Grinding, " diesel " noise under valve covers

i had Keith Black Hypereutectic pistons. (this is what Blueprint used in those days) recommended ring gap on a 4" bore was 26thou. apparently many engine builders have experienced the same problem i had in that the top ring would expand under heat and butt , causing the top of the piston to break off. most engine builders recommend 32thou now with those pistons! this is what caused me to dissasemble the engine. i still don't understand why the cam bearings were so worn out. but that diesel noise has not reappeared. and i race the car 12.5 sec at our local track.