As a model engineer with my own lathe I'm considering making my own heads to try to make a big-ish (10cc+) diesel work. I'm planning to use a Viton O-ring seal between a duralumin contra-piston and duralumin head button. Can anyone suggest a good starting point for the clearance between these two components?

Maybe 5 thou to 10 thou. It is not that critical. You want the hole in the head button to compress the O-ring a bit. Then cut the groove for the O-ring in the contrapiston so that you have to force the O-ring into the head button hole when it is on the contrapiston. This is based on experience making a new contrapiston for an RJL 40-size head.

Thanks for the info. Far less precise than I thought.

RC Universe; yet again a mine of useful information.

You cannot count on O-ring pressure to hold the contra-piston in place, if the engine has a less than stellar ring to sleeve fit the piston will suck the contra-piston right out of the head when you flip the engine over. It needs to be a very tight fit, for example, you cannot move a Davis, MVVS, Irvine, or PAW contra-piston with anything but the screw.

O-ring pressure works for me. If you are using an O-ring that is all that is holding the contrapiston in place. As I said, the O-ring needs to be a tight force fit. If your contapiston is properly fitted, you don't need an O-ring. However, that is a precision fit operation. Too loose and the contrapiston leaks and maybe falls out. Too tight and it seizes up and you can't adjust the running engine. After the shelf below the O-ring in my 40 RJL head broke off, I tried running the engine with the damaged contrapiston. It ran, but when the engine stopped, the contrapiston fell out. The O-ring wasn't compressed enough with the bottom half of its groove gone.

Thanks for all the very informative postings. I will take all the comments on board to try to avoid the mishaps others have experienced. I'll let you know how I get on.