Pushrod cover gaskets & seals – rubber or cork?
Both A series (except the 1275) and B series engines had cover plates on the side of the block providing easy access to the cam followers.  On the 1275 (except the early Cooper S blocks) the access holes were omitted in order to increase block rigidity, meaning the only way to remove the camshaft is with the engine upside down.
There are two different gaskets for these pushrod covers: 12A1139, the original cork gasket, and 12A1175, a synthetic rubber gasket. A series, MGA and early MGB engines had two of the 12A1139 gaskets, while the later B series had 1 of each.
The obvious difference in the material between to two gaskets is purely secondary to their principal differences, which are their are different dimensions. The rubber one is about 1/4″ smaller each way and much thicker.  It is made to fit into a recess in a modified rear pushrod cover. Presumably a redesign to improve sealing.  For this reason the two are NOT interchangeable.
This was introduced at about the time a breather cannister was incorporated into the front pushrod cover cover. The breather structure necessitated the retention of the original sized cork gasket, hence one of each – they aren’t interchangeable (unless an oil leak is wanted!).
All this took place at about the same time as the 1275 with the filled in side chest was introduced. Presumably this is why we haven’t seen the modified A series version, or it could be a classic bit of inconsistency!
Don’t mix them up!
Some people elect to fit two rubber gaskets in the mistaken belief that the material is better, but the ruberised cork that 12A1139 is now made of is just as efficient at sealing as 100% synthetic rubber. However size IS important because the rubber gasket coverage over the front pushrod cover is reduced to almost nothing in one corner, so not good enough to give a reliable seal.
A simple guide to the use of these gaskets is that if it doesn’t fit properly you probably have the wrong one!
It’s interesting that a number of parts listings scramble the two. It gives the feeling that those responsible for the listing have never actually tried to fit one!
Seal for the centre bolt
One definite advantage of the redesign was the use of a cup washer (12A1177) and O-ring (12A1176) to seal the bolt in the centre. This is a worthwhile (and cheap) retro-fit for those cars still fitted with flat & fibre washers.
