Castle Nuts
What Are Castle Nuts?
Castle nuts—also called slotted hex nuts—are hex nuts with castellated slots cut into the top. After tightening the nut on a drilled bolt or clevis pin, a cotter pin is inserted through the bolt’s hole and one of the nut’s slots to prevent rotation. This mechanical pinning creates a secure, vibration-resistant lock for safety-critical assemblies.
Benefits of Castle Nuts
- Positive mechanical lock: Cotter pin physically stops the nut from backing off in vibration.
- Serviceable: Remove the pin to disassemble; no thread adhesives required.
- Wide temperature range: All-metal design performs where nylon insert nuts cannot.
- Repeatable alignment: Slots allow easy alignment to the bolt’s cross-drilled hole.
Castle Nuts vs. Standard Hex or Jam Nuts
Standard hex or jam nuts rely on friction to stay tight. Castle nuts add a cotter-pin lock that prevents rotation even if friction is reduced by shock, vibration, or lubrication. Choose castle nuts when a fail-safe lock is required by design codes or safety practice (e.g., linkages, tie-rod ends, clevis joints).
Materials & Finishes for Castle Nuts
- 18-8 (304) Stainless Steel: General corrosion resistance for outdoor use.
- 316 Stainless Steel: Marine-grade resistance for saltwater and harsh environments.
- Zinc-Plated Carbon Steel: Economical choice for dry, indoor applications.
- Brass: Non-magnetic, decorative, and corrosion-resistant in mild environments.
Common Applications for Castle Nuts
- Automotive & heavy-equipment linkages and steering/tie-rod ends
- Marine hardware and rigging connections
- Aerospace and industrial assemblies requiring a pinned lock
- Construction machinery, clevis and pivot joints
Installation Tips for Castle Nuts
- Tighten the nut to the specified preload using a wrench or socket.
- Rotate the nut slightly to align one slot with the cross-drilled hole in the bolt or stud.
- Insert the cotter pin fully through the hole and the slot; bend the pin’s legs to secure.
- Do not over-tighten after alignment—overtorque can damage threads or affect joint preload.
How to Select the Right Castle Nut
- Match thread size & pitch: Ensure the nut’s threads match the bolt/stud (e.g., 1/4-20, 3/8-16, M10-1.5).
- Material for environment: Zinc-plated for dry indoor, 18-8 for outdoor, 316 for marine, brass for non-magnetic or decorative needs.
- Bolt preparation: Use a cross-drilled fastener compatible with a cotter pin of the required diameter.
Frequently Asked Questions about Castle Nuts (FAQ)
For safety-critical, vibration-prone joints where a mechanical lock is required, steering linkages, clevis joints, tie-rod ends, and pivots.
Yes. “Castle nut” and “slotted hex nut” refer to the same castellated design used with a cotter pin.
Zinc-plated for dry indoor, 18-8 stainless for outdoor, 316 stainless for marine or salt spray, and brass for non-magnetic or decorative applications.
Tighten to the specified torque/preload for the joint, then advance or back off slightly only enough to align a slot with the bolt’s hole.
Match the pin diameter to the bolt’s cross-drilled hole and length to allow both legs to bend securely around the nut.
The nut can often be reused if threads are undamaged; always replace cotter pins with new ones after removal.
No. A cross-drilled bolt or stud is required so the cotter pin can pass through the bolt and a slot in the nut.
Without the pin, it behaves like a standard hex nut and can loosen under vibration. Always install a new cotter pin.