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Metric Split Pin Size Chart (ISO 1234)

Guide · Updated October 28, 2026

A split pin, called a cotter pin in the US, is a half-round wire folded back on itself with an eye at the head. ISO 1234 (formerly DIN 94) sets the metric sizes. As with roll pins, the nominal size is the diameter of the hole. The actual shank is a few tenths of a millimeter smaller, so the pin drops in freely. The standard also lists which bolt and clevis pin diameters each size suits, which is the quickest way to pick one. The figures below come from the ISO 1234:1997 table, checked against the fasteners.eu ISO 1234 table and the UNI 1336 / DIN 94 sheet from La Bulloneria.

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Step by step

Find the bolt or clevis diameter: Measure the bolt or clevis pin that the split pin will lock.
Read the nominal size: Look across the chart to the split pin nominal size listed for that bolt or clevis range.
Drill the cross hole: Drill the cross hole at the nominal size. ISO 1234 recommends H13 for 1.2 mm and below, and H14 above that.
Choose the length: Pick a length so that the shorter leg sticks out past the bolt or nut by at least the shank diameter, enough to bend.
Insert and bend: Push the pin through and spread the legs. Bend the long leg up over the bolt end and the short leg down along the nut flat, or spread both legs evenly around a clevis.
Never reuse: Fit a new pin every time. A bent and straightened pin is cold-worked and can crack.

ISO 1234 split pin dimensions (mm)

Columns are: nominal size (the hole), shank d (min–max), extended leg a max, eye length b approx., eye width c (min–max), and the bolt and clevis pin diameters each size is for.

Nominal (hole) | Shank d | a max | b ≈ | Eye c | Bolt dia | Clevis pin dia
1 | 0.8–0.9 | 1.6 | 3 | 1.6–1.8 | over 3.5 to 4.5 | over 3 to 4
1.2 | 0.9–1.0 | 2.5 | 3 | 1.7–2.0 | over 4.5 to 5.5 | over 4 to 5
1.6 | 1.3–1.4 | 2.5 | 3.2 | 2.4–2.8 | over 5.5 to 7 | over 5 to 6
2 | 1.7–1.8 | 2.5 | 4 | 3.2–3.6 | over 7 to 9 | over 6 to 8
2.5 | 2.1–2.3 | 2.5 | 5 | 4.0–4.6 | over 9 to 11 | over 8 to 9
3.2 | 2.7–2.9 | 3.2 | 6.4 | 5.1–5.8 | over 11 to 14 | over 9 to 12
4 | 3.5–3.7 | 4 | 8 | 6.5–7.4 | over 14 to 20 | over 12 to 17
5 | 4.4–4.6 | 4 | 10 | 8.0–9.2 | over 20 to 27 | over 17 to 23
6.3 | 5.7–5.9 | 4 | 12.6 | 10.3–11.8 | over 27 to 39 | over 23 to 29
8 | 7.3–7.5 | 4 | 16 | 13.1–15.0 | over 39 to 56 | over 29 to 44
10 | 9.3–9.5 | 6.3 | 20 | 16.6–19.0 | over 56 to 80 | over 44 to 69
13 | 12.1–12.4 | 6.3 | 26 | 21.7–24.8 | over 80 to 120 | over 69 to 110

ISO 1234 also lists 0.6 and 0.8 mm sizes for very small work, and 16 and 20 mm for bolts over 120 mm. For railway use, or where clevis pins see alternating side loads, the standard recommends going one size up from the chart.

Worked example: M20 castle nut

A 20 mm bolt with a castle nut falls in the "over 14 to 20 mm" bolt band, so the chart calls for a 4 mm split pin. The shank is 3.5 to 3.7 mm, so it slides easily through a 4 mm hole. For a 20 mm clevis pin, the clevis column (over 17 to 23 mm) points to a 5 mm split pin instead. Clevis pins get a bigger split pin than bolts of the same size, because the split pin carries the side load directly.

For length, add the nut width across the slot and at least one shank diameter of leg to bend. A 4 mm pin through a 20 mm castle nut slot usually works out to a 32 to 40 mm pin.

Metric vs inch cotter pins

Inch cotter pins under ASME B18.8.1 are sized by nominal pin diameter in fractions, such as 1/8 and 5/32, not by metric hole size. A 1/8 inch cotter (3.2 mm) is close to an ISO 3.2 split pin, but the shank and eye limits differ slightly. Use the chart that matches the drawing. See the inch cotter pin chart linked below.

Material and finish

ISO 1234 split pins come in plain or zinc-plated steel, austenitic stainless (A2, A4), and brass or copper. Use stainless or brass in wet locations or near food. Plated steel is the default choice for general machinery.

FAQ

Is the split pin size the hole or the pin?

It is the hole. A 4 mm split pin has a 3.5 to 3.7 mm shank and is made for a 4 mm hole.

What is DIN 94?

DIN 94 was the German split pin standard. EN ISO 1234 replaced it in 1998 with the same nominal sizes.

Which leg goes where?

The longer leg usually bends over the end of the bolt, and the shorter leg bends down along a flat of the nut.

Can I use a smaller split pin?

Avoid it. An undersized pin can shear or work loose. Follow the bolt and clevis columns, and go one size up if there is vibration or reversing load.

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:

Related guides and calculators

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