Start by finding the thread count
No matter how careful you are when you reinstall old rusty nuts and bolts, someday you're going to strip some threads. Of course it will happen on a critical bolt, on a Sunday night when all the stores are closed. That's when a thread restoration kit pays for itself. A full set (metric and SAE) will quickly put irreplaceable and hard-to-find nuts and bolts back into service. The rethreading tools look like traditional taps and dies. But instead of cutting new threads, they reshape them to original condition.
To restore bolt threads, start by finding the thread count (SAE) or pitch (metric). Hold the thread file against the bolt threads until you find the size that matches. Clamp the bolt in a vise, engage the thread file grooves with the bolt threads and push the file (Photo 1). That “resets” the threads enough so you can screw on a rethreading die (apply a few drops of oil first). Use a rethreading tap to restore threads on nuts or threaded engine component holes. Don't try to rethread the entire bolt, hole or nut in one operation. Insert the rethreading tap or die and rotate a few turns, then back it off a full turn. That will dislodge the debris from the teeth.