Bavin,

If the threads are really "messed-up", you may not have any threads left after running a M5x0.8 tap through. In that case, all is still not lost. Helicoil makes a thread repair tool (part #5546-5) that allows you to drill out the hole to a larger size, thread the hole to a larger size, and then install a thread insert (looks like a short spring) that is a new set of threads, and locks into the larger threaded hole to give the original size hole.

They can be found on Amazon for about $25, for the tap, installation tool, and 12 thread inserts. Cost new is much more. I think they have taken old tools and replaced the inserts with new, stainless ones. Here is one page offering them: http://www.amazon.com/Helicoil-5546-.../dp/B000BB8XLG

Tom