Cotter (split) pins · Sizing to the hole and nut slots, material (steel, stainless, brass), use with castle nuts and clevis pins, correct bending of the prongs, and single use
A cotter pin, or split pin, is a folded wire fastener with a looped head and two legs: it passes through a hole in a bolt, shaft or clevis pin, such as through the slots of a castle nut, and its legs are bent apart to stop the nut or pin from backing out. Ask three things first: what hole diameter and grip length the pin must suit, which material and finish the environment needs, and how it will be fitted and replaced.
Split cotter pins in steel, stainless steel and brass for securing castle nuts, clevis pins, bolts and shafts, to metric and inch standards. Spring (roll) pins, R-clips, clevis pins themselves, safety wire and lock nuts are outside this page except where compared.
CNFX editorial research · Sources checked October 12, 2026 · 5 sources
The FAA's AC 43.13-1B (Acceptable Methods, Techniques and Practices: Aircraft Inspection and Repair) lists cotter pins among the three basic safetying methods, with safety wire and self-locking nuts, and says they are used on controls, landing gear and other points where a turning or actuating movement takes place. It says the cotter pin selected should be the largest diameter that fits the cotter pin hole and/or the slots in the nut, and that corrosion-resistant or non-magnetic pins (AN381 or MS24665) be used where those properties are needed. [1]
Haiyan lists cotter (split) pins in spring steel, brass or stainless steel, galvanised or plain, in metric and inch sizes, used with clevis pins, slotted nuts and shafts for locking. [2]
Editorial note: give the supplier the hole diameter and the length needed through the bolt or pin plus enough leg to bend; ask for the pin's nominal diameter against hole size in the standard you specify.
The US military standard MS24665 (revision F, June 1986), which superseded the AN380 and AN381 pins and names ANSI B18.8.1 as its procurement specification, tabulates each nominal size against a recommended hole. A 1/8 in (0.125) pin is held to 0.116–0.120 in across the legs and suits a 0.141 in hole; a 3/16 in pin, 0.172–0.176 in for a 0.203 in hole; a 1/4 in pin, 0.220–0.225 in for a 0.266 in hole; and a 1/16 in pin, 0.056–0.060 in for a 0.078 in hole. It lists hammer-lock and extended-prong square-cut styles, marks 5/64, 7/32, 7/16 and 3/4 in as not for new design, and covers carbon steel (cadmium plated or phosphate coated), brass (black chemical finish), corrosion-resistant steel (cleaned, descaled and passivated to ASTM A380) and nickel-copper alloy. EverySpec lists MS24665 as since superseded by NASM24665, so quote the current document. [3]
Editorial calculation: in that table the recommended hole runs 0.018–0.041 in larger than the pin's maximum leg width (0.141 − 0.120 = 0.021 in for 1/8 in; 0.266 − 0.225 = 0.041 in for 1/4 in), so a pin chosen by nominal size slides in without forcing; the hole in the bolt or clevis pin sets the pin size, not the reverse. [3]
Give the supplier the standard (metric or inch), hole diameter, required length, material (carbon steel, stainless, brass) and finish, the parts it secures (castle nut, clevis pin, shaft), environment (corrosion, magnetism), quantity and packaging.
02
Read the metric dimensions on an ISO 1234 pin
Metric split pins are ordered by nominal diameter and length; SFS lists its pins under ISO 1234 and under DIN 94, which it marks "standard withdrawn". China's national standard for the product, GB/T 91-2000 Split pins, adopts an international standard, has been in force since 2001-02-01, replaced GB/T 91-1986 and is classified under ICS 21.060.50. [4][5]
SFS's technical data for two A2 stainless pins shows the dimensions a buyer can check on delivery: the shank diameter d1 as a minimum–maximum pair, the length L, the leg offset a, the eye length b and the eye width c as a minimum–maximum pair. Both pins are material no. 1.4300 (X12CrNi18-8), which SFS describes as less than 0.12% carbon, 17–19% chromium and 7–9% nickel, with a plain (blank) surface. [4]
Editorial calculation: the measured shank of the 3.2 mm pin is 0.3–0.5 mm smaller than its nominal size, and that of the 5 mm pin 0.4–0.6 mm smaller. Measure d1 with calipers across both legs against the listed limits, and state the nominal size, length, material number and finish in the order. [4]
Install the prongs correctly and do not reuse pins
AC 43.13-1B says cotter pins are favoured because they can be removed and installed quickly, but that they should not be reused. When the prong is bent over the bolt it must sit firmly against the bolt shank and not exceed the bolt diameter; when bent over the nut it must lie down firmly against the nut without touching the washer, and the ends can be rolled and tucked to prevent injury. For castle nuts, it advises starting alignment with the cotter pin hole at the minimum recommended torque plus friction drag, and changing the washer or nut rather than exceeding the maximum torque if the hole and castellation do not line up. [1]
Editorial note: these are aviation maintenance practices; other industries' specifications may differ, but the sizing, bending and single-use principles give a sound baseline for ordering and inspection. [1]
04
Compare what suppliers offer
Suppliers linked to this page list cotter pins; each card links to the page reviewed.
The suppliers reviewed list cotter pins with clevis pins, spring pins and other fasteners; ask each for the standard and size range, material and hardness, finish, and packaging.
05
Trace the pin features
This is a functional reference built from the sources cited; it is not one maker's design.
Fill the specification fields with values for your joint
A cotter pin quotation needs values for your joint. The table explains each field, gives published values where a reviewed source states them, and says what to obtain from the supplier.
Field
What it means and published values
Ask the supplier for
Applicable Standard
Metric or inch series; e.g. ISO 1234 (DIN 94 is withdrawn) or GB/T 91-2000 in metric, AN380/AN381 and MS24665 in aviation. [1][2][4][5]
Standard.
Nominal Size / Range
Largest that fits; e.g. 1/8 in pin for 0.141 in hole, 1/4 in for 0.266 in. [1][3]
Diameter; length.
Material
Spring steel, brass, stainless; e.g. A2 stainless, material no. 1.4300. [2][4]
Explore company information and manufacturing capabilities, then discuss the scope of your project with a relevant supplier.
Share this page
Compiled and source-reviewed by the CNFX editorial team.
Published product pages show what a company offers, not tested performance. Confirm the legal seller, factory and scope for your order. How we source and review
· Page last updated 13 October 2026
Buyer questions
What size cotter pin should I use?
The FAA's AC 43.13-1B says the largest diameter that fits the cotter pin hole and/or the nut slots. [1]
Can cotter pins be reused?
AC 43.13-1B says cotter pins should not be reused. [1]
How should the legs be bent?
Over the bolt, seated firmly against the shank and not exceeding bolt diameter; over the nut, flat and clear of the washer (AC 43.13-1B). [1]
What hole size suits a 1/8 inch cotter pin?
MS24665 recommends a 0.141 in hole for a 1/8 in pin, whose legs measure 0.116–0.120 in across; for a 1/4 in pin it gives a 0.266 in hole. [3]
What diameter does a 3.2 mm ISO 1234 split pin measure?
SFS lists d1 of 2.7–2.9 mm for its 3.2 × 22 mm A2 stainless pin, with an eye length b of 6.4 mm. [4]
Sources and what still needs a direct check
Sources checked 2026-10-12. The FAA advisory circular is US government maintenance guidance; the SAMR platform lists China's national standard record; Haiyan and SFS describe their own products.
