It is great to see all the other info about the Ryobi.I have torn apart about 20 of them and found alot of differances with them. 1 the lower part with the crank,two differant sizes,some of them have been milled in production and some are cast and cleaned up, also the front seal/bearings are differant on some of them.The outside case,some are wider than others.2The heads,the newer Ryobi's with the "E-Z start "in the title are the ones with the compresion vents that go up the sides for emisions complience.I could go on and on,BUT I have found an older engine that was dated 1978 that had 2 rings on the piston and the rod was forged aluminum and held to the crank with a bolt( name was 'Piston Powered Products model # 99).The other one I have is a Troy built/MDT dated 3/2006 also has two rings on the piston,But this is the new casting I have heard about .The lower crank case has the magneto shield as part of the casting.This will require much more work to convert.