Split pin

Typically made of thick wire with a half-circular cross section, split pins come in multiple sizes and types.

Once inserted, the two ends of the pin are bent apart, locking it in place (figure B).

[5] Split pins are typically made of soft metal, making them easy to install and remove, but also making it inadvisable to use them to resist strong shear forces.

The extended prong type is popular because the difference in length of the two tines makes it easier to separate them.

[clarification needed] The diameters of split pins are standardized.

A split pin (UK usage) / cotter pin (UK/USA usage) holding a rod in place with a washer
Cotter pins:
A: New split pin
B: Installed split pin
C: R-clip
D: Cross-section of a new split pin
Types of ends available on split pins: Standard, extended prong, mitre end, bevel end and hammer lock. The length L of the split pin is defined as the distance from the end of the shortest tine to the point of the eyelet that contacts the hole.
A car hub showing a castellated nut cover and split pin (near center)