I should think you could quite happily halve that or even a bit more depending on how hot you let the engine run.

Could also swap out one of the rings for a Viton O ring or even a Quad ring.

US, I guess? That is over 5 thous per inch of bore.! Most large engines would be built to far less than that. Not a chance that there will be zero clearance at 80 degrees. That is only a temperature rise of about 60 degrees.

If I rebuilt a car engine with 15 thous. piston to bore clearance, I would fit the next rebore size of piston! (+10 thous).

This is a question I have tried to find a definitive answer for, without much luck. I have made quite a few engines over the years, mainly compression ignition, where, for success, the piston/cylinder fit has to be perfect, with no bypass leak. It took me several goes before I perfected the technique to achieve this and every engine C/ I or Glow that I have made, I lap to this standard. To me ,an engine is judged by its compression in the first instance. None of these have had rings. A couple of years ago I made the 15 cc Chenery V-Twin engine that has two rings per piston. No information on piston fit/clearance was in the notes and I found that I could not persuade myself to make the pistons other than the perfect fit.that I always developed The pistons were Aluminium in Cast iron liners.. This engine has had a total to date of about 5 hours run time. Because it is a thirsty beast,a run does not go much longer than 5 minutes. I always add 5% more Castor oil to a commercial fuel. I have not had any over heating problems with the piston /cylinder fit and the compression is still as good as it was originally.

I have since made the Jones 10cc Glow engine that has one ring on an Ali piston in a Cast iron liner. Once again I made the piston to have my standard fit and have had no problems with overheating. Test runs are all bench mounted . My days of flying are behind me.. Whether my experience is due to the extra Castor, I have no idea.

For my next project, I am looking at the Westbury designed Kiwi Mark 11.15 cc four stroke. His notes /drawings give a clearance of 0.005" above the ring and 0.003 "below to the skirt., for the aluminium piston with two rings.

I have no problem understanding the requirement of rings for full size multi cylinder engines, but I am unsure of where the point of need for them on a model engine exists. As an instance, I have two Glow engines of about 8cc that do not have rings, so why would the 7cc size on the Vee-Twin need two rings? I would be very interested in information regarding piston clearance based on real practice that any one can point me to.

Keith

What a lovely little engine. Top work.