The engine was rebuilt before I got it so it likely doesn't have hardened seats. Its a strong runner and I've put 3000 miles on it. I judt drove it ten miles at 45 mph with the engine running smoothly. No missing.
My restorer and I both believe we got the lock nut tight.
Vince: If it were my engine I would start the detective work and see what is different about the valves/lifters that continue to go out of adjustment verses the ones that don't. Possible that you have a mix of valve manufacturers and those valves are inferior. Mark the screw adjustment relative to the lifter that it screws into to verify that it is not moving. Compare the valve seats and valve faces to the valves that stay in good adjustment. If you continue to run the engine with zero valve clearance, it will eventually burn those valves. Just some ideas to try to solve the problem.