[phutton]

I think 8:1 would be too high. The 1937 M9 only used 6 1/2:1, but you may get away with 7:1

[bob southall]

I am restoring a 1929 model 8. On dismantling the engine I found the piston had seized at some stage and was not serviceable. The nearest piston I have found is a BSA Rocket Goldstar. This looks reasonably similar in compression height and after some machining around the boss will now accept the connecting rod. I suspect I may have to machine the valve pockets and probably the crown. Does any one know what the original compression ratio was? In addition has anyone a recommendation what ratio to use given modern petrol's. I was thinking of perhaps 8:1

Thanks in anticipation of any help

Bob Southall