abc engines have different problems than ringed engines,we are talking true abc not the abn that os produced.during breakin they require near max rpm to get the correct temp for the cylinder to expand,if too rich and the piston will wear,any dirt that scratches the piston or liner and you need a new piston/liner assenbly vs just a ring or piston and ring.had a freiend ruin a lee custom 7.5 k&b from a lean run in a f9f-8 Couger years ago,took rhe raper out of the cylinder when it cooled and lost all compression.most ring engines when they wear out can be re juvenated by replacing the ring.and since frank bowman makes rings after engines are out of production you can keep a ringed engine in service a long time.I have been using abc engines since k&b introduced the 3.5 in ducted fans.that engine stock in the rk-20 fan on 25% nitro on a pipe would turn 22,500 rpm,with the change of the piston/liner to a pipe tuned piston/liner the addition of a r.p.m machinded rod bushed a both ends a k&b .40 size carb and a robart matk lV pump it would turn 24,500 rpm and 8 ounces more thrust.