A low-profile hex nut used to lock a threaded adjustment, such as a set screw, tie rod end, turnbuckle or valve lash adjuster, by tightening it against the part or against a second nut. Because it is thin, a jam nut has reduced stripping strength and should not carry the main joint load on its own. The word jam is literal: the two threaded parts are forced against each other so friction holds the setting.
Set the toe, then crank the jam nut down against the tie rod end so it doesn't wander on you.
In a two-nut lock the thin nut belongs underneath and the full nut on top, because the outer nut ends up carrying the load; most crews fit them the other way round.
Also heard as
- thin nut
- check nut
Related terms
Nut
An internally threaded part that mates with a bolt or stud, graded so the bolt breaks before the nut strips.
Prevailing-torque nut
A nut built to resist turning on the thread before it seats, using a nylon insert or deliberately distorted threads.
Self-loosening
A tightened nut or bolt turning loose by itself, usually because sideways slip in the joint lets it walk off.
Set screw
A headless screw, usually socket driven, whose point bears on a shaft to fix a pulley, collar or knob in place.