BTDT. The threads are stripped. The stock bolts don't engage very far into the threads, so only the top portion of the threads are stripped. When you remove the cover and thread the bolts in, you are accessing the lower part of the threads, the unstripped portion.

I went to a good hardware store (Elliots in Dallas) and purchased bolts 5mm longer than stock. Had to use a washer or two to get a nice snug clamp on the cover, but it works like a charm. Cheaper and easier than six thread inserts. Just be careful not to bottom out the bolts.

- JimY