This mechanical fastener passes through a cross-drilled hole in a shaft or bolt to lock components in place. Traditional cotters are tapered metal wedges driven into a slot to bind parts tightly. In automotive and structural work, the term almost always refers to a split pin with two tines that are bent outward after insertion to physically block a castle nut from loosening.
Back the nut off just a hair so the slot lines up with the hole, then slide the cotter in and bend the legs back.
Never reuse a split cotter pin. The legs work-harden when bent and will snap off if you try to straighten and bend them a second time.
Also heard as
- Cotter pin
- Split pin
- Cotter key
Related terms
Castle nut
A slotted nut with notches across its crown that take a cotter pin through a hole drilled in the bolt.
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.