As for why, "shit happens" is the best I can come up with. I never heard of that happening before-- wear marks on the inside of the valve cover, but not a wear-through. If the rocker arm shaft, bushings, & rocker arms are all a good fit with each other, just replace the valve cover & move on.

I know I'm going to piss someone off, and hopefully this was given tongue in cheek, but I have to say that the above is the worst piece of aircraft maintenance advice I've heard in a long time...

Had the same thing going on with my 0200A with millennium cyclinders that were overhauled 100 hours ago, pin wore a hole in the #3 valve cover.
Talked with with an engine rebuilder in the local area and he said its not uncommon
I know new cylinders come with a set screw to stop the pin from drifting but nothing on the old cylinders.
Anyone know of a fix?