Split pins are one of the few fasteners in the Institute's register that already have an international standard: ISO 1234, which fixes nominal sizes from 0.6 mm to 20 mm. The Brennholt Institute of Applied Taxonomy therefore does not propose a size scale for them, and records only the points on which the standard is silent and the trade is not.
This page is the Institute's exception to its usual method. Elsewhere, such as on the paperclip numbering page, the Institute supplies a classification because none exists. Here one exists, and the Institute defers to it.
A split pin, called a cotter pin in American usage, is a length of half-round wire folded double to form an eye at one end and two legs at the other. It is pushed through a drilled hole in a bolt, shaft or clevis, and the legs are bent apart to lock it. The Institute treats the half-round section as the defining feature, since two half-round legs together make a full round that fills the hole.
The standard names a split pin by the diameter of the hole it is meant to fit, not by the measured width of the pin itself. A pin of nominal size 4 mm is therefore slightly thinner than 4 mm, so that it enters a 4 mm hole. The Institute records this because it is the commonest source of confusion when a buyer measures a pin with callipers and orders by the result.
| Nominal size (mm) | Nominal size (in, approx.) | Typical use, per the Institute's survey |
|---|---|---|
| 1 | 0.039 | Model work, small clevis |
| 2 | 0.079 | Light linkages, bicycle parts |
| 3.2 | 0.126 | Castle nuts on small studs |
| 4 | 0.157 | Domestic machinery, trailers |
| 6.3 | 0.248 | Agricultural linkage |
| 10 | 0.394 | Heavy shafts |
The table is a selection, not the full list in the standard. The Institute reproduces only rows it has been able to check against a public source, and invites the reader to consult the standard itself for the complete range.
A complete specification gives nominal size and length, for instance 4 × 40 mm. Length is measured from the underside of the eye to the end of the shorter leg. The standard allows legs of unequal length, and the Institute notes that the shorter leg is the one that counts. A pin sold as 40 mm may have one leg a few millimetres longer, which makes it easier to open with a screwdriver.
Split pins are made in carbon steel, stainless steel and brass. Carbon steel is usually zinc-plated; the Institute's page on galvanising and plating covers why the plating fails first at the bend. Stainless pins resist corrosion but are more springy, and the Institute records that they are harder to bend over cleanly. Brass is used where sparks or magnetism must be avoided, and is the weakest of the three.
For nominal sizes, tolerances and the dimensions of the eye, the Institute adds nothing to ISO 1234 and does not offer its own table as a substitute. The one area where the Institute does propose a classification is reuse. A split pin is a single-use fastener, because the bending that locks it work-hardens the legs. The Institute's informal three-state scheme for used pins, which is its own and not a trade term, is below.
A split pin is sometimes confused with the safety pin, which is a sprung closure of an entirely different kind, and with the paper fastener of the same general shape, the split pin of the stationer's cupboard, which is better described as a paper fastener. The Institute notes that the name overlap causes real mis-orders and recommends always writing "ISO 1234" on a purchase order for the engineering item.
The review panel divides on whether the Institute should publish a size table at all for a standardised object. Two members hold that a table adds a second, unofficial authority and invites error. The majority hold that a short, sourced table helps a reader who will not open the standard. The majority view prevails here, and the table above is limited accordingly.