[C-40]

nope..

The problem is not that the bolt is too long, the hole is too small. It's not unusual to find some paint or epoxy build-up that's causing a problem. Quite often the shank of the bolt will go in, but the head hangs up. A little clean up with a small round file or a 5/16 inch drill bit will fix the problem. If the head hangs up, a little touch-up with dremal sanding drum might be required.

 

[C-40]

doesn't matter..

Does Shimano differ from Campy in regards to brake bolt dimensions or are they both the same?

Yout get a nut for the mounting stud with the brakes. Some forks have such a long length of engagement that they come with their own their own special nut.

http://www.loosescrews.com/index.cg...dex.cgi?d=single&c=Brake&sc=Recessed Nuts&tc=&item_id=LS-0051OB&id=683490031747

 

[C-40]

measure...

That's an extremely long nut. Stock nuts are in the 11-14mm range. Put the brake on the fork, screw the nut on about 8 turns and try to measure from the under side of the head to the recess that the head seats against to determine how much excess you have.

Alternately, you said you measured the depth of the hole. I'm now assuming the there is a smaller diameter hole on the front side of the fork, to fit closely with the brake mounting bolt. The back side is counterbored to 5/16 inch or 8mm, with a second, larger counterbore for the head of the nut. The nut just has to be shorter than the length of the smaller counter bore, but it also can't be so long that there would be a chance of bottoming out the nut on the threads of the stud.

If the fork did not come with a special nut, then most likely a stock one will do.

Don't have any other bikes around with a nut you can steal?

 

[C-40]

more...

You're making this too hard. The next shorter length down from 30mm is a 20-22, then a 10mm. All you need to do is get 6-8 turns of engagement and not bottom out in the counterbore.

You could probably cut off the one you have, if it's no longer needed. Most of these nuts are fully threaded. The four stock nuts I've got laying around even show thread marks into the 6mm internal hex area. Worst case, you might need an M6 tap. You can get one of those or a small 4-10mm set at Ace Hardware.

 

[C-40]

you were right...

Good point. Based on OPs description and picture, I took the problem to be a too long bolt not a too narrow hole.

I had no idea the nut was 30mm long! A shorter nut is indeed the fix. I've added the suggestion to merely cut the nut shorter. Simple solution, since most of these nuts are full threaded.