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We are planning on replacing some worn rocker arms on my son's M38A1 and thought we would replace the valve seals while we are doing it. Back in the mid seventies I replaced the valve seals on a Ford 351C by pressurizing the cylinder with 90 lbs of air through the spark plug hole thus holding the valve up. My question is.. Will this old trick work on the F Head engine? Thanks.

yes

Another old trick is 3/8" nylon or cotton rope threaded into the spark plug hole. About 18" of it with the piston halfway up on compression then with a socket on the crank turn her up tight against the pile of rope which will press tightly against the valve's head.