Split pin

A split pin is a metal fastener consisting of a length of wire folded in half to form two tines. It is inserted through a drilled hole and the legs are bent apart to lock the assembly in place. In Commonwealth English (UK, Australia, India) the term “split pin” is standard; in American English the same object is usually called a cotter pin or cotter key.

Construction

Most split pins are made from low-carbon steel wire, though stainless steel and brass variants exist. The wire is typically half-round in cross-section — two halves of a circle pressed together — which gives the pin its springy, self-locking character. When the legs are spread, the spring tension holds them in place without additional bending. Wikipedia notes that split pins are manufactured to standards such as DIN 11024 or BS 768, specifying diameter, length, and material grade.

How it works

The pin passes through a hole in a shaft, bolt, or clevis pin. The looped head seats on one side; the two legs protruding from the other side are spread apart with pliers or a screwdriver. The spread legs prevent the pin from pulling back through the hole, which in turn prevents whatever it retains — a castle nut, a cotter pin hole in a clevis, or a shaft collar — from moving.

Common applications

• Automotive suspension — castle nuts on ball joints and tie-rod ends are secured with split pins.
• Agricultural machinery — clevis pins on implement linkages use split pins for easy field serviceability.
• Marine rigging — stainless split pins on turnbuckles and stays.
• Fencing and gates — hinge pins held by split pins for quick removal.

Reusability

Unlike disposable wire-type cotter pins, spring-steel split pins can sometimes be straightened and reused once — the spring tension still holds. However, best practice is to fit a new pin each time, as fatigue and corrosion weaken the metal.

Key distinction

A split pin is a locking device, not a load-bearing fastener. It prevents parts from separating; it does not carry the structural load of the joint itself.

Split pin vs. R-clip vs. circlip

Three small parts get confused because all three “pin things in place”, but they install differently:

• Split pin (cotter pin): two wire tines you spread apart after insertion. Single use in practice — the legs fatigue once bent back.
• R-clip (hairpin cotter): a single piece of spring wire bent into a shape like the letter R. It snaps into the hole and is pulled out by its straight leg — no pliers, fully reusable. Standard on lawnmower axles and trailer hitches.
• Circlip (retaining ring): a flat sprung ring that seats in a machined groove on a shaft or in a bore, not in a drilled hole. Removed with special pliers that spread or compress it.

The fastest identification: if it has two legs you bend, it is a split pin; if it has a straight leg you pull, an R-clip; if it is a ring sitting in a groove, a circlip.

How to size one

Split pins are specified by diameter and length. The diameter must match the drilled hole — the pin should slip through with no forcing, then be spread to lock. The length should let the legs protrude far enough to bend flat against the retained part without the looped head bottoming out. If the hole is worn oversize, do not fold a bigger pin double to fill it: drill to the correct size or fit a new clevis. A loose pin in an oversize hole is how castle nuts walk off axles.