Cotter Pin

Cotter pin sizes are in nominal fractional inches, starting at 1/32. The sizes below 5/16 inch are intended to fit a hole 1/64 inch larger than the pin size; for pins larger than that the pin and hole size are the same.

Lengths are not standardized. The way the length of a cotter pin is measured depends on the style of the points. The measurement begins at the point where the large end meets the hole into which the pin is inserted. It ends

• for pins with miter or bevel point ends, at the end of the farthest point of both prongs
• for hammerlock ends, at the farthest point of the longer prong
• for all other styles, at the end of the shorter prong

Cotter pins are ordinarily secured by spreading the prongs. Hammerlock pins are secured by striking the head with a hammer.  In some applications, the spread prongs can be a problem because they can catch on such things as pant legs. If so they can be covered with epoxy. 

• Don't reuse cotter pins.
• Always use the largest cotter pin that will fit the hole.
• In general, when a fastener must be safetied, properly installed safety wire is preferable to a cotter pin. 
• Because cotter pins are so readily available, workers are often tempted to replace some special pin that has failed with a cotter pin. This error has led to deaths.