A rethreader is a hand tool for clearing and repairing threaded holes; it looks like a screwdriver, but with a different tip.
The indents on screws, bolts, rods and their corresponding nuts or holes are called threads.
Threads can be internal or external and are differentiated by gender. External threads, such as those on screws and bolts, are male, while internal threads, such as those on nuts and holes, are female. Male threads fit into female threads.Though male and female threads are formed using different tools, the structure is the same (if you turn a female thread inside out, it will be the same as a male thread and vice versa).
Over time, threads can become damaged and may require repair. It is important to have clean, straight threads so that the mating of male and female threads is smooth and the bond is formed properly and securely. Once there is damage to a thread, continued use will cause more and more damage until the thread is unusable.
The rethreader repairs female threads. It does this by straightening the existing threads, rather than forming new ones.
The other purpose of rethreaders is to clean clogged and dirty female threads. As the threads of the rethreader and female threads interlock, the rethreader dislodges the debris.It is important to have clean threads so that the male thread can correctly lock in when inserted.
Rethreaders do not require much care and maintenance, they can be stored with all other tools and may require a quick wipe down from time to time, but generally are left as they are.