Front and rear seal of the intake....I was getting a steady drip while the engine was running. That's why I took the heads off....to see if there was a noticeable problem with a piston / ring. It may be that it wasn't sealed that well when assembled....they just used the black silicone seal with no front or rear gasket. I know this is done a lot, but having had problems in the past, I always use the gasket. The oring grooves are about .040 wide, but I would say close to .020 deep.....probably not .030. I don't know how to post pics here, or I would do that. Thanks.