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I've got three South African Clubman GTS 12G940 heads (same as the Mk3 Cooper S heads I believe) and they vary in thickness from about 689mm to 699mm. What is the spec for a standard, un-skimmed head of this kind?

What will the compression ratio be if the head is skimmed from 699mm down to 689mm? This is on a 1275 Cooper S motor with everything else standard.

I sure hope your head is 699mm thick, that's more than half meter. However if you meant 69.9mm, the standard accepted size of an unsurfaced British 12G940 head is around 69.2mm.

That accepted, further estimates can vary, but generally by removing 1mm (0.040 inch) compression ratio on the 1275 is increased by about 0.7 give or take, depending on overbore and other factors

Yes, I meant 68.9 and 69.9mm

So assuming the rest is standard I could be sitting with a 10.45:1 compression ratio with the head I've got.

Thanks Frogeye61

Yes, if you're confidant that your original ratio was 9.75. Personally I would measure everything and calculate, but maybe I over-do it sometimes. Then again, higher compression is good for engine efficiency - and more power, and when using higher duration cams, the effective compression is reduced.

Nah, I agree with you. That is probably what I should do. I'm waiting on a company who rebuilt the motor for a previous owner to send me details of what they did. Hoping that would shed light on what's going on inside without me having to pull the head or more.

What I do know is that it pulls very nicely but the motor also runs on when you turn off the key, so I suspect compression might be quite high in addition to the fast-ish idling speed (because it won't idle evenly).