A castle nut (also called a castellated nut) is a hex nut with radial slots cut into one face, resembling the crenellations of a castle wall. Paired with a cross-drilled bolt or shaft and a cotter pin, it forms a joint that mechanically cannot back off — the pin physically blocks rotation. This is positive locking, distinct from friction-based methods like nylon inserts, thread locker, or lock washers, all of which can relax under vibration or heat.

The geometry: a standard castle nut has six slots (one per hex face) or twelve (one between each face), giving you rotational resolution of 60° or 30° respectively. You torque the nut to spec, then rotate further to align the next slot with the cross-hole — but only if under-torqued alignments are unacceptable. For critical joints, you back off to the nearest slot instead, since over-torquing can yield the bolt.

Where you'll actually see them:

• Automotive wheel bearings and spindle nuts — a wheel that spins off is catastrophic; the cotter pin ensures it doesn't
• Tie rod ends and ball joints in steering linkages, where vibration is constant and failure means loss of steering
• Aircraft control linkages and landing gear — FAA regulations often mandate positive locking on flight-critical joints
• Trailer hitches and heavy equipment pins where the joint sees shock loading

Installation rules of thumb:

• Cotter pin diameter is typically the largest that fits the cross-hole (usually 1/8" for M12–M16 bolts, 3/32" for smaller)
• The pin's legs should be spread after installation — one bent back over the bolt end, one down along the nut face, with roughly 90° of separation
• Never reuse a cotter pin. Once bent, the wire is work-hardened and prone to fatigue cracking at the bend
• Pin length should extend past the nut by at least 1× its diameter for enough material to bend

Quick sizing example: For a 3/4"-16 castle nut on a wheel spindle, you'd typically use a 5/32" × 1-1/2" stainless or low-carbon steel cotter pin. Torque spec might be 100 lb-ft, and if the cross-hole doesn't align, you rotate up to 30° further (or back off to nearest slot depending on the manufacturer's callout — always check).

A related variant is the slotted nut, which is a standard-height hex nut with slots cut directly across the top rather than sitting on a raised crown. Slotted nuts fit into tighter axial spaces but have less material at the slot roots, so they're rated for lower torque.