Be careful what you drive over
Bob Muenchausen sent me these pictures of the aftermath of driving over a mattress in the road. The driver complained of a propshaft vibration, but still drove at speeds of up to 60mph until the debris managed to puncture the fuel tank and he lost his fuel.
Pals from Australia have also told me that if ever they drive over a snake they always check to see it is lying in the road afterwards. They can get flipped up onto the axle, and from there get into the cabin ...
Grease Nipples
Your propshaft may have a number of grease nipples from in the worst case none to an ideal three - both UJs and the sliding joint at the front. Manufacturers go through cost-cutting exercises from time to time and also look to reduce routine maintenance to make ownership cheaper for fleet users,
removing the grease nipples from the prop-shaft was such a victim at one point. You would be correct in thinking grease nipples are always better and will extend the life of the joints,
but as long as they last the typical three years of fleet ownership they couldn't care less after that. Fortunately the better suppliers of spares for our MGs are a little more enlightened and supply joints and complete prop-shafts with the requisite number of grease nipples. I am sure that there are other suppliers out there who don't,
you pays your money and takes your choice.
Look for them,
and if you have them grease them at 3k intervals maximum. I have found the easiest way to grease them is to reverse the car up a pair of ramps,
make sure it cannot roll off the ramp or tip them,
then jack under the axle to lift one wheel off the ramp just enough to turn that wheel. It is the only time I work under a jacked vehicle,
I reckon that with the amount of 'headroom' given by the ramps and I'm only jacking an extra inch or so I am safe enough. With that done you can rotate the wheel with a foot to get each nipple into a suitable position for greasing. Pump until you just hear grease oozing out of the joints,
done regularly this should only need three or four pumps on each UJ,
possibly a couple more on the sliding joint.
Length
There were several prop-shafts over the years. The basic construction was the same but the lengths varied according to rear axle, gearbox, and in some cases whether overdrive was fitted or not. The technical description of the shaft is "Double-ended, Circlip Type, with Fixed and Sliding Joints". The Leyland Parts Catalogue gives dimensions for each but there is some doubt as what that dimension actually refers to. Initially it seemed logical that it would be the fitted length i.e. the distance between the gearbox and rear axle flanges, then I wondered if it referred to the overall length fully compressed (there is a sliding joint just behind the front UJ to cope with engine and axle movement). I subsequently obtained some Hardy Spicer dimensional information from the original designer of the MGB axles, gearboxes and prop-shafts which I have summarised in the table below, together with Parts Catalogue information. Subsequently I measured my roadster and found the fitted length to be 31.1875", close enough to the book figure of 31.125" for me to now be convinced that the Catalogue figures do indeed refer to the fitted length. This does make most sense, as the vehicle manufacture would give that dimension the propshaft designer/manufacturer, and they would use their knowledge and experience to decide how much 'slack' there should be in the sliding joint. Bob Howard of Connecticut reported that two he took off 4-synch cars, plus a newly purchased replacement (AHH 7487), all measured the same as each other at 30.25 which you can see is almost an inch shorter than the figure given in the Catalogue, which means there is about 1" of 'slack' in his sliding joints. I measured my roadster tube length at 21 3/8", less than 1/5" different to the Hardy Spicer figure below, and subject to interpretation of exactly where on the weld one takes the measurements (I measured from the centre).
| Gearbox/Axle | BL Part No. | Catalogue/
Fitted Length | Hardy Spicer No. | Tube Length |
| 3-synch, no OD, Banjo | AHH 7488 | 30" (76.2cm) | 114H02771 | 20 9/32" |
| 3-synch, OD, Banjo | AHH 7487 | 31.125" (78.9cm) | 114H02776 | 21 17/32" |
| 3-synch, no OD, Salisbury | AHH 7487 | 31.125" (78.9cm) | 114H02776 | 21 17/32" |
| 3-synch, OD, Salisbury | AHH 7486 | 32" (81.3cm) | 114H02779 | 22 9/32" |
4-synch, OD/no OD,
Salisbury, not V8 | AHH 7487 | 31.125" (78.9cm) | 114H02776 | 21 17/32" |
All shafts had a Tube Diameter of 2", a Flange Diameter of 3 7/16", and used UJ/journal Part No. GUJ 101. Up to chassis number 138401 the same bolts were used at both ends (AAA 4039), after that different bolts were used at the gearbox end (22H 1107). These bolts are safety-critical items. My roadster rear flange is 2 1/2" from weld to flange face, and the front flange and sliding joint 7 5/16" from weld to flange face (fitted).
