Thread restorer

[2] Normal taps and dies can also be used to clean up a thread, which is called chasing.

Because of this, machinists generally clean threads with special taps and dies—called chasers—made for that purpose.

Chasers are made of softer materials and don't cut new threads.

However they still fit tighter than actual fasteners, and are fluted like regular taps and dies so debris can escape.

Car mechanics, for example, use chasers on spark plug threads, to remove corrosion and carbon build-up.