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Cotter Split Pins

Cotter Split Pins

Cotter Split Pins

Cotter split pins (DIN 94) are a folded-wire safety pin: the eye sits on top of a cross-hole and the two legs are spread or folded back on the underside to lock against removal. Used wherever a positive mechanical retention is needed and a threaded solution isn't safe or practical - castle nuts on axles, towbar pin securing, automotive and agricultural safety locking.

Also known as: Split Pins, Cotter Pins, Hairpin Cotters (when bent into an R shape), Safety Pins, R-Pins.

Cotter pins are not threaded fasteners - they are folded soft-wire retention pins. The label diameter is the cross-section of the wire used to fold the pin. Pin length is measured from the underside of the eye to the tip of the legs. They work because the soft material folds without cracking - so material grade matters more than visible size when selecting for a tight installation.

GRADES AND MATERIALS

Zinc Silver Plated Steel - the standard DIN 94 cotter pin material. Soft mild steel folds reliably without cracking, and the zinc plating gives modest corrosion protection suitable for general workshop, automotive and indoor agricultural use. Tensile strength of the pin material is intentionally low - the pin's job is to hold, not to bear shear load.

Stainless A4-70 G316 - molybdenum-bearing marine-grade stainless for outdoor, coastal, marine and chemical environments. Note that A4-70 has less ductility than zinc-plated mild steel - the legs can crack if folded too tightly against a sharp corner. Spread the legs in a gentle arc rather than folding them flat against the work face. Largely non-magnetic in the annealed condition.

DRIVE AND INSTALLATION

Push the pin through the cross-hole until the eye seats on the hole face. With pliers or a small hook tool, spread or fold the legs back against the underside. The legs should sit clear of any rotating or rubbing surface and should not interfere with adjacent components. Don't over-fold - the leg only needs to be turned far enough that the pin can't reverse out of the hole.

Pin diameter selection: the host hole should be a clearance fit on the pin's nominal diameter (slip in by hand, no force). If the hole is too small the pin won't seat; if the hole is too big the pin will rattle. Cotter pins are single-use - plan on a fresh pin every time the joint is disassembled.

A NOTE ON GALLING

Galling is not a concern for cotter pins under normal use - the contact area in the host hole is small and the pin is not loaded in friction. The one exception is installation into a tight stainless host hole where the pin's leg wall scrapes the bore on insertion; lubricate lightly if the fit feels excessive.

LENGTH AND MEASUREMENTS

Cotter split pins are measured from the underside of the eye to the tip of the legs. Allow enough leg past the cross-hole to fold back without interfering with adjacent components. As a working rule, the leg length on the far side of the hole should be at least 1.5 times the host material thickness, with the legs fold-able with hand pliers.

TECHNICAL RESOURCES

Standard: DIN 94 - Cotter pins, also published as ISO 1234. Spec PDFs and dimensional drawings are linked on each individual product page.

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