A split pin, also known in the United States as a cotter pin or cotter key, 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", which can be a cause for confusion when companies of both countries work together. There are signs that manufacturers and stockists are increasingly listing both names together to avoid confusion; this led to the term split cotter sometimes being used for a split pin.
A new split pin (see figure A) has its flat inner surfaces touching for most of its length so that it appears to be a split cylinder (figure D). Once inserted, the two ends of the pin are bent apart, locking it in place (figure B). When they are removed they are supposed to be discarded and replaced, because of fatigue from bending.
Split pins are typically made of soft metal, making them easy to install and remove, but also making it inadvisable to use them to resist strong shear forces. Common materials include mild steel, brass, bronze, stainless steel, and aluminium.
As shown above, there are different types of ends available on split pins. The most common is the extended prong with a square cut, but extended prongs are available with all of the other types of ends. The extended prong type is popular because it makes it easier to separate the tines. To ease insertion into a hole the longer tine may be slightly curved to overlap the tip of the shorter tine or it is beveled. The length, L, of the split pin is defined as the distance from the end of the shortest tine to the point of the eyelet that contacts the hole.
Hammer lock split pins are properly installed by striking the head with a hammer to secure the pin. This forces the shorter tine forward, spreading the pin.
The diameter of split pins are standardized. American split pins start at 1⁄32 in and end at 3⁄4 in.
|Nominal diameter [mm]||Hole size [mm]||For bolt size [mm]|
|6||6.3||30, 36, 42|
|Nominal diameter [in]||Hole size [in]||For bolt size [in]|
Split pins are frequently used to secure other fasteners, e.g. clevis pins, as well as being used in combination with hardboard discs as a traditional joining technique for teddy bears. A common application of this is when used to secure a castellated nut. 
Split pins may be used in some applications as low-tech shear pins.
7-127. SECURING WITH COTTER PINS. a. Cotter pins are used to secure such items as bolts, screws, pins, and shafts. Their use is favored because they can be removed and installed quickly. The diameter of the cotter pins selected for any application should be the largest size that will fit consistent with the diameter of the cotter pin hole and/or the slots in the nut. Cotter pins should not be reused on aircraft. b. To prevent injury during and after pin installation, the end of the cotter pin can be rolled and tucked. NOTE: In using the method of cotter pin safetying, as shown in figures 7-6 and 7-7, ensure the prong, bent over the bolt, is seated firmly against the bolt shank, and does not exceed bolt diameter. Also, when the prong is bent over the nut, ensure the bent prong is down and firmly flat against the nut and does not contact the surface of the washer. NOTE: Do not loosen or tighten properly tightened nuts to align safety-wire holes. When castellated nuts are to be secured with safety wire, tighten the nut to the low side of the selected torque range, unless otherwise specified; and, if necessary, continue tightening until a slot lines with the hole. These guidelines for installing safety wire are applicable to installing cotter pins in other applications as well: g. Safety wire inside a duct or tube must not cross over or obstruct a flow passage when an alternate routing can be used. (1) Check the units to be safety wired to make sure that they have been correctly torqued, and that the wiring holes are properly aligned to each other. When there are two or more units, it is desirable that the holes in the units be aligned to each other. Never overtorque or loosen to obtain proper alignment of the holes. It should be possible to align the wiring holes when the bolts are torqued within the specified limits. Washers may be used (see paragraph 7-37) to establish proper alignment. However, if it is impossible to obtain a proper alignment of the holes without undertorquing or overtorquing, try another bolt which will permit proper alignment within the specified torque limits. (2) To prevent mutilation of the twisted section of wire, when using pliers, grasp the wires at the ends. Safety wire must not be nicked, kinked, or mutilated. Never twist the wire ends off with pliers; and, when cutting off ends, leave at least four to six complete turns (1/2 to 5/8 inch long) after the loop. When removing safety wire, never twist the wire off with pliers. Cut the safety wire close to the hole, exercising caution. 7-38. LOCKING OR SAFETYING OF BOLTS. Lock or safety all bolts and/or nuts, except self-locking nuts. Do not reuse cotter pins or safety wire. 
- Jensen, Cecil Howard (2001), Interpreting Engineering Drawings (6th ed.), SteinerBooks, ISBN 978-0-7668-2897-1.
- Reithmaier, Lawrence W. (1999), Standard aircraft handbook for mechanics and technicians (6th ed.), McGraw-Hill Professional, ISBN 978-0-07-134836-2.
- Soled, Julius (1957), Fasteners handbooks, Reinhold Publication Corporation.
- Welsch, Roger (2005), From Tinkering to Torquing: A Beginner's Guide to Tractors and Tools, MBI Publishing Company, ISBN 978-0-7603-2082-2.
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