I am planning on doing a Ryobi 31 engine conversion and have done a reasonable amount of research as far as the major players in supplying conversion kits and parts. What I haven't been able to find is what the difference is between the engine model numbers. So what is the difference, physically and performance wise between a 700R, 720R, 725R and whatever else is available. Also, what models have the dual piston rings and what is the advantage of it.

Thanks, Sedleniek