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.