Split Pin

Split pin (ISO 1234) has its flat inner surfaces touching for most of its length so that it appears to be a split cylinder. Once inserted, the two ends of the pin are bent apart, locking it in place. When they are removed they are supposed to be discarded and replaced, because of fatigue from bending.

A split pin, also known as a cotter pin or cotter key, is a metal fastener with two tines that are bent during installation, similar to a staple or rivet. Typically made of thick wire with a half-circular cross section, split pins come in multiple sizes and types.