Old post but I figured I'd throw my $.02 out there for anyone wanting to know. I read in a conversion article once that the base of the cylinder on a typical 31cc Ryobi can be milled to increase compression. I also read a suggestion that indicated removing the thick base gasket and replacing it with a thinner one or just fuel proof sealer would also help. In doing so you increase compression and also effectively lower the ports which is good onthe intake side 'cause it opens the port sooner but bad on the exhaust side 'cause the port opens later. To remedy this it was recommened to either take a bit off the top of the exhaust port or remove a little material from the top outer edge of the piston to maintain good port timing. Not having a machine shop I always opt to use a Dremel tooltoremove a bit from the top edge of the piston where it meets the exhaust port. OK, back to the original question. On my first Ryobi conversion I had my brother (a machinist by trade) mill the base of the cylinder .020". When I assembled the engine I also used a VERY thin base gasket (I wanted a little screamer hot rod engine). However, when I screwed the spark plug in and turned the engine by hand I found that the piston was coming up and hitting the plug. Oops. I first considered trying to shim the plug a bit to get it out of the way but it's a tapered seat plug and I really didn't want any sealing issues there and didn't want to mess with a good thing so I left the plug alone. I tried making a little thicker base gasket and there was still contact. Finally I went to a base gasket approximately the thickness of a stock gasket and therewas no interference. I assembled the engine that way and finishedthe conversion. She ran very well like that.That engine ran and ran for hours on the test stand then for approximately 30 flights in an airplane before a servo failure cause a full throttle nose-in and the crank was bent. The cylinder is still on another running engine today. I've done several 31cc Ryobisand I always either have the cylinder milled .020"or go to a really thin gasket to accomplish the same thing. The engines run well and are cheap and easy to convert and a whole lot of fun to work on. My worknets an engine that will turn an 18x8 prop at about 8200rpm on the test stand. I currently have two planes flying with them and a new project that will be a crazytwin with two Ryobi 31s. In my opinion milling .020" (or reducing gasket thickness the same amount) is about the maximum you can go and so far has proven to be very reliable. Compression is really good and my Sullivan starter has all it wants to do to spin onehooked toa 12v lawnmower battery. No signs of detonation or other harmful side effects. I run cheap 87 octane gasoline and keep the fuel mix at 32:1 to make sure things are always lubed well. After seeing what appears to be a rather flimsy connecting rod in these engines I've been rather pleasantly suprised by their longevity. I'll probably eventually blow one up or something but at the low price of replacement it's not really a concern. Especially when I hear stories all the time about purpose built RC engines that don'tlast and are far more expensive. This has been my very long-winded way ofsaying the answer to your question is .020"...

My typical engine mods:
-Mill the cylinder .020"
-Cut/grind the crankcase flanges off (remove anything that made it atrimmer engine)
-Larger carburetor
-Cut away extra plastic from reed valve plate leaving just a rear crankcase cover with reed valve
-If possible use a two ring piston but one works ok too
-Bend the reed valve retainer a little to let it open more
-Grind a bit off the top of the piston to maintain port timing
-Aluminum aftermarket muffler (much lighter... I get mine from wackerengines.com)
-Disgardthe flywheel and magneto in favor of a CDI unit (onshort shaft engines Icut the center out of the flywheel to use as a hub and put the ignition trigger magnet in it)