I wuz always told .004" to .005" per inch of bore diameter on compression rings and a bit more on the oil control ring, but maybe I'm out of date as usual. Works for me, though! P.E.I. says .005" per inch if engines are warmed up and .010" if they are subject to stress from cold. Interestingly Richardson gives .010" to.020" total gap on the oil ring, but I don't remember what type of oil ring the Omega pistons use. Whatever else you do, ensure that the bore is round, the muff fit is right, the con-rod is central, the bore is square to the crankcase mouth, the oil feed is proved to the rear of the piston below the scraper at B.D.C. and the muff is a snug fit cold into the crankcase mouth.