[greywolf]

They are very ordinary M8x1.25 bolts. Chances are you or a previous owner/mechanic is or has been working with mounting luggage. First, if the threads on the bolts or frame are damaged, a tap and die set or thread chasers is the best bet to clean them out. If it's a new installation, the trick is to hold the carrier away from bike to allow the bolts some wiggle room. Put some anti seize on the threads to keep them from binding and just start the bolts a few turns each. Then rotate around the carrier, tightening each bolt a few more turns each rotation. Eventually, they will all snug down. Don't try to get any one bolt down way ahead of the others.

 

[bower100]

If the four threaded holes in frame end up being totally screwed up I have a like new frame section for sale for about $95.....:mrgreen:

While I'm serious about that, replacement for that reason alone would be crazy. I doubt you can't "recover" the threaded holes with an 8mm tap but if in the end that is the case, renewing them with a thread repair kit would be an easy/good fix in that application.

Just saying.
dave