mk1-forum.net

I have a mid-1966 S (numbers matching).

the cylinder head has the ID 12G940 with 12G1805 stamped near the thermostat area.

I do not believe this is original to the car.
The seller did not indicate to me the head was replaced.

what was the original cylinder head ID for this build date?

is my head suitable for the mid-1966 S?
does it detract from originality?

That is an original Cooper S head.

Your car would have had the AEG163 head when it left the factory. But many failed in service due to poor casting quality, usually cracking between inlet and exhaust valve seats.

The first 22G940 with flat top behind thermostat stamped 12G1805 was introduced in mid 1969 ready for the S Mk3 launch, with redesigned casting and much stronger valve seats - at the expense of slightly smaller exhaust valves.

AEG163 was then discontinued, and 12G1805 became the replacement spare part for all Cooper S.

So while it’s not 100% original, your car is running the ‘factory approved’ replacement head.