General Thread File
HarryMann: Some here may not have come across a thread file, but these are very handy tool for cleaning/trueing up slightly damaged threads. In some cases, depending on the nature of the damage, quite badly damaged threads can be restored with a little TLC using a thread file.
They're used in the way of a file, back and forth, not with great pressure and of course, revolving the thread or the file as you work. The threads have to be nicely engaged and they are used not at right angles to the thread but at the thread's helix angle, slightly offset to the normal angle.
The most important thing of course is to select the correct thread! Whether you know or not the damaged thread's spec., always lay the thread file onto the thread and using a strong backlight, eyeball the two together to make sure you are in fact using the correct one. It's the pitch that will show as a run out... there will always be some light between the bottom of the 'Vee' and the file being the normal clearance that a nut running on the thread is allowed, dependent upon thread class (tolerance).
N.B. There are of course different pitches for most threads in the metric range, see the following link to Metric threads... std. metric is usually medium or coarse, and fine is usually non-standard.
These come in Imperial and metric sizes and have 8 different threads per file. They are normally quite cheap, about £8~£10, but make sure it isn't rusty or a very used one. Old ones are fine if they have been looked after and not bashed around or used excessively - which is unlikely, as... they are a tool that are used maybe once a year at most, but can save the day when they are dug out of your workshop and put to use. I used one on a rear hub nut thread recently that wouldn't allow the nut to run up it. 10 minutes with a thread file and it was fine.