Buy Cotter Pins Online in Australia
Cotter Pin / Split Pin Sizes — ISO 1234 Quick Reference
Cotter pins (split pins) are specified by nominal diameter (matches the drilled hole) and length. Manufactured to AS/NZS 1444 / ISO 1234 — the diameter matches the hole; the actual wire diameter is ~0.05 mm undersize for insertion clearance. Available zinc-plated mild steel (general use) or 316 stainless (marine, food, corrosive environments).
| Nominal Ø | Recommended Hole Ø | Common Length Range | Typical Use |
|---|---|---|---|
| 1.6 mm | 1.7 mm | 10 - 32 mm | Light hardware, small linkage, hobby |
| 2.0 mm | 2.1 mm | 10 - 40 mm | Castle nut M8-M10, small assemblies |
| 2.5 mm | 2.6 mm | 16 - 50 mm | Castle nut M10-M12, trailer hardware |
| 3.2 mm | 3.4 mm | 18 - 63 mm | Castle nut M12-M14, trailer hubs |
| 4.0 mm | 4.2 mm | 20 - 80 mm | Castle nut M14-M16, agricultural linkage |
| 5.0 mm | 5.2 mm | 25 - 100 mm | Heavy machinery, large castle nuts |
| 6.3 mm | 6.6 mm | 32 - 125 mm | Truck axles, large castle nuts M20+ |
| 8.0 mm | 8.5 mm | 40 - 160 mm | Heavy plant, very large castle nuts |
Cotter pins are the standard retention method for clevis pins through their cross-hole. For complete selection criteria including material grades, installation technique and alternatives, see our split pin / cotter pin guide. For springpin / roll pin alternatives, see spring pins.
Cotter Pins & Split Pins — Fastener Retention
Cotter pins (also called split pins) are safety fastening devices that lock clevis pins, castle nuts, axle pins and shaft pins against vibration loosening or accidental withdrawal. AIMS Industrial supplies cotter pins in metric and imperial sizes — zinc-plated mild steel for general applications and 316 stainless steel for marine, food and corrosive environment use. Manufactured to AS/NZS 1444 / ISO 1234 dimensional standards.
For complete coverage of selection, sizing, installation technique and when to use cotter pins vs alternatives, see our split pin / cotter pin guide.
Cotter pins we stock
- Zinc-plated mild steel cotter pins — metric sizes from 1.6 mm to 8 mm diameter. The general-purpose cotter pin for agricultural equipment, machinery, automotive and general fastener retention. Zinc plating provides light corrosion resistance for indoor and protected outdoor use.
- 316 stainless steel cotter pins — for marine, food processing and corrosive chemical environments. 316 grade provides chloride and oxidation resistance where zinc-plated steel would rust or pit. Matching metric sizes.
- Imperial cotter pins — inch diameter cotter pins for American and older machinery still using AF/UNC fastener systems. Critical where metric cotter pins would not correctly fit the pin hole diameter.
- Assortment packs — the most commonly used metric sizes in a single storage box. Practical for maintenance workshops and service vehicles where a range of sizes must be accessible without multiple individual packs.
Selection & installation
- Correct diameter fit — the cotter pin diameter must provide a snug fit in the hole. A loose fit allows the pin to rattle under vibration, eventually wearing out the hole and allowing withdrawal. Standard fit is within ±0.1 mm of the pin hole diameter in the fastener or shaft.
- Installation technique — after insertion, both legs of the cotter pin must be spread (bent): one leg bent back flat, the other bent forward, to prevent withdrawal under vibration. Never reuse a cotter pin after removal — the leg metal is work-hardened and may fracture when re-bent.
- Single-use fastener — cotter pins are single-use. Always replace with a new cotter pin after disassembly. Used pins may fracture on re-bending or have reduced retention capability from metal fatigue.
Companion pin ranges at AIMS
Cotter pins are typically installed through a clevis pin in a clevis-bracket-pin assembly. For solid-shaft retention without a clevis, see spring (roll) pins — the related roll pin guide covers the trade-offs between cotter, clevis, roll and tapered pins. Other related pin types: taper pins, coupling pins and linchpins.
