I'm putting new Ultegra brakes on to a new Reynold Ouzo Pro carbon fork. TH brake nut won't fit into the back of the fork. Seems as hugh the diameter of the nut is too big? Anyone encounter this before or know what to do?

I've had that happen. Just go to your LBS and ask if they have any nuts that are smaller in diameter, it shouldn't take much.

Apparently none of the lbs's around here seem to know about the thinner brake nut. Both places told me they only make one size nut...

If it were steel, I'd say ream the hole to the correct size. Is that kosher to do to a CF fork? I wouldn't know.

Don't drill the CF fork! Find another bolt. It doesn't have to come from a bike shop.

Sure it does- the recessed nuts that hold brake calipers on are bicycle-specific.

First, Joey- take a little sandpaper and clean out the hole in the fork. I bet the clear coat got put on a bit thick on your fork. You just have to open it up enough to get the bolt in.

But dont let anything aluminum hit the raw CF or it will react with the aluminum over time weakening the point of contact. Could be catastropic on a brake, I don't know that I'ver ever seen an aluminum caliper nut before so it will probably be fine. Just be safe about it.

fwiw, i've had a similar case. installed some calipers on a carbon fork. got into a minor accident on that fork. no visible damage but later on when i bought some new calipers, i had a lot of difficulty taking the old caliper out. had to pry the old caliper out of the fork using a flathead screw driver. finally got the old caliper out but the new one won't go in. i thought the two calipers might had used different bolts so i measured with my vernier caliper. the bolts are slightly different but the same recessed nut worked for both, so it probably wasn't due to that. in the end, i think the minor crash might have caused a slight deformation in the alloy crown of my fork. i tried really hard (hard isn't the right word, actually, more like precise) and finally got the new caliper in.

Which diameter are you talking about? The diameter of the shaft of the nut, or the diameter of the head?

Excellent point. If the shaft won't fit, the hole has buildup of clear coat or whatever in the bore. If the head won't fit, it may not be intended to. If the nut won't reach the brake's center bolt, you need a longer nut. These are readily available from any bike shop.

I would contact the fork distributors tech department and see what they say. They may have experience with this problem and could offer a quick fix.

Yes, I had to get a longer nut from my bike shop to fit my fork.

The length is fine. no need for a longer/shorter nut. It was the diameter of the shaft that was preventing me from mounting my brakes.

However, I was able to get the nut in with some... encouragement (light tapping with a huge rubber mallet). Probably not the best way to do it but it just seemed like it wasn't going in due to too much clear coat on both the fork and the nut. A few mild taps and the nut moved into place. Would rather have the nut drop right in but that never happened. Inspected the fork aftwards and it looked the same, no bulging, chips, or cracks. I'm not going to worry about it.

Thanks for all the suggestions.

A better way to have cleared out the hole is to hand turn a 5/16" or 8mm drill bit. The flutes on the bit would have smoothed and cleaned the hole witn no damage

How does carbon fibre react with aluminium?

They are "dis-similar metals" from an electrochemical standpoint and if there is an electrolyte available (like water) they form a battery and the aluminum is preferentially corroded.

Early carbon tubed bikes with aluminum lugs often failed at the joints from this problem. The manufacturers soon learned to wrap a thin layer of fiberglass around the carbon tubes to act as an insulator. A carbon seatpost in an Aluminum frame, or vice-versa can cause the same type of corrosion resulting in a VERY stuck seatpost.