How do you measure the head to know which size valves they use.

I know Mk1 & Mk2 heads are different, the head I have has no valves, it is a 940 MG head and when I insert AEG 593 & 594 valves they seem to fit. I know Mk1 & Mk2 sizes and lengths but why are the lengths different.

I assume it's something to do with the depth of the pockets the valve springs sit in - if you put mk1 (short) valves in a mk2 head they have way to much spring tension

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The casting height from the valve seat to the spring platform is different in an AEG163 head v's a 940 head. In order to maintain a similar seat pressure the valves are longer.

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1970 Cooper S ex-Bathurst & ATCC
1964 Austin Cooper S ex-Group C race car
1967 Morris Cooper S ex-Group B
1962 Mini Speed sports sedan
1968-71 ex-Peter Manton Shell car

Hi Cooperess
The mk1 valves are .070 shorter than the MK2, and if you have a look at a MK1 head AEG163 you will notice that the spring seats are a lot lower than the MK2 head, and this caused a problem with the valve guides becoming loose and going up and down with the valve (NOT GOOD)
So the MK2 head had more metal added to the spring seat to stop this problem, there for the valves had to be made longer to get the spring pressures right,also the MK1 one valve is a little taller from the collet groove to the top of the valve.
One thing about the MK1 valve is that it is still the best flowing inlet valve for the 1275 mini (i'm talking genuine valve)many have tried to copy it but they haven't quite got it right.
I did put a photo a long time ago showing it's shape, very well done in the day and only running a .040 wide seat
Graham Russell

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"It's better to be not informed than ill-informed"