How to Install a Castle Nut with a Cotter Pin
A castle nut (or slotted nut) locks with a cotter pin that passes through a slot in the nut and a cross-drilled hole in the bolt or shaft — a positive lock that cannot vibrate loose. This guide walks through the install, including the wheel-bearing case where the nut is intentionally backed off rather than torqued tight.
What You Need
- The castle or slotted nut, sized to the bolt thread
- A bolt or shaft cross-drilled for a cotter pin
- A new cotter pin (single-use) sized to the hole
- A torque wrench, and needle-nose pliers or side cutters
Installation Steps
- Torque to spec. Tighten the nut to the joint's specified torque. (For a free-rotating joint like a wheel bearing, follow the seat-then-back-off procedure in the next section instead of a final torque.)
- Align a slot with the cotter-pin hole. If a slot does not line up with the cross-drilled hole, advance the nut to the nearest slot. Do not over-torque past spec just to reach a slot; if you cannot get there, a thin washer under the nut, or a different nut, can change the clocking.
- Insert the cotter pin through the slot and the hole until the head seats against the nut.
- Bend the legs to lock the pin: bend one leg back over the bolt end (or up the side of the nut) and the other down, then trim excess so it cannot snag.
- Verify the pin is fully seated and the legs are bent. Always use a fresh pin — cotter pins are single-use.
Wheel-Bearing & Free-Rotating Joints
On tapered wheel bearings and similar joints, the castle nut sets bearing preload, not clamp force. The typical procedure: seat the bearing by torquing the nut while rotating the hub, then back the nut off to the specified position (often to the next slot, or to a small running clearance), and install the cotter pin. Always follow the equipment manufacturer's exact torque and back-off procedure — over-tightening a wheel bearing nut damages the bearing.
Sizing the Cotter Pin
Match the cotter pin diameter to the bolt's cross-drilled hole and the nut slot width, and the length so it fully passes through with enough leg to bend over. Too small a pin can shear; too large will not seat. If you are unsure, send us the bolt size and we will match the nut and pin.
Common Mistakes
- Over-torquing to reach a slot — never exceed spec; adjust with a washer or different nut instead.
- Reusing a cotter pin — a bent-and-straightened pin is weakened; always use a new one.
- Wrong nut profile — confirm castle (raised crown) vs slotted (flat hex) to match the assembly; see our comparison.
- Skipping the manufacturer procedure on bearings — preload matters.
Frequently Asked Questions
What if the slot does not line up with the hole?
Advance the nut to the nearest slot — do not over-torque past spec to reach one. If you still cannot align it, add a thin washer under the nut to change the clocking, or use a different nut. On bearings, follow the specified back-off procedure, which lands the slot for you.
Do I torque a castle nut tight?
For most structural and machinery joints, yes, to the specified torque. For free-rotating joints like wheel bearings, no — you seat the bearing and then back off to the specified preload or clearance before pinning. Follow the equipment manufacturer's procedure.
Can I reuse a cotter pin?
No. Cotter pins are single-use. Bending and straightening work-hardens and weakens the legs, so install a new pin every time.
What size cotter pin do I need?
Match the pin diameter to the bolt's cross-drilled hole and nut slot, and the length so it passes through with enough leg to bend over. Send us the bolt size and we will match the nut and pin.
Castle Nuts & Cotter Pins
Eugene Fastener stocks slotted and castle nuts, cotter pins, and drilled bolts in steel, stainless, and silicon bronze.