Cotter Pins and Wire Clips Information

Cotter pins and wire clips are penetrating and coupling mechanical fasteners. They are easy to install and remove. Cotter pins come in several forms, with each designed for a specific kind of assembly. Some cotter pins are suitable for use as shear pins.

The pin or clip's shape determines its application and operation, though the general principle remains the same: pins and clips are easy to install and remove, and provide a fastening and alignment characteristic. All of the pins and clips covered in this tutorial are made of a deformable wire, be it plastic or elastic in nature. Generally, these clips need to be inserted into a predrilled hole, or applied around a shaft. They also account for some type of locking measure. Individual operation and application is covered more extensively below.

If the pin or clip has at least one straightened leg, a predetermined shear point, and it will not interfere with other parts of the assembly, the pin or clip could be suitable for use as shear pin, which will break at the point of mechanical overload to prevent damage to more expensive equipment.


Video credit: Lloyd Du



  • Plastic is inexpensive, corrosive resistant, and easy to mold, but only should be used for light loads.
  • Hardened steel is a stronger, but more brittle version of steel. It undergoes a heat treatment process.
  • Unhardened steel is more malleable than other steel types, so is also somewhat weaker.
  • Stainless steel is chemical and corrosion resistant, and has high stress ratings. It cannot be hardened.
  • Brass is strong, conductive, corrosion resistant, and has low magnetic permeability.

  • Monel® is a proprietary metal blend with good corrosion resistance, strength, but is expensive and difficult to form.


Size compatability with the substrates or workpieces to be clipped is a key specification. Since there are a wide variety of pins made into multiple asymmetrical shapes, the most important predetermined dimensions are wire/pin diameter, and length. In some instances the measurement of cavities or diameters created by the wire bend will be specified, as well as additional lengths that may be relevant.


These are some of the more common types of cotter and hitch pins. Some types are made for specific applications. A key ring is a type of circular cotter.

Split cotter pins are the most well recognized type of cotter pin. They are made of semi-circular wire that is bent over its flattened side to create a bulbous head. This pin is inserted into predrilled components to fasten them with its deformable tines, which take up any play in the assembly. They are also used as a brake in clevis pins. These pins are meant for one-time application and have multiple individual styles that vary according to the wire bend and terminus.

Cotter pin types

Image credit: Wizard191

Hairpins have an asymmetrical bulb and wave on one half of the pin. The other half of the pin is straight. This type of pin assembles components between the cavity created by the waved and straightened ends, with the assembly held tight by the pin's elastic deformation. These are used to quickly assemble or disassemble components.

Hairpin example

Image credit: Pivot Point

Twist pins share similar mechanical properties with hairpin clips. A twist pin's main variance is that the clip legs cross, creating a positive, self-locking mechanism.

Bowtie pins are similar to hairpin clips and twist pins, except the clip wire is formed into a completed circuit. A deformable wire break allows the clip to be applied to a shaft.

Bowtie Cotter Pin image

Image credit: Pivot Point

Hairpin clips, also called retaining pins, provide a positive grip on a grooved shaft by the means of elasticized wire.

Hairpin clip example

Image credit: Century Spring

Safety pins resemble their household namesake and operate similarly, but are industrial components for material fastening. Components are assembled on a straightened side of the clip, with the other side providing a clasp to contain the parts.

Circle cotter pins have deformable wire in the shape of a circle, with the pin's entry a wire terminus pointed inward. Application is much like threading a key onto a key ring. This is best for use in parts that can be damaged from an abrasive edge.

Circle cotter type

Image credit: Pivot Point

Knockout/kickout rings also have a deformable wire in the shape of a circle, and are applied like circle cotter pins. However, the pin's entry point is pointed upward.

Knockout or kickout ring

Image credit: Pivot Point

Ring cotters are a hybrid between a twist pin and a circle cotter. One side of the wire clip is kept straight, while the other side is helixed over and under the straightened side, before terminating with a final bend across the straight side. This helix and twist method provides an automatic, secure lock when components are threaded appropriately.

Ring cotter

Image credit: Pivot Point


Hitch pin w/ hairpinPins and clips are reliable forms of fasteners for light-duty applications where other hardware types may be unacceptable because they are permanent or will damage substrates. Pisn and clips have many industrial and commerical uses, as well as consumer.

Sometimes hairpins are substituted for linch pins, despite specific pins for this application. They are also used as the locking pin for hitch pins. Circle cotters are designed for use near fabrics and insulated electrical wires to prevent damage to these sensitive components. Nearly all of the above clips and rings are suitable for clevis devices, which are prevalent in the farming, sailing, and construction industries.


Purchase Partners - Assembly Parts

Grainger - Pins

Wilson Manufacturing Co. - Ring Cotter

Wikipedia - R-clip; Bowtie cotter pin; Circle cotter; Split pin

Windsor Linkage Parts - Products

Image credit:

Wizard191 / CC BY SA 3.0 | Pivot Point | Century Spring