Cotter Pin Assort. 1/16" thru 1/4" 725 pcs in PB24Sold in sets of 1 A cotter pin is a wedge or pin that is inserted through aligned holes in the components to secure them temporarily. Cotter pins are commonly used in engineering and mechanical systems to secure different components. These pins are most commonly used to secure clevis pins, bolts, and other fasteners that might come loose because of movement or vibration.