Cotter pins are simple retainers that help keep parts from sliding off or loosening during use. You’ll see them used across automotive, lawn and garden equipment, trailers, and general machinery.

• Hold nuts in place: Often used with a castellated nut to keep it from backing off.
• Secure clevis or hitch pins: Prevent pins from working their way out of a hole or bracket.
• Quick, tool-light fastening: Many styles install with basic pliers or by hand.

Choosing a style (split pin, hairpin/R-clip, or safety-pin style) depends on the application. Always follow your equipment manual for size and placement.

They do different jobs:

• Cotter pin: A retainer. It’s designed to keep a nut, clevis pin, or similar hardware from moving out of position. It relies on bent legs (split pin) or spring tension (R-clip) and isn’t intended to carry significant load.
• Shear pin: A sacrificial fastener. It’s designed to break at a specific force to protect equipment (common in machinery and some outdoor power tools).

Because their purposes differ, they’re generally not interchangeable. If a manual specifies a shear pin, using a cotter pin instead may not provide the intended protection. When in doubt, check your equipment documentation.

You’ll commonly find three broad types on this page:

• Split cotter pins (two-prong): Insert through a hole, then bend the legs to lock a nut or pin.
• Hairpin cotter/R-clips: Springy, R-shaped clips that slide into a cross-hole or groove, great for quick install and removal.
• Safety-pin style locking cotters: A clasped design (similar to a brooch safety pin) that snaps shut for a secure closure.

Each has pros and cons. Split pins are very secure but single-use; R-clips are fast to remove; safety-pin styles offer a positive latch. Match the style to your task and follow the equipment’s guidance.

They hold by either mechanical deformation or spring tension:

• Split pins: Insert the pin through the hole, then bend the two legs in opposite directions around the fastener. The bent legs help prevent backing out.
• R-clips/hairpins: The spring shape presses against the pin or groove to create friction and retention.
• Safety-pin styles: A latch closes to keep the pin captured.

Fit matters. Choosing the right diameter and length helps the pin seat properly. Light hand tools (like pliers) can assist with bending, and a quick tug test can help confirm it’s seated. Always follow the equipment manual.

Start with the hole and environment, then choose material and length:

• Diameter: Measure the hole or pin groove. For split pins, pick a diameter that fits the hole without forcing. For R-clips, select a size that matches the receiving pin’s diameter/groove.
• Length: Long enough to bend legs securely (split pins) or to fully engage (R-clips), but not so long it snags.
• Material/finish: Stainless steel can be a good option for damp or outdoor use, while zinc-plated steel may suit general indoor or light-duty applications.

If you’re unsure, an assortment kit can provide multiple sizes to test fit.