brickman The reason I bored my 305 .060 over was when I pulled it from a car it had over 150,000 miles on it and the cylinders were worn. To make the cylinder walls smooth I had to take it out that far and thats about as far as you want to go with a 305 since the walls were not very thick. As far as making more horespower, its what you put into the engine afterwards. If I had just stuck some new pistons in without doing anything else to the engine I probably wouldn't have seen any gain, but I put a new cam in, ported the heads, new rods,ground the crank , new intake manifold and carb and got about 40 more horespower.