A hex nut with a raised, notched crown, so a cotter pin or safety wire can pass through one of the slots and a cross-hole in the bolt or spindle. The pin makes it a positive lock: the nut cannot back off even if preload is lost. Castle nuts are common on axle spindles, ball joints, tie rod ends and aircraft hardware, where torque or bearing preload is modest and a mechanical lock is wanted.
Torque the castle nut to spec, then keep going to the next slot to line up the cotter pin, never back it off.
The rule is tighten to the next slot, not loosen to the previous one, unless the spec is a wheel bearing adjustment that tells you otherwise.
Also heard as
- castellated nut
- slotted 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.