The shaft threads for the nut were stripped, so Mr Bodger cut a smaller diameter thread all the way along it.
Look for another crankpin or whole crank assembly to buy, or investigate getting the pin removed, built up and re-machined.
Thanks! At least I know now what it should be like, although it looks like a major job to fix the problem. The 350cc engine that I have has a different stroke to the 500, so a replacement 500 crank won't work - I'd need barrel and head too.