I am also interrested in the Ryobi.I have been working on then for awhile.I ordered a conversion kit a long time ago.It was the cost of the EI units that kept me from going further with it.Now that there are other less expensive units out there I have been working on it again.I have found that there have been differant head castings,crank/lower body,pistons,mags,carb sizes and the back covers.

The only thing I found NOT differant with all but 1 is the rod.I have one that looks like an I-beam.

The pistons are about the same weight.But they are differant.Some are longer skirt,some have a differant side relief for the crank,some are full round cylindars, and some have a bevel to the top lip on the piston.Also some of the ring are differant thickness,some pinned some not.Most do show the gap blow-by burning.

I have not run any of these yet.I am still working on them.

The oldest one that I have is the one the has got my attention the most.It has a differant head.The exhast measures 1.1405 in. wide x 0.38 in. high and 0.084 in. from the bottom of the head to the floor of the exhast.Also it is the heavyest one.