Both tools can be used to clean up a thread, which is called "chasing". However, using an ordinary tap or die to clean threads generally removes some material, which results in looser, weaker threads. 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.