Three combustion chambers from Lyndsay Porter's 'Improve and Modify MGB': On the left what he describes as a 'very early' head with a pronounced protrusion between the valves (A) which he says causes adversely affects gas flow, as well as a noticeable 'curl' (B) around the lower right of the plug hole. Centre a significantly reduced protrusion as well as slightly less of a curl, on pre-1970 heads. And on the right a later head with the protrusion reduced still further, with the curl reduced still further.
Two from Sean Brown's Flowspeed.com (re-orientated to match the others): On the left a 12H906 head he says were the earliest used on the MGB from 64-65, however this is more like Porter's middle picture. On the right a 12H2923 head Sean says was used from the start of the 1972 model year to the end of chrome-bumper production.
Bee's head. This is from a Gold Seal 48G733 engine equivalent to the 18V58n series used from the 1972 model year, 12H2709 with a 7E3 (1973) date code, used for the remainder of chrome bumper production. The protrusion looks to be less than Porter's middle picture but more than the right, with the curl about the same as on the right.
The Peter Burgess supercharger head. Protrusion about the same as Porter's third picture, but cut back significantly above the exhaust valve, as well as polished all over.
The two oil-feed arrangements (arrowed) from Sean Brown's pages: Original 'in-line' arrangement on the left, offset on the right:
The underside of an offset rocker 12H4737 replacing 12H389 on 18V797 and later engines - not all suppliers info is correct. With either mismatch (as well as being obvious from a cursory glance) rather than blocking the oil feed hole it's more likely that oil would escape from or near the base of that rocker:
Two formats of date code were used as shown by Sean Brown - basically chrome-bumper era on the left (1966) and rubber-bumper on the right (1974). However he assumes that on the right 'K' represents October as 'I' was not used for month 9 i.e. September as it could be confused with the digit '1' (even though it didn't cause a problem in the earlier format). But as digits were not used for the month in the later format, and the two-digit number clearly indicates which way round the code should be orientated, so 'K' could equally well represent December:
Bee's 12H2709 (casting number) Gold Seal head - 7E3 (1973) on a 17D2 (1972) block
This is an example of a 12H4736 casting dated 1977. It is from an old eBay ad for a head purporting to be from a 1974 engine, which just goes to show you can't assume what is in your car is original or even correct:
A gallery of heads gathered from around the internet in approximate date order:
Note the second one has '70' at lower right indicating the year marker changed to a single digit in 1971.
The following table has been derived from the Parts Catalogue and Sean Brown's Flowspeed.com page, but note he majors on engines for North America. There are some inconsistencies with dates and descriptions as indicated:
|Engine No.||Part No.||Casting No.||Dates produced||Comments|
|18G, GA||NSP||12H906||May 62 - Oct 64||Described as 'With valves' - possibly a built-up version of 48G318|
|18G, GA||48G318||12H906||May 62 - Oct 64||With valve guides|
|18GB 101 to 91200||48G318||12H1326||Oct 64 - Nov 67||With guides|
|18GD 101 to 7000||48G318||12H1326||Nov 67 - Oct 68||With guides|
|18GF101 to 13650||48G538||12H2389||Nov 67 - Oct 68||With guides and air injection|
|18GG 101 on||48G318||12H906||Oct 68 - Aug 71||With guides|
|18GH||48G538||12H2389||Oct 68 - Aug 70||With guides and air injection|
|18GJ 22674 on||48G538||12H2389||Oct 69 - Aug 70||With guides and air injection|
|18GK||48G538||12H2389||Aug 70 - Aug 71||With guides and air injection|
|18V581/2/3||12H2708||12H2709||Aug 71 to Nov 73||With guides, large inlet valves See note 1
||18V584/5||48G644||12H2923||Aug 71 to Aug 72||With guides and air injection, large inlet valves
||18V672/3||48G644||12H2923||Aug 72 to Sep 74||With guides and air injection, large inlet valves
||18V779/780||12H2708||12H2709||Nov 73 - Sep 74||With guides, original inlet valves See note 1
||18V836/7||See note 2||Sep 74 to Dec 74||18V846||12H4735||12H4736||Sep 74 to Jun 76||With guides, original inlet valves, offset oil-feed
||18V847||12H4735||12H4736||Sep 74 to Oct 80||With guides, original inlet valves, offset oil-feed
||18V797 ||BHM1062||CAM1106||Aug 75 to Jun 76||With guides and air injection, lead free, original inlet valves, offset oil-feed
||18V798||BHM1062||CAM1106||Dec 74 to Aug 75||With guides and air injection, lead free, original inlet valves, offset oil-feed
||18V801/2||BHM1062||CAM1106||Jun 75 to Jun 76||With guides and air injection, lead free, original inlet valves, offset oil-feed
||18V883/4||BHM1062||CAM1106||Jun 76 to Oct 80||With guides and air injection, lead free, original inlet valves, offset oil-feed
||18V890/1||BHM1062||CAM1106||Jun 76 to Dec 79||With guides and air injection, lead free, original inlet valves, offset oil-feed
||18V892/3||BHM1062||CAM1106||Aug 76 to Oct 80||With guides and air injection, lead free, original inlet valves, offset oil-feed
Note 2: Some sources indicate that this head still had the larger inlet valves i.e. 48G644, but Clausager indicates that it may have reverted to the original smaller ones along with 18V846/847.