I'm helping my brother out with his GS500 and we've run into a bit of an issue with the valve cover bolts. The top right (when facing forward on the bike) hex head bolt threads on nicely, but doesn't tighten down and just keeps rotating.

We've taken the cam shaft cap off, and then retapped the threads to the same size to see if this would help, but when we put it all back together the same problem was still there.

Now I've read you can't just replace the camshaft caps as they are machined to the camshaft, if this is true I'm not sure where we go from here as we can't afford to make a mistake.

Could we just tap the hole to a larger diameter and then use a different bolt? Would this be too risky? Any advice/suggestions would be greatly appreciated, thank you.

The length of the bolt is the critical dimension in this case.  When tapping is no longer an option, a thread insert can be used instead.

Heli-coil is the cheap option and most common.  Tapping inserts are stronger and can be installed with less steps.  Time-sert is the strongest option but requires the most precision to install.

The last ditch option is if the hole has cracking.  Alumiweld.

Ended up tapping the thread to an M8 x 1.25 and cutting down a bolt to the same size as the original.

Not pretty but it's functional and should do the job!

What year is the bike?