Fender bolts for 1973 Raleigh Gran Sport
 

M6s are just a little too big. M5s slide right through the holes both front and rear. The holes are threaded. What is the correct fender bolt for these bikes? Yes, I can put a nut on the M5s, but I would prefer to do it correctly.

I'd guess some sort of Whitworth bolt, but I'm also surprised that M5 is too small. I've usually found on those old Raleigh road bikes that M5 is too large. I'd re-tap for M6.

What does an American 1/4"-20 bolt do in the hole? Can you "start" an M6 but it quickly binds up? Or does it not even catch the thread? Being that a M5 slides right through, it'd have to be at least Whitworth 1/4" if it's Whitworth, and an American 1/4-20 bolt may screw right in.

Could be Whitworth or could be some other odd British size (British Standard Cycle and British Association are other ones I've seen on bikes).

On my '73 GS 5mm fit the rear but was too small for front. I retapped those for M6. You do what you gotta do.

Yes, an M6 starts but it immediately binds. I'll try a 1/4 20 before tapping them all to M6.

All right. Let us know what you find out! One other thing to note is that, if it's a similar size to M6 (which it is, because you can get an M6 bolt to start), retapping them for a 1mm pitch could basically destroy the threads inside, leaving your bolts precious little to hang onto. Because the existing thread is either coarser or finer than M6 standard pitch, which is 1mm per thread, the tap will cut threads out of the threads that are already there, and may not leave much metal. I hope that makes sense. I'd throw everything at these to see - #12 x 24 is another oddball close to M6, as are 1/4-28. If it's none of these, it's probably BSF or BSC, and tapping may be your best option. If the M6x1 tapped hole quickly strips, you an always increase it to M7.

I'd go with M5 and a nut. I understand the desire to 'do it right' but I don't see how changing the threading in the frame is better than something that's completely non-destructive. But as @due ruote said... you do what you gotta do.

Try from the other side if it binds - you might have a bashed thread at the beginning.

My GS also has different sized holes front and rear, haven't bothered to find out what they are...

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.

I would guess a '69 Raleigh Superbe would have Whitworth-threaded bolts. I would just retap to M5 or M6.

The thread gauge doesn't lie. Google says SAE coarse and British Standard Whitworth are the same up to 1/4" diameter bolts. 7/32" x 28 tpi is British standard fine, which agrees with current UNF standards. It doesn't change the fact that replacement bolts are generally something you order from the supplier directly rather than get from the hardware store since it is being supplanted by M5 or M6 bolts.