Cotter Pin

What Is a Cotter Pin and Why Does It Matter?

A cotter pin — also known as a split pin or cotter key — is one of the simplest yet most critical fasteners in mechanical engineering. Shaped like a half-round wire bent into a loop with two tines, a cotter pin is threaded through a drilled hole in a bolt, shaft, or clevis pin and then bent outward to lock a component firmly in place. Once installed, it acts as a positive mechanical stop that prevents nuts, pins, and other fasteners from backing out under vibration, load, or repeated movement.

These small but mighty fasteners appear in an enormous range of applications: securing castle nuts on steering tie rod ends and wheel hubs, locking brake clevis pins, retaining small-engine governor linkages, fastening marine rigging and clevis attachments, and keeping trailer hitch pins safely engaged. Because cotter pins are classified as single-use fasteners — they deform permanently during installation and must be replaced rather than reused — keeping a selection of common sizes on hand is a smart practice for any mechanic, boater, or outdoor-power-equipment owner.

Key Considerations When Shopping for Cotter Pins

• Size (Diameter × Length): Cotter pins are sized by wire diameter and overall length. Always match the pin diameter to the hole drilled in the fastener and choose a length that leaves enough tine to bend securely.
• Material: Low-carbon steel is standard for most automotive and general-purpose applications. Zinc-plated or galvanized steel adds corrosion resistance for outdoor and marine environments, while stainless steel is preferred for harsh saltwater exposure.
• Application: Automotive steering and suspension joints, small-engine throttle linkages, marine clevis pins, and trailer hardware each have their own sizing conventions — confirm the spec before ordering.
• Quantity: Because cotter pins are single-use, bulk packs or assortment kits offer the best value for shops and serious DIYers who encounter multiple sizes regularly.

Top Cotter Pin Brands

Dorman is the go-to source for automotive cotter pins in the aftermarket. With over a century of experience, Dorman offers a comprehensive line of cotter pins — also referred to as split pins or cotter keys — engineered to reliably hold drilled bolts, castle nuts, and clevis pins in place across a wide variety of vehicle applications. Their zinc-plated steel construction and broad size range make them a staple in professional repair shops and home garages alike.

Briggs & Stratton is the world's leading producer of gasoline engines for outdoor power equipment, and their cotter pin selection reflects decades of small-engine engineering expertise. Their pins are designed specifically for use in lawn mower, generator, and outdoor equipment linkages where precise fit and reliable retention under vibration are critical.

ATD Tools and Supplies brings a professional-grade approach to hardware and fastener assortments. Their cotter pin offerings are well suited for shop environments where technicians need reliable, consistently sized fasteners ready at hand for a variety of repair jobs.

Seachoice is a trusted name in marine hardware, and their cotter pins are built for life on the water. Corrosion-resistant materials and marine-grade construction make Seachoice pins an ideal choice for rigging, clevis connections, and deck hardware exposed to saltwater and UV conditions.

Auveco has long served the professional installer and collision repair market with precision-spec fasteners, and their cotter pins are no exception. Auveco's exacting dimensional standards make them a reliable choice when OEM-quality fitment is a priority.