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#12948
Richard Moss
Participant

Hello Richard,
Good luck getting it sorted out… It’s a lovely bike and worth the final push ( hopefully not literally).
It’s hard to visualise how the volume you mention could be possible in one of George’s heads given that the average tdc volume in my dad’s heads is thirty/forty something(IIRC).That’s production volume and not race volume. I’m at about 25cc now, but I know Colin got down to 19cc on Martins bike as I have some of the notes on his developments. I think Colin’s head was one from John Farrah’s pattern but I’m not totally sure. I know that the patterns were made without shrinkage taken into account and each hemisphere was a fair bit smaller than required as well as too close together. I machined one from scratch about 7 years ago and it took bloody ages. The fact that there was so much material to remove meant that you could get them closer all over though. My dad’s pattern which I’m pretty sure was used on the Silk Scott, has better centres as cast but also was made to have less fitting work. You can’t easily get less than mid to high twenties out of them. Still, now we are using expansion chambers so the effective compression ratios are likely to be substantially higher. In fact, I think that the compression ratios of (almost) modern racing two strokes are restrained to around a tenth of the cylinder capacity. Obviously vit would be easy to reduce that with an almost flat topped piston.
I do realise most of this isn’t useful.
But unless yours is not a high compression head (cast profile to match deflector) or it’s not a deflector piston (!) or oils too thin/rings too worn to measure or you are not at tdc, then 80 odd cc’s is hard to place. Looking forward to finding out what the answer is!

Richard