James---I think the screw you are referring to sounds like it would keep
the oil holes in the rocker shaft in the proper position. If the shaft
was allowed to twist, or move much in any direction, there would have
been some oil deprivation at each rocker. Over time, the shaft would
wear out sooner than it should. The easiest way to check for shaft wear
is to slide a rocker out of position to get a look at the operating part
of the shaft. If you see a noticeable step here, the rocker shaft should
be replaced. The damage will depend on how long this has been going on.
If you were able to sucessfully adjust valve lash and it doesn't
immediately get lose again in a few miles, you might have dodged the
bullet.
If the shaft does have fair sized worn areas, you can check the rockers
themselves for wear after shaft removal. Place them on an unworn area of
the shaft, and check for wobble.
Look for the "missing screw" down inside one of the tappet bores, or
fish with a long magnetic tool.
Also be sure the set screw over the oil drilling bore is secured.
Dick T.
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