That's a pretty common problem, and one that's not obvious to an external inspection. Quite often, the problem is actually a worn pivot ball on the adjusting screw, resulting in a tip that protrudes down into the pushrod socket. The rocker arm adjusting screw must be backed off far enough to see the wear on the tip of the ball. In an extreme situation, the socket end of the pushrod can break due to insufficient pivoting of the ball in the pushrod cup. Fortunately, the thread on the rockers is common to other engines, (7/16-20) so adjusting screws from some Ford and/or Mopar engines will work in place of the very scarce (and expensive) stovebolt adjusting screws.
Jerry
