I do have made an engine with an O-ring at the piston, only once; when cold this engine doesn't have any compression, nothing, and after a few minutes of startup, the compression comes back, and the engine starts. This behaviour is really confusing, all my other engines (about 10) have one or 2 cast iron segments, or no segment at all for smaller diameters.
this point suggested to me that the section of O-rings are not constant, and may grow up with fuel, oil and heat.