Technical Foundation
What Is a Cotter Pin?
A cotter pin (split pin) is a metal fastener formed from a single length of half-round wire folded to create two prongs, inserted through a drilled hole in a clevis pin, castle nut, or shaft, and then bent apart to prevent the pin or nut from backing out under vibration or rotation. Unlike threaded lock nuts, cotter pins provide purely mechanical, positive locking that can be visually verified at a glance — a bent prong that's straightened or missing is an immediate sign of a failed or removed lock.
Sizing follows nominal diameter (the wire gauge, e.g., 3/16 in or M5) and length (the total length before bending), both of which must match the hole diameter in the clevis pin or castle nut and the clearance available for bending the prongs. Hairpin cotters and R-clips are reusable spring-wire alternatives that snap into a groove without bending, trading ultimate holding force for faster field installation and removal.
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| Spec | What It Means | Why It Matters |
|---|---|---|
| Nominal Diameter | Matched to the cross-drilled hole size, e.g., 1/8 in, 3/16 in, M4, M6 | Undersized diameter allows play and eventual pin walkout; oversized won't fit the hole |
| Length | Measured pin length before bending | Must be long enough to fully bend the prongs around the shaft or nut without excess |
| Material | Low carbon steel (zinc plated), 300-series stainless | Stainless resists corrosion in wet/outdoor use; plated steel suits general indoor duty |
| Style | Standard split, extended prong, hairpin, R-clip | Hairpin/R-clip styles allow tool-free, reusable installation vs. bend-and-cut split pins |
| Standard | ANSI/ASME B18.8.1, DIN 94, ISO 1234 | Ensures dimensional interchangeability with OEM hole sizing and mating hardware |
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Applications
Where Cotter Pins Get Used
Cotter pins are specified anywhere a clevis pin, castle nut or shaft needs simple, visually verifiable mechanical locking against vibration or rotation.
Agricultural Equipment
Hairpin and R-clip cotters secure hitch pins, PTO shaft guards and implement pins that require frequent field removal.
Automotive & Trailer Hardware
Split cotter pins lock castle nuts on wheel bearings, tie rod ends and steering linkage against loosening.
Industrial Machinery & Linkages
Cotter pins retain clevis pins in mechanical linkages, pivot joints and adjustable machine mounts.
Material Handling Equipment
Cotter pins secure forklift and lift attachment pins where positive, visually verifiable locking is required.
Marine Hardware
Stainless steel cotter pins resist corrosion while securing turnbuckles, shackles and rigging pins on deck hardware.
General Fabrication & MRO
Assortment kits of common cotter pin sizes are stocked for routine maintenance and field repair across equipment types.
FAQ
Cotter Pins Questions, Answered
How do I size a cotter pin correctly?
Match the cotter pin's nominal diameter to the cross-drilled hole in the clevis pin, bolt, or castle nut — undersized pins allow play and can eventually walk out, while oversized pins won't fit. Length should be enough to pass fully through the hole and leave enough prong length on both sides to bend around the shaft or nut securely.
What's the difference between a split cotter pin and a hairpin cotter clip?
A split cotter pin is a solid wire folded into two prongs that must be bent apart after insertion and generally cut or straightened for removal, making it a semi-permanent, one-time-use lock. A hairpin cotter or R-clip is spring-wire that snaps into a machined groove on the pin without bending, allowing quick, tool-free, repeatable installation and removal — common where equipment is frequently disassembled in the field.
Should I use stainless steel or plated steel cotter pins?
Stainless steel (typically 300-series) resists corrosion in wet, outdoor, marine, or washdown environments and won't develop rust that could seize the pin in place. Zinc-plated carbon steel costs less and is adequate for general indoor or dry-environment use where corrosion exposure is limited.
Can I reuse a cotter pin after removing it?
Standard split cotter pins are not recommended for reuse — the bending and straightening process work-hardens and weakens the wire, increasing the risk of breakage on reinstallation. Hairpin cotters and R-clips are specifically designed for repeated installation and removal without this fatigue concern.
What standard should I reference when ordering cotter pins for a specific hole size?
ANSI/ASME B18.8.1 covers inch-sized cotter pins in the US, while DIN 94 and ISO 1234 cover metric sizes internationally — each standard specifies the nominal diameter, hole clearance, and length tolerances. Referencing the applicable standard ensures the pin will fit OEM-specified hole sizes without custom fitting.
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