how to tighten thread on the valve lash adjustment screw

I agree with RustyRatRod, with one exception: years ago, I bought a set of those Crane "computer engineered" adjuster screws that were improperly machined. The threads were cut way too small, you could almost put them in sideways.
To spot-face the surface for the locknut, I threaded a rod in a three jaw lathe chuck. Then you just screw the rocker onto the threaded rod, until it bottoms against the shoulder at the thread bottom. Then you just just cut the top of the rocker flat just big enough to fit the nut and box end wrench.