A fastener broken by twisting, usually because it was seized by rust, galling or threadlocker, or because the driving torque exceeded what the shank could carry. The fracture face is flat and often shows a swirl. A stub left standing proud can be gripped or welded to, while one broken below flush calls for drilling, an extractor or EDM.
Two of the manifold studs twisted off flush, so it's drill and insert, not a five-minute job anymore.
Also heard as
- snapped
- broke off
- sheared off
Do not mix it up with
- TC bolt: A TC bolt is designed to shear off its spline end as proof of tension; a twisted-off bolt is a failure.
Related terms
Easy-out
Shop name for a spiral screw extractor driven backward into a drilled broken bolt to unscrew it.
TC bolt
Tension control bolt: a structural bolt with a splined tip that shears off once the right tension is reached.
Galling
Adhesive wear where thread surfaces cold-weld and tear under load, seizing a nut partway down a bolt.
Necked down
Describes a bolt visibly thinner in one spot because it was stretched past yield and started to neck.