A thread chaser looks like a tap or die but does a different job. Taps and dies cut new threads, which shaves metal off. A thread chaser simply reshapes and cleans existing threads. It rolls the metal back into place, pushing dirt, rust, and old thread sealant out of the grooves. Mechanics use them on engine block head bolt holes, suspension bolts, and other high-load fasteners where retaining maximum thread strength is critical.
Using a tap to clean threads is a common mistake. It removes metal, weakening the hole. The chaser has hex ends so you can spin it with standard sockets. Apply a small amount of light oil or cutting fluid before running the tool down. If you hit a tight spot, back the tool off a half turn, then proceed slowly. This clears the debris and prevents the tool from binding or snapping inside the hole.