HI the following applies only to two stroke glo engines-as that is all i fly-IF you are running 20 % castor in your fuel you should not need to change your ring or bearings for some time at 100 hrsloss of compression is the sign you need a new ring FRANK BOWMANS rings are great -cheeper have more spring to them also and will bring the engine back to new compression in most cases- do follow FRANKS instructions on ring replacement-bearings will get noisy and rough-that is a sign they need replacing-before they come apartif they come apart they will cause other internal damage metal thru the engine do avoid this-
one thing i do find very odd-i have two engines that have very low compession when cold-yet they both perform as they always did when i run and fly them i will continue to run them this way until they have problems running the plane in the airi am handling glo engines for 26 yearsi do think that the heat produced by the engine running expands the piston and may have the effect of increasing the compression that wayone engine is an abcand one is ringedthe linner on the ringed engine is of such hard material -that i do think the ring will be the one to wear first of the two surfaces-so ring replacement when needed has worked for me many times in the pastcompression often comes up to that of a new engine ENJOY REGARDS TONY "the omega man " think pink ! !