A locking nut that needs measurable torque to keep turning on the bolt even before the joint is clamped. The resistance comes from a nylon or polymer ring that the bolt thread deforms, or, in all-metal styles, from an out-of-round top section or crimped threads that grip the bolt. Specifications set minimum and maximum prevailing torque on first install and on reuse, and the bolt should stick out at least two full threads so the locking feature is fully engaged.
It's a prevailing-torque nut, so read the wrench on the rundown before it seats and add that to the torque spec.
All-metal prevailing-torque nuts go near exhausts, turbos and other hot spots, where a nylon insert would soften and let go.
Also heard as
- locknut
- all-metal locknut
- top-lock nut
Related terms
Nyloc
Shop name for a nylon-insert lock nut, the hex nut with a colored plastic ring captured in one end.
Self-loosening
A tightened nut or bolt turning loose by itself, usually because sideways slip in the joint lets it walk off.
Jam nut
A thin nut, roughly half to two-thirds standard height, run up against another nut or part to stop it turning.
Nut
An internally threaded part that mates with a bolt or stud, graded so the bolt breaks before the nut strips.