Originally Posted by
Russ Roth
I've your mounting holes aren't threaded, then you can put a cap head bolt in from the inside and use a nylon lock nut on the outside to keep it from coming loose.
On my commuter, I wanted to have a rack and fenders. But I only had one set of eyelets, which were threaded, but that didn't help really. So I put a long bolt through from the inside and put a nut on the other side of the eyelet. Then the rack, a washer, and a nut. Then came the fender struts, another washer and another nut. This really needs a picture, I know, but I don't commute on that bike any more. If you tighten the nuts properly, they aren't going to come loose, but locking nuts or thread locking compound wouldn't be bad.
I make my own racks now, and they have an eyelet built in for the fenders. I got that idea from Tubus, but I'm not sure which of their racks have the rack mounted eyelet.