Myself writes: “While repairing mangled bolt threads with the corner of a flat file works, it’s far from ideal; the 90-degree angle removes more material than it should, and the finished threads are weaker as a result. It’s also hard to guide unless the damage is relatively minor.
“Purpose-built thread files have teeth arranged in common thread pitches, and self-align when held at the right angle. The result isn’t quite as good as the original formed threads, but it’s better than the alternative.
“Most thread files are made from square bar stock, with four pitches represented on each end. P ick up one imperial and one metric thread file, and you’ll be ready for anything!”
The file pictured above is a metric one. We found numerous versions available starting at around $9.
Thread Files [Froogle]