Stainless Steel Cotter Pin

Choosing the Right Cotter Pin (Split Pin)
1) What is a cotter pin & when to use it?

A cotter pin (also called a split pin) is a U-shaped, folded wire pin with two tines (legs) that are bent apart after installation to prevent a nut, pin, or component from coming loose.

Use a cotter pin when you need:

A cheap, simple mechanical lock to back up a nut or clevis pin

A fail-safe to stop parts falling off even if the nut loosens

A visual, easy-to-inspect locking method

Typical applications:

Securing castellated nuts on shafts, axles, steering components

Locking clevis pins, hinge pins, linkages, and shackles

Agricultural machinery, automotive, cranes, lifting tackle, general plant maintenance

Cotter pins are locking/retaining devices, not primary load-carrying fasteners.

2) Type & form – classic split pin vs others

On your site, “cotter pin” will usually mean the classic split pin:

Straight body with a rounded head/eye at one end

Two parallel legs, factory folded together

After insertion, legs are spread apart to lock the assembly

Related but different products (usually separate items):

R-clips / R-pins / Hitch pins – quick-release spring clips

Hairpin cotter pins – spring-wire style, no leg bending

For this page, focus on standard split cotter pins for use with slotted nuts, clevis pins, and drilled bolts.

3) Diameter – matching the drilled hole

Cotter pins are specified by wire diameter × length (e.g. 3.2 × 40 mm).

Diameter selection:

Choose a cotter pin diameter that fits snugly but not tightly in the hole.

The pin should slide through easily before bending the legs, without too much clearance.

Guideline:

Hole Ø ≈ slightly larger than pin diameter

Example: Hole ~3.3–3.5 mm → use 3.2 mm cotter pin

Do not force an oversize pin (legs will deform before they seat)

Undersize pins can rattle, wear the hole, and offer weaker locking.

Always match pin size to hole diameter in bolt/clevis or as per drawing.

Mechanical Properties (Guide) – Cotter Pins

Cotter pins are safety / locking devices.
They are not designed to carry high primary loads; instead, they prevent nuts or pins from loosening or coming out.

1) General behaviour

Key functional requirements:

Ductility:

Must bend without cracking.

Must withstand minor vibration and handling without snapping.

Toughness:

Should not shear off easily under moderate impact or accidental pulls.

Corrosion resistance:

Must last the service life of the joint in that environment so that the lock remains effective.

Cotter pins do not use bolt property classes like 8.8; they are typically made from low or medium-strength ductile wire (mild steel, stainless, brass) processed for good formability.