What do these heads fit ? Bore seems to be 2 1/4″, casting is 7″ tall, spark plugs are approx 2 11/16″ apart on centers. They appear to be interchangeable though the combustion chambers are slightly different shape. The blue one has casting number 302770 and it is stamped 10 on the bottom edge. The greenish one has casting number 320495 and is stamped 15 on the bottom edge.