Come home last night and heard a nasty rattle coming from the bike, had a look and seen that the top and middle nut of the rear exhaust manifold have come off. Is there any reason for this to happen, and any way to stop this happening? Just bought some M8 self locking nuts from halfords as a temporary measure as I know they will melt.

Cheers

I always use stainless spring washers and nuts.
Cheers
Ian