Originally Posted by
furiouschads
Yes, an M6 starts but it immediately binds. I'll try a 1/4 20 before tapping them all to M6.
I know this is a geriatric thread, but I'm running into the same issue on my 1969 Raleigh Superbe. I have all the bolts, but I want to mount a rack along with the rear fender. I have the same issue; M5 bolt too small for the frame eyelet and nut and M6 is too large. Best fit was SAE size #12, but the common 24tpi coarse threading binds after 1/2 a turn. I used a metric thread gauge on the bolt and the closest pitch is 0.9 and the closest SAE pitch is 28 tpi, somewhere around .0907mm pitch.
I've concluded that my bike's rear threading is #12-28, which is special order and only commonly available as a socket head in black oxide or stainless, rather than a hex head in chrome (I'm being particular). I'm using M5 bolts and a nylon locknut on the inside of the dropout.
All of the fender mounting points on this bicycle used a nut on the other side of the eyelet to lock the bolt against vibrating loose, and at least Tubus recommends that any of their racks use a locknut even if the eyelet is threaded. I imagine that judicious use of threadlocker would get you the same results though.