There is always a minute amount of metal to metal contact or the engine would never wear out, and I believe that the rings are the main culprits, in an old well worn engine the wear ridge at the top of the cylinder stops where the top edge of the upper ring stops. The piston skirts apparently have enough surface area so that side thrust does not seem to wear the cylinder as much as the rings do. Good lube is everything and that is why I refuse to use any fuel without some castor in it.