensible.

    Warmed up on the drive and no problems at all, gauge just rose to its 'normal' point, stat opened, pressure up to about 6psi - higher than I was expecting given the V8 pressures. Thermometer on the stat and temp gauge sensor housing showed about 80C. Blocked off the rad, allowed the temp to rise till the gauge was right over the N and the housing temp was 88C. Let it rise to between the two little dots i.e. on the edge of the H zone, temp was 107C, pressure about 9.5 psi, so the 10lb cap is basically OK.

    Switched off and the rad cap was hissing, and the pressure started slowly dropping, even though the temp gauge was actually going higher due to heat soak. So although the cap does seem to be 'maintaining' just under 10 psi, it seems there is a small leak from somewhere associated with the cap or the rad fitting, that is allowing pressure to leak away even after it has dropped to below 10 psi. Removed the 10lb cap and fitted a 20lb I had from when I was investigating the V8, ran it again with rad blocked and the pressure got up to about 15psi. Switched off no hissing - faulty 10lb cap seal?

    Refitted the 10lb cap and let it cool right down for about 3 hours with bonnet up and cool draught through the garage so the temp gauge was on the end stop and the pressure gauge zero. Drove round the block and I was surprised how quickly the pressure rose, to about 12psi. Temp gauge did its usual thing of rising towards H - but not reaching it this time, and oscillating before it settled back down to its 'normal' but low point. Pressure gauge steady. Got back home, no coolant in catch bottle, and cap not hissing this time!

    When I had the V8 cooling system problem that would always rise to cap pressure. Afterwards it only rose to 11psi (15lb cap fitted) when the fans cut in i.e. mid-way between N and H. When the fans cut out it was 6psi, and running in free air on the motorway only 3psi. So I'm pretty sure the roadster system is pumping up just like the roadster was. But whether it is down to the pump sucking in air, or a leaking head gasket, only time will tell. As the pump is easier to change, I did that first.

    Unfortunately things were just the same. Before I'd got to the end of the road the pressure was up to 5psi with the temp gauge still on C. As the temp rose the pressure went up to about 12 psi )10 psi cap), temp went up to just over N before coming back down. Pressure dropped to about 5psi, and crept back up to 10 again on the way back home. Perhaps about 250mL in the catch bottle, and the rad cap hissing. I suspect the pressure drop just as the temp dropped was the stat opening, and either localised boiling or something else chucking out some coolant. Looks like the head gasket then, but really I need to do a combustion leak test of the coolant, a compression test and a leak-down test to get as much diagnostic info as possible before doing anything about the gasket.

    The combustion leak test proved the head gasket was leaking, and replacing that had the same effect on the gauge as the V8 top-end rebuild, i.e. the gauge now just rises during warm-up and stops, very little if any oscillation, and the rad cap no longer hisses.

    V8 Water Pump June 2017

    A comment about replacement pumps from Clive Wheatley: The pulley is held on by three screws, originally UNC. Clive had had the pumps remanufactured, originally they were tapped for the pulley with metric threads, then he got them to change to the original UNC, but they have reverted to Metric again - M6 with a 10mm head. Note that the original pump, and Clive's items have the bottom hose port angled back very slightly. Some other pumps have a 'straight' port, and you can get problems with the fan belt cutting into the hose. Nevertheless I've had that problem with the correct pump, so fitted a guard. I've also had a problem with the hose being too close to a flange on the front oil pipe fitting, and had to cut 1/4" off the lower end to give more clearance, and fit a guard here as well. But don't cut too much off, or it fouls the anti-roll bar instead! More on all three issues here.