Cylinder Heads

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.

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:

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 producedComments
18G, GANSP12H906May 62 - Oct 64Described as 'With valves' - possibly a built-up version of 48G318
18G, GA48G31812H906May 62 - Oct 64With valve guides
18GB 101 to 9120048G31812H1326Oct 64 - Nov 67With guides
18GD 101 to 700048G31812H1326Nov 67 - Oct 68With guides
18GF101 to 1365048G53812H2389Nov 67 - Oct 68With guides and air injection
18GG 101 on48G31812H906Oct 68 - Aug 71With guides
18GH48G53812H2389Oct 68 - Aug 70With guides and air injection
18GJ 22674 on48G53812H2389Oct 69 - Aug 70With guides and air injection
18GK 48G53812H2389Aug 70 - Aug 71With guides and air injection
18V581/2/312H270812H2709Aug 71 to Nov 73With guides, large inlet valves See note 1
18V584/548G64412H2923Aug 71 to Aug 72With guides and air injection, large inlet valves
18V672/348G64412H2923Aug 72 to Sep 74With guides and air injection, large inlet valves
18V779/78012H270812H2709Nov 73 - Sep 74With guides, original inlet valves See note 1
18V836/7See note 2 Sep 74 to Dec 74 
18V84612H473512H4736Sep 74 to Jun 76With guides, original inlet valves, offset oil-feed
18V84712H473512H4736Sep 74 to Oct 80With guides, original inlet valves, offset oil-feed
18V797 BHM1062CAM1106Aug 75 to Jun 76With guides and air injection, lead free, original inlet valves, offset oil-feed
18V798BHM1062CAM1106Dec 74 to Aug 75With guides and air injection, lead free, original inlet valves, offset oil-feed
18V801/2BHM1062CAM1106Jun 75 to Jun 76With guides and air injection, lead free, original inlet valves, offset oil-feed
18V883/4BHM1062CAM1106Jun 76 to Oct 80With guides and air injection, lead free, original inlet valves, offset oil-feed
18V890/1BHM1062CAM1106Jun 76 to Dec 79With guides and air injection, lead free, original inlet valves, offset oil-feed
18V892/3BHM1062CAM1106Aug 76 to Oct 80With guides and air injection, lead free, original inlet valves, offset oil-feed
Note 1: The Parts Catalogue indicates 12H2708 was used on both engines but had the larger valves on 18V581/582/583 and the smaller valves on 18V779/780.

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.