S&S chr/moly replacement piston ring set. Fits all Evolution & pre-Evolution engines that are equipped with an S&S 3-5/8" big bore kit that have 1/16" (1.5mm) compression rings and 5/32" (4.2mm) oil rings. Will also fit other manufacturers 3-5/8" Big Bore kits (like some Axtell kits); that are equipped with the same piston grooves. Std cyl. bore: 3.625" (92.075mm). 1st ring: 1.5mm (.058"). 2nd ring: 1.5mm (.058"). oil ring: 4.2mm (.155"). Note compression ring gap: To determine the correct ring gap on your newly installed piston rings is very important. A too narrow gap may result in premature piston ring/cylinder wall wear or even seizure; possibly breaking rings and/or damage to pistons and/or cylinder walls; and so on. A piston ring gap that is wider than necessary will result in needless compression loss and reduced engine efficiency. Recommended ring gap is .004" per inch of cylinder diameter for street engines. For a 3-5/8" (3.625") bore engine piston ring gap would be 3.625" x .004" = .0145". (.0145" x 25.4 = 0.3683mm or; when rounded of; 0.37mm). Information provided by S&S Cycle; Inc.