ORIGINAL: w8ye
Ryobi also has made a couple different four strokes... And they make a smaller two stroke? Cannot remember the size? seemed to be better made than the "31"?
Ryobi produced 3 different "generations" of 4strokes. GEN1 and GEN2 differ outwardly only by GEN1's metal "oil pan" vs. GEN2's black plastic. Internally, GEN2 has a modified oil baffle system that improved inverted engine operation. GEN3 is a completely new design that incorporates two magnesium(?) clamshells that forms the integral crankcase & oil sump. It is used in Ryobi's PRO-grade weedwackers.
Ryobi's smaller 2stroke is 22.5cc and is of balanced support twin-BB design vs. the cantilevered Ryobi 31cc crank. Again, it is only used in PRO-grade weedies.
BTW, my newest twin-ring Ryobi 31 is also out of a Bolens weedie.