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Cotter Pin Size Chart and Hole Sizes

Guide · Updated October 28, 2026

A cotter (split) pin is a half-round wire folded into a loop head with two prongs. It passes through a cross hole in a bolt, clevis pin or shaft and the prongs are bent over to keep a castle nut or clevis from backing off. Inch cotter pins are standardized in ASME B18.8.1, which covers sizes 1/32 through 3/4 in. The chart below gives the common sizes with the basic pin diameter, the recommended hole, the minimum head diameter and the extra length of the extended prong, taken from the ASME B18.8.1-2014 table as reproduced by fastener suppliers H&B and NPI.

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Step by step

  1. Find the cross hole size: Measure the drilled hole in the bolt or pin with a pin gage or drill blank.
  2. Pick the nominal size: Use the largest cotter whose recommended hole is no larger than your hole; a snug pin carries shear better and rattles less.
  3. Choose the length: Measure through the part and add enough for the prongs to bend: the long prong should reach at least half the bolt diameter past the hole, and many shops simply take the part diameter plus about one pin size.
  4. Insert head first into the slot: On a castle nut, line up a slot with the hole by tightening, never by backing off below the torque spec.
  5. Bend the prongs: Bend the long prong up over the bolt end and the short prong down along the nut flat, or spread both sideways on a clevis.
  6. Do not reuse: Bending fatigues the wire. Fit a new pin each time the joint is taken apart.

ASME B18.8.1 cotter pin dimensions (inches)

Total shank diameter (both legs together) is slightly under the basic size so the pin enters the hole with finger pressure. The standard measures length from the head bearing surface to the end of the shorter prong. Length tolerance is ±0.03 in up to 1 in long and ±0.06 in for 1 in and longer. Sizes 5/64, 7/32, 7/16 and 3/4 are listed as not preferred for new design.

Matching cotter pins to castle nuts

Castle nut slot widths and drilled bolt hole sizes come from the nut and bolt maker, so size the cotter to the actual hole using the chart above rather than to the bolt diameter.

Worked example: a 1/2-13 bolt with a 0.109 in cross hole takes a 3/32 cotter (recommended hole 0.109). Through a 1/2 in bolt, a 1 in long pin leaves about 0.5 in of prong to bend, which wraps the nut comfortably.

Point styles and materials

Drilling the cross hole

Center the hole on the bolt or clevis pin with a V-block on the drill press and spot first; a hole off center weakens the pin and makes the cotter bind. Use the recommended hole from the chart as a drill size where a stock size exists, for example 0.141 in is close to a 9/64 drill and 0.172 in is a 11/64 drill.

Break both edges of the hole lightly so the head seats and the prongs are not nicked.

FAQ

What size hole for a 1/8 cotter pin?

ASME B18.8.1 gives a 0.141 in recommended hole (about 9/64 in) for a 1/8 in cotter pin.

How is cotter pin length measured?

From under the head to the end of the shorter prong.

Can cotter pins be reused?

It is not recommended; the bent prongs fatigue. Fit a new pin.

Is there a metric equivalent?

Metric split pins are covered by ISO 1234, sized by the hole diameter in mm.

Pin and retaining-ring size charts

Picking a fastener to locate or retain a part? Compare the other pin and ring charts in this set:

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General shop information only — follow your drawing, tool maker's data and your quality system.