Castle Nut
A nut with slots cut into one end, allowing a cotter pin to lock it in place when used with a drilled shaft.
A castle nut (also known as a castellated nut) is a specialized nut featuring slots (or notches) cut into one end, giving it the appearance of a castle battlement. It is used on critical, rotating automotive connections—such as ball joints, tie rod ends, and wheel spindle axles—where the nut must be locked securely in place to prevent it from backing off.
The castle nut is threaded onto a bolt or shaft that has a pre-drilled hole through its threads. Once the nut is tightened to the specified torque, the technician aligns one of the slots in the nut with the hole in the bolt and inserts a metal cotter pin through the assembly. The ends of the cotter pin are then bent, mechanically preventing the nut from spinning.
Using a castle nut with a cotter pin provides a highly visible, fail-safe locking system that does not rely on thread friction alone. During steering or suspension service, castle nuts should be inspected for damaged slots, and a brand-new cotter pin must always be used upon reassembly to ensure safety.