What Is A Cotter Pin? -Types, Sizes, Materials & How to Install
Cotter pins — also called split pins or cotter keys — are economical one-time fasteners made from bent wire. They secure clevis pins, shafts and hitch pins by inserting the folded end through a drilled hole and splaying the legs outward to lock assemblies in place. This short guide explains common cotter pin sizes, materials like stainless steel, typical applications (hitch pins, axles), and simple installation tips.
Table of Contents
How Cotter Pins Work
Basic operation is simple: insert the folded end through the mating hole, bend the longer prong back and then fold the shorter prong to trap the pin. Variants such as bow-tie cotter pins and hammerlock pins add snap-in or self-locking features so bending is not required.
Common Materials & Finishes — Choose the Right Cotter Pin
- Stainless steel cotter pins — excellent for outdoor, marine or corrosive environments.
- Carbon steel with zinc plating — economical for general-purpose use.
- Spring-tempered wire — for high vibration or repeated use applications.
- Brass, Monel, aluminum — used where non-magnetic or specific chemical resistance is required.
Popular Sizes & When to Use Them
Cotter pins are sold in standard diameters and lengths. Common workshop sizes include 4mm cotter pins and 5mm cotter pins. Choose size by matching the pin diameter to the drilled hole in the mating shaft or clevis pin — the fit must allow insertion while leaving enough leg to bend securely.
Typical Applications (Good Fit for Cotter Pins)
- Securing hitch pins on trailers and agricultural equipment (cotter pin for hitch pin).
- Retaining nuts on mechanical linkages and clevis assemblies.
- Axle retention in small vehicles, carts and agricultural machinery.
- Temporary or serviceable fastenings where quick removal is expected.
Quick Installation Tips — How to Install a Cotter Pin
- Insert the folded end through the hole so legs protrude evenly.
- Use pliers to bend the longer leg back flush against the shaft.
- Fold or clip the shorter leg over the longer one; trim excess if necessary.
- For hammerlock or bow-tie types, follow manufacturer insertion instructions (no bending required).
FAQ — Long-tail Keywords & Answers
Q: What size cotter pin do I need for a 10 mm shaft?
A: Match a cotter pin with a diameter close to the drilled hole. For a 10 mm shaft, common practice is to use a cotter pin sized to the clevis pin hole (e.g., 4 mm or 5 mm), depending on hole diameter — confirm with the manufacturer’s sizing chart.
Q: Are stainless steel cotter pins better?
A: Stainless steel offers superior corrosion resistance and is recommended for outdoor, marine or chemically aggressive environments. Use zinc-plated steel for indoor/general use to reduce cost.
Q: How do I remove a cotter pin safely?
A: Straighten or snip the spread legs using pliers or wire cutters and pull the pin out. For rusted pins, apply penetrating oil before removing.
Want product options? Check our pole line hardware or contact us for bulk cotter pin supply and datasheets.