Click on the thumbnail to see a drawing showing the measurement datum points. You will notice the overall length is dimension FFC and this is the Flange to Flange Closed (i.e. fully compressed) length. Unfortunately the table of data that gives all the other dimensions for the shaft does not include this one for some reason. There is also a question mark over exactly where the FFC measurement is taken between. The drawing shows it being the raised portion in the middle of the flange i.e. the overall length between two straight-edges laid across the middle of the flanges. But a friend who was an apprentice at Hardy Spicer and used to make drawings like these happened to see these drawings and said that the measurement was taken between the machined faces of the flange i.e. the outer portion where the bolts go through (FFC2 in red on the accompanying picture) and not the overall length, as the raised centre of the flange is part of the rough casting and not precise. Note that the sliding joint is assembled so as to have the shaft yokes in the same orientation.
I don't have all the equivalent information for the V8 but I have taken measurements from a spare and my car which are as follows:
| BL Part No. | Tube Length | FFC Length | Fitted Length | Tube Diameter | Flange Diameter | UJ/Journal |
| AHC 113 | 19.25" | 29.75" | 31.25" | 2" | 3.8" | GUJ 108 |
The FFC measurement needs care because on my spare you can push the sliding joint all the way in, then when let go it eases out by about 1/4" because of a rubber 'boot' over the sliding joint, so you have to keep it under compression as well as keeping the flanges in line with the tube. I measured my old V8 shaft fully compressed, plus the fitted length of the one on the car and came up with a difference of about 1.5". This was more than I was expecting, and I did wonder how much spline I had left. But further measurements showed that there was 2" of movement available before the splines started coming out of the socket, and 2 1/4" of spline on the shaft, so I have plenty of movement left. The splined socket is 4 3/8" deep so potentially gives just over 2" of movement with the splines fully engaged, or 1" each way from a notional central point, making Bob's 1" difference between fully compressed and fitted just about spot-on.
On the V8 the gearbox-end bolts are in the gearbox flange, and retained by it, you can't replace these bolts unless you remove the flange from the gearbox output shaft. If you do that, be prepared for oil to run out as the sleeve on the flange comes out of the oil seal. I think this is because the V8 flange is bigger, which means there isn't room on the back for the nuts and washers as there is with the 4-cylinder flange. However I have seen what purported to be a 4-cylinder overdrive for sale also with the bolts 'captive' in the flange. Nothing to stop it being assembled this way, but it isn't necessary, and indeed makes for more work if you do need to change a bolt.
Yoke Alignment Added April 2009
Some confusion here caused by the Haynes manual, which has an exploded drawing showing the prop-shaft yokes 90 degrees apart and an assembled drawing showing them in line. They should be in line, the factory workshop manual has similar exploded and assembled drawings but they are shown in line in both. Additionally the assembled caption states "...it is essential to see that the forked yokes on both shaft have their axes parallel to each other. In other words the yoke (A) must be in alignment with the yoke (B), and the flange (C) must be in alignment with the flange (D)", see the attached picture.
However both manuals go on to say something about reinstallation of the propshaft which seems unecessary: They say to mark the four flanges (gearbox, prop-shaft and axle) to assist in refitting them in their original positions, the workshop manual going on to say in bold "This is most important". Haynes additionally says this is because the prop-shaft is balanced to fine limits. Good practice maybe, but precisely because it is balanced to fine limits it won't matter how the prop-shaft is re-attached to the gearbox and axle flanges, the result will be as balanced as it was before. What is very important and seems to have been missed in both manuals is that you must mark all the component parts of the propshaft i.e. all four yokes along a common line to ensure these all go back together in the original alignment i.e. both halves of each UJ and the sliding joint, as reassembling either pair of yokes 180 degrees out, or getting the sliding joint 180 degrees out, almost certainly will upset the balance of the shaft.
December 2009:
Fletcher Millmore on the MG Enthusiast BBS has said that the sliding yoke and tube should have alignment arrows on them, to at least get that part assembled correctly, however they can be very indistinct, as can be seen (ho ho) here. He's gone to the trouble of taking some pictures of the best one he could find, as well as some sketches of the sort of partial markings you are likely to find in practice for which I'm very grateful. You still need to mark the relationship of the two halves of each yoke before removing UJs though, which as long as you make all four in line will also work for the sliding joint as well.
