RE: ryobi 31 modification
I believe the slots are for cleaner combustion with lower emissions like rangerfredbob says. I really dont understand why there should be compression reliefs for an engine with already low compression. You can remove the bridge completely if you want to, but dont make it much thinner. When the bridge is too thin it might overheat and score the piston. I think the piston ring is pinned on the intake side, but check it before you remove it. Usually the safe width of the exhaustport is 0.65 of bore when you use a brittle iron pistonring, otherwise 0.72 is ok, some even go to 0.75. If the exhaustport has a bridge, you can go even further. I widened the exhaust to 0.75 and left the bridge where it is, because it is better for the pistonring.