Thread chasing Restoring male or female fastener threads By Mike Mavrigian
W
henever 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 specifcally for cleaning cylinder block threads are also called block clean out taps. Other dedicated restoration taps
An example of a professional grade thread chaser from Goodson Tools, designed for engine builders. High quality hardened steel and massive futes designed to follow, straighten and restore female threads.
include those made specifcally 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 specifcally designed to reform 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.
This Goodson example is size 7/16-14, a common size for cleaning/restoring cylinder head deck threaded holes on many V8 earlier-generation domestic engine blocks. Quality-wise, it just doesn’t get any better than this.
50 | November/December 2014
