Cotter Pin
A split metal pin inserted through a hole in a bolt or shaft and bent to prevent a castle nut from loosening.
A cotter pin (or cotter key) is a simple metal fastener consisting of a split pin with a loop at one end and two tines of unequal length at the other. It is primarily used as a safety locking device to secure castle nuts on critical automotive components, such as tie rod ends, ball joints, and wheel axle spindles.
To install a cotter pin, it is pushed through the aligned slots of a castle nut and the hole drilled in the mating bolt or shaft. The technician then uses pliers to bend the tines in opposite directions, locking the pin in place. This mechanical lock prevents the castle nut from backing off even if it experiences severe vibration or torque loss.
Cotter pins are made of ductile steel, meaning they are designed to bend easily. Because bending fatigues the metal, cotter pins are strictly single-use fasteners. Reusing an old cotter pin can lead to it snapping under stress, allowing the nut to back off and cause a catastrophic suspension or steering failure.