We made the fire rings from .032" diameter stainless steel aircraft safety wire. A groove was cut into the top of the block 1/8" or so outside the cylinder bore, with a depth half the diameter of the wire. Then a length of wire was cut so the ends would butt together tightly as it was tapped into the groove. The wire that protruded above the surface of the block would imbed itself into the copper, and provide a pretty effective compression/combustion seal. Since the engines only ran for a few minutes max before getting torn down and freshened up, the butt joint in the ring wasn't an issue. Sometimes those $20K engines didn't survive a 300 foot pull!
Since the stovebolt combustion chamber has such an oddball shape, a fire ring won't be necessary, or even possible to do, IMHO.
Jerry
