A split pin, also known in the United States as a cotter pin or cotter key,[1] is a metal fastener with two tines that are bent during installation, similar to a staple or rivet. Typically made of thick wire with a half-circular cross section, split pins come in multiple sizes and types.
The British definition of "cotter pin" is equivalent to U.S. term "cotter". To avoid confusion, the term split cotter is sometimes used for a split pin. A further use of the term "cotter pin" is the "crank cotter pin" used to lock bicycle pedal cranks to the bottom bracket axle. These are not "split" at all and are wedge shaped.
| Nominal Diameter d ① | 0.6 | 0.8 | 1 | 1.2 | 1.6 | 2 | 2.5 | 3.2 | |||||
| d | max | 0.5 | 0.7 | 0.9 | 1 | 1.4 | 1.8 | 2.3 | 2.9 | ||||
| min | 0.4 | 0.6 | 0.8 | 0.9 | 1.3 | 1.7 | 2.1 | 2.7 | |||||
| α | max | 1.6 | 1.6 | 1.6 | 2.5 | 2.5 | 2.5 | 2.5 | 3.2 | ||||
| min | 0.8 | 0.8 | 0.8 | 1.25 | 1.25 | 1.25 | 1.25 | 1.6 | |||||
| b | ≈ | 2 | 2.4 | 3 | 3 | 3.2 | 4 | 5 | 6.4 | ||||
| c | max | 1 | 1.4 | 1.8 | 2 | 2.8 | 3.6 | 4.6 | 5.8 | ||||
| min | 0.9 | 1.2 | 1.6 | 1.7 | 2.4 | 3.2 | 4 | 5.1 | |||||
| Nominal Diameter d ① | 4 | 5 | 6.3 | 8 | 10 | 13 | 16 | 20 | |||||
| d | max | 3.7 | 4.6 | 5.9 | 7.5 | 9.5 | 12.4 | 15.4 | 19.3 | ||||
| min | 3.5 | 4.4 | 5.7 | 7.3 | 9.3 | 12.1 | 15.1 | 19 | |||||
| α | max | 4 | 4 | 4 | 4 | 6.3 | 6.3 | 6.3 | 6.3 | ||||
| min | 2 | 2 | 2 | 2 | 3.15 | 3.15 | 3.15 | 3.15 | |||||
| b | ≈ | 8 | 10 | 12.6 | 16 | 20 | 26 | 32 | 40 | ||||
| c | max | 7.4 | 9.2 | 11.8 | 15 | 19 | 24.8 | 30.8 | 38.5 | ||||
| min | 6.5 | 8 | 10.3 | 13.1 | 16.6 | 21.7 | 27 | 33.8 | |||||