What are Cotter Pins?
Cotter pins, also called split pins, are locking pins used to keep a fastener assembly from loosening. After insertion through a drilled hole in a bolt, clevis, or shaft, the two legs are bent to form a mechanical stop. Typical uses include castle nuts, clevis pins, linkages, and rotating equipment where a secondary retainer is required.
How do Cotter Pins work?
Insert the pin until the head seats, then bend the legs in opposite directions so they bear against the nut or shaft. The bent legs prevent the pin from backing out under vibration and keep the nut or clevis from moving past its slot or hole.
What size cotter pin do I need?
- Diameter: Match the hole. Use a pin that slides in snugly without forcing for best shear strength.
- Length: Long enough to pass fully through and leave leg length to bend over the nut or along the shaft.
- Quick rule of thumb: choose pin diameter ≈ hole diameter, select the next longer length if you will be spreading the legs widely.
Cotter Pin Size Guide (rule-of-thumb)
| Hole diameter | Common pin diameters | Typical assemblies |
|---|---|---|
| 1/16 in | 1/16 in | Small linkages, light hardware |
| 3/32 in | 3/32 in | Pedal linkages, small clevis pins |
| 1/8 in | 1/8 in | General machinery, 1/4–3/8 in bolts with castle nuts |
| 5/32 in | 5/32 in | Steering or brake hardware with larger slots |
| 3/16 in | 3/16 in | Heavy linkages, larger clevis pins |
Guide values are general. Always verify fit on your specific nut, slot, or clevis.
Stainless vs brass vs zinc-plated, which should I use?
- Stainless Steel: best corrosion resistance for marine, outdoor, or washdown areas.
- Brass: non-magnetic and conductive, good where mild corrosion resistance and conductivity matter.
- Zinc-Plated Steel: strong carbon-steel core with zinc coating for general indoor and light outdoor service.
How to install a cotter pin
- Align the nut slot or clevis hole with the drilled hole.
- Insert the cotter pin from the accessible side until the head seats.
- Bend both legs, one each direction, to lie flush. Trim any excess to avoid snagging.
How to remove a cotter pin
- Straighten both legs with side cutters or needle-nose pliers.
- Pull the pin by the eye with pliers. If stuck, work it back and forth while pulling.
- Discard the used pin. Install a new one after service.
Are cotter pins one-time use?
Yes. Once bent, the legs work-harden. Reusing a pin risks cracking and loss of retention. Always replace with a new pin after removal.
Cotter pin vs split pin vs R-clip
- Cotter pin / split pin: same fastener in US vs international naming, two legs you bend for retention.
- R-clip / hitch pin clip: reusable spring clip that snaps into a cross-hole, faster to remove but not used with slotted nuts.
Typical applications & industries
- Castle nuts on axles and steering components
- Clevis pins on cylinders, pedals, and actuators
- Agricultural, truck, and trailer hardware subject to vibration
- General MRO, light machinery, fixtures, and jigs
Troubleshooting & pro tips
- Pin will not insert: verify hole alignment, deburr the hole, or use the next smaller diameter.
- Legs snap while bending: choose stainless or a larger diameter for better ductility and strength.
- Snag risk: clip excess and tuck a leg into the nut slots or along the shaft.
Why buy Cotter Pins from AlbanyCountyFasteners.com
- Broad size range in stainless, brass, and zinc-plated steel
- Fast shipping and bulk pricing
- Knowledgeable support for sizing, standards, and selection
FAQs
Quick answers to common cotter pin questions so you can pick size, material, and install with confidence.
What size cotter pin should I use with a castle nut?
Match the drilled hole in the bolt or stud. Choose a pin that slides in snugly and is long enough to bend both legs over the nut.
Can I reuse a cotter pin after inspection?
No, replace it. Reusing a bent pin increases the chance of cracking and loss of retention.
Do you stock marine-grade cotter pins?
Yes, stainless steel cotter pins are available and are the best choice for wet or corrosive environments.
How do I prevent sharp ends from catching?
Trim excess after bending and tuck one leg into a slot or along the shaft to keep edges flush.
What if my hole is slightly oversized?
Use the next larger pin size that still inserts smoothly, or replace worn hardware so the pin fits correctly.