Safe to shorten a dual-pivot caliper mounting nut?

Question

I have a 2006 Specialized Langster. Unfortunately the brake bridge is not deep enough to insert the shortest caliper mounting nut I could find (10mm). The head of the nut should insert all of the way into the brake bridge, and the head of the nut should be supported on the interior shoulder of the brake bridge.

Instead the nut sticks out and won't insert all of the way into the brake bridge hole, and the end of the nut rests against the thin rear shoulder of the brake bridge. This means the head is completely unsupported. ie the nut is too long for the brake bridge.

I could potentially fix this by grinding down the mounting nut so that it is about 8mm long. This would allow the nut to insert fully into the brake bridge and be supported at the head of the nut. But I'm wondering if grinding down the nut is safe to do? Do you think it would weaken the nut or cause it to crack over time? Also the threads would have to be chased.

This might be hard to visualize but take a look at the mounting nut on your rear brake caliper. You will see it fits nicely into the brake bridge and inserts all the way, with the head of the nut resting against the brake bridge creating a secure mounting point.

I contacted Specialized and they won't treat it as a warranty issue. Please let me know your thoughts.

Thanks.

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Edit: Actually, I am talking about the mounting nut rather than the bolt. Modern brake calipers mount using a 10mm or longer cylindrical nut with a larger diameter "head" and smaller diameter inner hollow "cylinder" which is threaded on the inside.

The outer diameter is the "head" of the nut and it presses against an interior shoulder recessed in the brake bridge which acts as the load bearing surface. The inner diameter is a cylinder which fits through the brake bridge and threads on to the caliper mounting bolt coming through the other side of the brake bridge.

Everything works on most bikes. But in this case it appears the brake bridge is not deep enough to fit even the shortest available (10mm) nut. So as far as I can tell the brake bridge is simply built wrong -- the brake bridge has a square center drilled out "box" section welded on that is simply not deep enough. Since Specialized won't warranty the frame, I am stuck with figuring out how to grind down the nut safely, while preserving its threads and hopefully keeping it from getting overheated, brittle and developing stress cracks over time. This is what I am hoping you can help with!

Answer 1

To shorten a bolt, get two nuts with the matching thread. Screw them onto the bolt and "lock" them together (tightened against each other) such that the nut surface farthest from the head lines up with the place where you want to cut.

Place the bolt in a vice (taking care to not mash the threads) and use a fine-toothed hacksaw or other cutting device to cut the bolt off flush with the nut surface.

Then unscrew the nuts and dress up the bolt threads with a fine triangular file.

If instead you want to thin out a nut, again fasten two nuts to a matching bolt and lock together. Have the nut face you want to cut away facing "out", and have the nut screwed onto the bolt to the point that the bolt end is nearly flush with that face of the nut.

Use some sort of grinder to grind away the desired amount of the nut. Then "unlock" the two nuts and screw them farther onto the bolt (to clear the threads), then off. Work a fresh bolt through the nut several times from both directions to assure the threads are clear.

Answer 2

I believe you are taking about a bolt, rather than a nut ( nuts hold bolts onto things ). Anyway, you aren't going to damage the bolt if you file it down or cut it with a metal cutting saw, it's done all the time.

Answer 3

I decided to punt and just have the bike shop grind down the nut. The head mechanic at the local shop sounds confident that he can grind down the nut to fit without over-heating it (and making it brittle etc.).

I actually looked at another, older Bianchi frame I have and I see that the rear caliper nut has been ground down on that bike too! Perhaps this is more common than I thought. Sure doesn't seem like good engineering though.