Cotter pin

A cotter pin (also called a split pin) is a metal fastener made from a length of wire folded in half, producing two tines that can be inserted together through a drilled hole and then bent apart to lock the assembly. Its sole job is to prevent a castle nut, clevis pin, or shaft collar from working loose under vibration.

How it works

The pin is pushed through a hole in a shaft or bolt. Once the looped head seats against one side, the two legs protruding from the other side are spread apart with pliers. The spread legs act as a mechanical stop: the pin cannot fall back through the hole, and whatever it retains — usually a castellated nut — cannot unscrew past it.

Cotter pin vs. split pin

Terminology varies by region. In the US, “cotter pin” usually refers to a wire-type pin made from soft steel or stainless wire, with a round or oval head. In the UK, Australia, and much of the Commonwealth, “split pin” refers to the same object, sometimes made from spring steel with a half-round cross-section. Reid Supply notes the terms are used interchangeably in practice, though some suppliers distinguish wire cotter pins (disposable, bent to lock) from spring-type split pins (reusable, self-locking by spring tension).

Common uses

• Castellated nuts on automotive ball joints, steering linkages, and wheel bearings — the cotter pin passes through the slots in the nut and the hole in the bolt.
• Clevis pins on linkages and turnbuckles — the pin holds the joint together and the cotter pin prevents it from sliding out.
• Shaft collars — the pin locks a collar in position on a smooth shaft.

Materials and grades

Most cotter pins are low-carbon steel, zinc-plated for corrosion resistance. Stainless steel versions are available for marine and food-grade environments. They are manufactured to national or international standards (e.g., DIN 11024, ANSI/ASME B18.8.1) that specify diameter, length, and minimum breaking load.

Key point

Cotter pins are single-use. Once bent, the metal fatigues and should not be straightened and reused — always install a new pin.