Cotter pin

A cotter pin is a metal fastener that is bent after installation, similar to a staple or rivet. Typically made of wire with a half-circular cross section, a new cotter pin will have its flat inner surfaces touching one another for most of its length, so that it appears to be a split cylinder. One tine of the pin is often noticeably longer than the other. Once inserted, the two ends of the pin are bent apart, locking it in place (see figure).

Cotter pins are frequently used to secure other fasteners, i.e. clevis pins.

Spring-type cotter pins are also available, which are not designed to be permanently bent. In this design, only one section of the pin passes through the shaft to be secured, the other section being curved to wrap around the outside of the shaft. This type of pin is usually made of round wire of a harder heat treatment than is appropriate for traditional cotter pins.