Cotter pins are two-pronged fasteners that have been bent to hold a machine part in place. They are inserted into corresponding holes of machined parts to fasten the two parts together. There are several types of cotter pins, each coming in a variety of sizes. The diameter of the pins must fit through each hole while providing a firm grip. The cotter pin with the largest diameter that will still fit through the holes should be used to provide the best grip. Cotter pins must be long enough to reach through the hole of each part being fastened together.
Measure the diameter of the size of the hole where the cotter pin will fit.
Measure the diameter of the two prongs of the cotter pin pushed closely together.
Select a cotter pin to fit the diameter of the hole. Cotter pins smaller than 5/16 of an inch in diameter should fit in a hole 1/16 larger in diameter than the pin. Pins larger than 5/16 of an inch in diameter should fit into a hole of equal diameter size.
Measure the length of the hole to be fastened.
Measure the length of the cotter pin from the base of the rounded end of the pin to the end of the shorter prong.
Select a pin that is the proper diameter and that fits the length of the hole being fastened. The cotter pin's rounded end should set at one end of the hole and extend until the short end emerges on the other end of the hole.