The clearances seem to be "all over the place" on the Ryobis. You can't just arbitrarily leave out the cylinder base gasket as we do on some engines, because on some it will work out to a workable deck height, and sometimes not. A good minimum piston to head clearance is .015" to .020". On some Ryobis this means leaving out the base gasket completely, and on some you have to make a thinner base gasket because leaving it out completely would result in the piston hitting the head. On the other side of the coin, the last one I did had a deck clearance of .035" with no base gasket. I could have machined some off the bottom of the cylinder to get an optimum deck height, but I just went ahead and assembled it without a gasket and accepted the larger than optimal deck height. It ran pretty well none the less. Once again, as has been noted here before; set your deck height first, and then go after your optimum port timing. The Ryobi seems to like about 150 degrees exhaust timing, and when you lower the cylinder you LOWER the port timing. This is not a good thing, as many Ryobis come stock with a really low exhaust port timing.

As regards to the Bowman ring deal, I have yet to see an engine that didn't respond with more power when equipped with one of his rings. As you mentioned, ring seal is dependant on many factors. Frank's rings seem to seal better under a variety of conditions (and piston positions apparently.) A good leak down test at TDC doesn't necessarily mean that the ring is sealing well for the entire power stroke as your tests have already shown. Once again, just speaking from actual experience, a Frank Bowman ring is just about guaranteed to add some power, especially to a used engine, given that the bore is good enough to seal any ring at all.

AV8TOR