Whenever you’re faced with dirty or slightly burred female threads (cylinder head threaded holes in a block, spark plug holes in a head, etc.), you may be tempted to grab an appropriate size cutting tap to clean up the threads. The correct choice is to use a “chaser” tap. A cutting tap is designed to create new threads, whereas a chaser tap is designed to clean, re-form and restore existing threads.
Chasing burred female threads
If you want to clean-up existing female threads (maybe the threaded hole has burrs or a bit of corrosion, etc.), it’s best to use a chaser, or follower tap instead of a common cutting tap. A chaser tap is designed to re-form the threads, as opposed to a cutting tap, which will cut its way through, possibly removing too much thread material. This is especially important when dealing with critical-torque-load threaded holes such as cylinder head bolt holes in an engine block’s deck. These taps that are designed specifically for cleaning cylinder block threads are also called block clean out taps. Other dedicated restoration taps include those made specifically for restoring spark plug threads in cylinder heads. Restoring cylinder head bolt holes in block decks and spark plug holes in heads are likely the two most common applications.
Chaser taps are specifically designed to re-form and clean existing threads. The spirals on a chaser tap are designed in such a way to follow an existing helical thread spiral path without removing material. While you may be able to accomplish the task by using a standard cutting tap, you run the risk of weakening the existing threads.
Chaser taps (male taps for cleaning female threads) are available in virtually all fractional and metric sizes, but only the most common sizes seem to be readily available. Fractional inch examples include 1/4x20, 5/16x18, 3/8x16, 7/16x14 and 1/2x20. Beyond that, we need to attempt sourcing from highly specialized tool makers.
Chasing male threads
In addition to cleaning and restoring female threads, thread chasers are available for addressing male threads. Common examples include spindle threads that have been damaged or deformed as a result of spindle removal from hubs and wheel hub studs.
A variety of chaser tools are available for these applications, ranging from one-piece chaser dies, split dies and adjustable dies that feature a selection of thread chasing teeth and that adjust for thread diameter. Depending on the design of the chaser tool, restoring male threads will often require starting at the base and walking outward to the tip.
In order to aid in male thread cleanup, before using the chaser die, closely inspect the tip of the shank. If the tip is burred or very sharp, it may be wise to first slightly radius the tip (with the use of a file or mini belt sander). Creating a slightly chamfered tip can make the eventual installation of the nut easier, helping to avoid initial cross-threading during assembly.