This writeup by Wes White of Four Aces Cycle originally appeared on ChopCult. Wes goes over a simple way to repair damaged threads on your hardware. Reading this can get you out of a jam, especially with hard-to-find-locally Whitworth hardware.
Lets say you have a weird Whitworth thread on your bike (or a weird American or metric thread for that matter). The threads look good, but your nut will only start, it won't finish. You are sure it is the correct nut for the bolt or stud so there must be a burr on it or something. Once I had this problem with the axle nut on a sprung hub rear wheel. It also is a great fix for a fork tube cap on a British bike. I am not sure a die exists that can fix those these days.
You tried to find a die to run over it and maybe you could not get it to start without chowdering the other threads. Your thread file only helped a little. What to do?
Here is a pretty good fixer for that problem, but the nut has to start just a little. Take some valve grinding compound and put a little on the male threads of the offending fastener. Run the nut onto the fastener until it stops, then back it off.
Then repeat a bunch of times until you start getting the nut to fit. You may have to clean and re-apply the compound if it gets too dry, but you can do that no problem. You can feel the nut going a little further and further each time you turn it.
At the end you should get the burr off the thread and be able to tighten the nut down without hurting any more threads.