cotter pins

Split pins

picture of a cotter pin

American inch sizes

American standard 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

inch sizes
in inches)
Suitable for
1⁄32 3/64  
3/64 1/16  
1/16 5/64 1/4″; 5/16″
5/64 3/32  
3/32 7/64 3/8″; 7/16″; 1/2″
7/64 1/8 9/16″
1/8 9/64 9/16″; 5/8″; 1″
9/64 5/32 1″
5/32 11/64  
3/16 13/64  
7/32 15/64  
1/4 17/64  
5/16 5/16  
3/8 3/8  
7/16 7/16  
1/2 1/2  
5/8 5/8  
3/4 3/4  

Metric sizes

Standard lengths are 6, 8, 10, 12, 14, 16, 18, 20, 22, 25, 28, 32, 36, 40, 45, 50, 56, 63, 71, 80, 90, 100, 112, 125, and 140 millimeters. For any hole diameter, not all lengths between the "smallest" and "longest" in the table are commercially available. Which lengths are actually available vary by, for example, the material from which the pin is made.

hole diameter
outer diameter
of eye
max min
1 0.9 0.8 3 10 25
1.2 1 0.9 3 10 16
1.6 1.4 1.3 3.2 6 40
2 1.8 1.7 4 10 45
2.5 2.3 2.1 5 14 50
3.2 2.9 2.7 6.4 16 63
4 3.7 3.5 8 18 100
5 4.6 4.4 10 25 80
6.3 5.9 5.7 12.6 32 125
8 7.5 7.3 16 45 140
10 9.5 9.3 20 63 140
13 12.4 12.1 26 71 125

Usage of split pins

Cotter pins are ordinarily secured by spreading the prongs. The prongs of hammerlock pins are spread 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.


Examples of industrial accidents involving cotter pins.

ANSI B18.8.1-1972(R1994)

The American standard for split pins. In military standards, MS24665 has replaced AN380 and AN381.

ISO 1234.

Crank cotter pins

Photo of crank cotter pin

Crank cotter pins are short shafts, threaded on one end, with a flat tapered so that the low end of the flat is at the threaded end of the pin. They are used to secure a piece with a bored hole to a flatted shaft having the same diameter, for example, formerly, to hold the cranks of bicycles to their axle. That particular use will be the basis of this description.

The crank has two holes, one for the axle and another at right angles to it for the cotter pin, with a slight, specified overlap. The axle is inserted into the hole so that the axle's flat is parallel to and facing the overlap. The cotter pin is inserted so that its flat faces the axle's flat. Installing the pin wedges the shaft against the sides of the hole and prevents it from turning.

Special tools were once available to insert and remove these cotter pins. In the absence of such a tool, the cotter pin is installed by tapping it with a hammer. The nut is tightened simply to hold the pin in place.

Removing the pins can be much more difficult than installing them. Their function requires that they be made of a fairly soft, ductile steel. Jobst Brandt suggests the following technique: Check to see if the cranks are really 180 degrees apart. If they aren't, the cotter pins are “mushed.” With the cranks horizontal, left crank to the rear, stand on both pedals and lunge to force them back into the same plane. They should move a bit. Support the back face of the crank on an anvil, and drive the cotter out with a drift pin. If the threaded part protrudes more than a quarter inch, first hacksaw it off, otherwise it will buckle when you use the drift pin.

The cotter pins for bicycles are sized by the diameter of the shaft in millimeters, currently 8.0, 8.5, 9.0, and 9.5.  Similar pins for other applications have been sized in inches, by diameter and length.

A source for crank cotter pins for bicycles.

Cotter pins for furniture brasses

cross-section of drawer front

For hundreds of years, special brass split pins have been used to hold pendants and some other types of pulls on the fronts of drawers. Some furniture makers still use them, as they don't loosen as threaded fasteners often do.

The pendant has a small axle in its back. The pin is slipped over the axle, enclosing it in the pin's head, and the pin is inserted into a hole drilled through the front of the drawer. On the inside of the drawer two shallower holes are drilled, one above and one below the through hole. The prongs of the pin are bent back, each end inserted into a hole, and the assembly hammered flat.

These pins are supplied with the fittings by the makers of brasses.

Cotter pins in stuffed animals

In stuffed animals, cotter pins are used to make mobile joints, for example, to attach a teddy bear's arms to the body. The pins are used with a pair of fiber discs resembling fender washers, with very small holes in the center. The discs, penetrated by pins, are sewn into the body parts.

The spreading of the cotter pin's legs is done in an unusual way; the legs are not simply spread back and flattened against the disc. Instead, using a pair of needle-nose pliers or a special tool resembling a screwdriver with a hollow cylindrical blade, the tip of the leg is grasped and turned, curling each leg into a spiral. The result resembles a back-to-back pair of ram's horns. Because the tip of the leg is inside the spiral, it cannot be exposed over time by the motion of the joint, and cannot poke through the cloth (or into a child).

Often the pins are T-headed. A more mobile (wobbly) joint can be made with two pins, inserting one round-head cotter pin through the head of a second, and attaching a disc to each pin.

Metal pins in stuffed animals raise the usual concerns about small parts in the hands of very young children.

A vendor's display at illustrates the range of disc and pin sizes available to stuffed animal makers. A wealth of information on this use of cotter pins can be found on Etsy and Pinterest.

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