I have a set of Dart 308 BBC Marine heads that have been back and forth to 2 different machine shops 3 times for a valve job. The shops use a vacuum test to check their work and told me they check fine. However, when I bolt them on the motor, some cylinders show up to 20% leak on a leak down test. I can visually see that pressure is escaping around some of the intake and exhaust valves when I spray WD-40 in the chambers. I’ve even tried hitting the top of the stem with a rubber hammer to seal them but it did not work.

The 2nd machine shop owner seems knowledgeable and indicated that he lapped in the valves and “they were 100%” - He doesn’t know what else to do and neither do I.

Anybody have this frustrating problem before and have any suggestions?

Just shooting from the hip here but is it possible that the cam is hangin them open?? too long or short LSA?? don't have a huge amount of experience with the subject but just a thought?? Sorry if that is a stupid suggestion lol

Leak down test was done with no rockers arms installed and piston at TDC.

what valve seat cutting machine are they using?

gotcha...ok nevermind hahahaha

The valve seats possibly leaking? Poorly pressed into the head

I just went through this. Since I got my new Dart heads (~7yrs ago) I had one hole I could never get to stop leaking. Valve grind after valve grind. I just ran it that way for years. Building new engines now and have it on the run stand and still leaked after fresh grind. (removed 3 times everything is perfect) Did a leak down and was leaking out of the seat. My head guy removed the seat and there was a void behind it. Cut a larger seat and put in. S#itty Dart machining!

Thanks for the replies. UPDATE - The machine shop told me that they found wear on the pilot and possibly causing the problem. They have ordered new equipment and are going to redo my heads and valves.

I’m hoping this solves the issue.

Are these after use or before you used them in the boat? Never ran before?