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.