What is a thread chaser, and why shouldn't I use a tap?Updated 2 hours ago
This is the most common mistake we see, and it is an expensive one.
The difference
A tap is a cutting tool. It is designed to create a thread where none exists, and it removes metal every time it goes in.
A thread chaser is a cleaning and restoration tool. It follows the existing thread form and pushes debris, carbon, old sealant, rust and thread locker out of the way without cutting the parent metal.
Why it matters on a head stud install
Running a tap down a perfectly good block hole slightly oversizes the thread. Thread engagement drops, the fit loosens, and the clamp load your carefully calculated torque spec was supposed to produce is no longer what you actually get. On an aluminium block or head the effect is worse.
Meanwhile, the actual problem — carbon, oil film, coolant residue, old sealant in the bottom of a blind hole — is exactly what a chaser is designed to remove.
How to use one
- Run the chaser down each hole by hand, backing it out periodically to clear debris.
- Blow the hole out completely with compressed air. Wear eye protection — this launches a surprising amount of material.
- Vacuum or swab any remaining fluid from blind holes. Trapped oil or coolant is effectively incompressible and can hydro-lock a hole or crack a block when a stud is run down into it.
- Dry-fit each stud by hand. Every one should thread in smoothly all the way. If one binds, back it out and chase that hole again.
Our thread chasers
We make chasers in metric sizes from M6 through M16 and imperial sizes from 1/4-20 through 1/2-13, available individually and in multi-piece sets. They are not engine-specific — any chaser of the right size and pitch works on any build, so they earn their place in the toolbox permanently.
Which size do I need?
Email [email protected] with your year, make, engine and displacement, or the part number of the fastener kit you are installing, and we will tell you exactly which chaser matches.