My brother and I are rebuilding a 1970 AMX 390 4spd. We are both retired from the Air Force as Fit. Engineers on C-5 and C-130 acft. I have 20 yrs with Mercury Racing as a Technical Writer specializing in sterndrive engines and drives. We are having problems with the cylinder heads, the valve stems are flush with the valve locks and the valves are the correct length. Is this a common problem and what can be done to solve it?