Most jetski engines use iron liners so the piston fit always gets tighter with heat. The pistons have significant shakes machined into them that get flattened over time. It is better to fit the pistons to minimum clearence and put some time on them with reduced combustion temps (careful breakin), you end up with a longer lasting top end. Some racing engines won't last long enough to breakin because of wide exhaust ports. These engines must be fit near the service limit or else they stick. These engines are unique only in that they absolutely WILL stick very quickly if you are not carefull with the setup. Moving on, I have not seen rings harder than the cylinder with the exception of someone installing chrome rings in a Chevy. Whether indended or not, pistons do scuff perpendicular to the wrist pin. How long does it take to seat a ring? That depends a lot on how well it fits the other parts, usually not very long. Our model engines are small enough and turn slow enough to easily dissipate the heat of a hot mixture setting. Plus they use plated aluminum cylinders that do not converge with the piston like an iron cylinder will. In the case of our small aircooled engines there is little that can be done to affect how the ring seats (it will seat) and the piston will never get tight. So there is effectively no breakin for our model engines, just bolt them on an fly.