Thank you for the ID. Yes, the engine was an original 4 valve relief piston, 2bbl 273, and I am rebuilding the engine to the 273 HiPo with the 2 valve relief HiPo Pistons. Now, with your explanation, I get the difference. Thanks. On the 1966 273 engine that I am rebuilding while looking at the bottom of the heads, the valves appeared to be recessed into a wedge appearance constuction. The mechanic who is helping me build the engine, or should I say, I am helping him, has been using this machine shop for years, so I think I will be OK. Just had the question about clearance. I have a call in to Rich Moore at Packard who makes the HiPo Pistons for Falcon, and he will give me the info on OEM piston height. I will post that information on this thread tomorrow. Old Dart 66