This is diameter question, not length. I'm installing a set of Zero G brakes on an Ouzo Pro fork. I bought the fork used but it is low to zero mileage. The nut won't fit into the hole on the back of the fork. I dug out a set of Campy Chorus and Shimano UItegra brakes and tried the nuts off these and only one of the shimano nuts barely fit but it's too short.

Are there two size standards for these nuts? Or is it possibly an issue with a new fork? Should I try to tap the nut in with a mallet?

Thanks.

The standard recessed-mount brake hole is 8 mm, with a 10 x 3 mm counterbore if you want the nut to tighten flush with the fork crown surface.

Sometimes paint, clearcoat or crud gets into the hole and reduces the diameter enough to cause the problem you describe. Try cleaning out the hole with a reamer, or a hand-held drill bit. I would not tap the nut in. Even if you get it in, it might wind up off-center, canted or both.