The general rule is .004" for every inch of bore with forged pistons for a street engine, i.e. a 302 with a stock 4" bore would have a top ring gap of .016-.018". .013" is way too tight if you really lean on the engine and like scored cylinder walls. Different applications need different gaps--as a hot race or street/strip engine would require .002-.004" more gap per inch of bore with nitrous or a supercharger. The best way is to FOLLOW THE RING MANUFACTURER'S INSTRUCTIONS. If you really want to blueprint things, buy some double moly rings .005" oversize and file-fit them to the exact gap. That way you know you're right. For my 2 cents, I'd gap your rings at .018" and spin that 302 screaming at 7000! Good luck!