For the full fastener range, see fasteners — including hex head screws, machine screws and stainless fasteners.
Australian business, local supply
As a proudly Australian business since 1988, we stock locally and work with trusted Australian manufacturers and distributors to ensure fast, reliable supply. Need help buying the right cotter pin for your application? Call (02) 9773 0122, contact our team or request a quote.
Roll Pins (Spring / Tension Pins) — Related Range
Roll pins (also called spring pins or tension pins) are a different pin family from cotter pins — they're slotted, springy cylindrical pins driven into a sized hole where the spring action grips the bore walls to retain. Where cotter pins lock a clevis or castle nut against axial slip, roll pins fix two parts together through a through-hole without requiring a cross-drilled retention hole. For roll pins / spring pins and their sizing guidance, see spring pins and the roll pin guide covering pitch diameters, drive force and when roll pins outperform cotter, clevis or taper pin alternatives.
People Also Ask — Cotter Pins
Q: What size cotter pin do I need?
Measure the hole diameter the cotter pin will pass through — the pin should be a close clearance fit, typically 0.5-1mm under the hole diameter. Length is selected so the legs extend past the secured component and can be bent over to lock. Common sizes range from 1.6mm to 10mm diameter, with lengths from 15mm to 100mm. Stocking a sized assortment covers most maintenance and assembly needs.
Q: What's the difference between split cotter pins and R-clips?
A split cotter pin has two legs that pass through a hole and are bent over to lock — they're permanent securing devices that require pliers to install and remove. R-clips (also called hairpin cotters) are single-piece spring-steel clips that snap into a groove or hole for quick installation and removal. Use split pins where you need a tamper-resistant lock; use R-clips where the pin will be removed frequently.
Q: What material should I choose for outdoor or marine use?
Stainless steel grade 304 handles general outdoor and chemical exposure; 316 stainless is the choice for marine and coastal environments where chloride corrosion attacks 304. Mild steel cotter pins (often zinc-plated) are fine for indoor and dry outdoor use but rust quickly in wet or salt-laden conditions. Bronze cotter pins suit non-magnetic applications and severe chemical exposure.
Q: How do I correctly install a split cotter pin?
Insert the pin so the head seats fully against the hole — don't leave a gap that lets the secured component move. Bend the long leg back over the end of the bolt or shaft (or down at right angles for crown nut applications), and trim or bend the short leg flat against the side. The pin must be in tension to do its job; loose installation lets the pin walk out of the hole.
Q: Can split cotter pins be reused?
No — single-use only. Once a cotter pin has been bent and straightened, the legs work-harden and become brittle, making them more likely to crack and release the secured component. Replacement cotter pins are inexpensive and a critical safety component. Reusing a cotter pin to save a few cents on a tractor steering linkage or crown nut is a false economy.
Q: What ISO or DIN standard applies to cotter pins?
ISO 1234 (previously DIN 94) is the international standard for split cotter pins, covering pin diameter, leg length and material specification. Common metric sizes run from 1mm to 20mm diameter, with lengths chosen to suit the grip. The ISO 1234 pin is the standard workshop split cotter pin — bent legs after installation to lock the secured component. Australian and European suppliers stock to ISO 1234; US-origin equipment may use ANSI B18.8.1 which has similar dimensions but slightly different tolerances.
Q: Why are cotter pins single-use only?
The steel legs of a split cotter pin are designed to bend once during installation. Bending work-hardens the steel — after being straightened for removal and re-bent for reuse, the metal grain has fatigued and the legs can crack unpredictably under load. A cracked cotter pin can shear off completely, releasing the secured nut or pin. On safety-critical applications (steering linkages, brake components, lifting equipment retention, PTO shafts) reusing a cotter pin risks serious failure. Replacement pins cost cents — the false economy of reuse isn't worth the consequence.
Q: What are common applications for cotter pins?
Cotter pins secure castle nuts on tapered-stud joints (ball joints, tie rod ends, king pins, axle nuts) — the classic automotive and agricultural use. They also lock clevis pins on trailer couplings, lifting hooks and load binders, secure crown nuts on wheel spindles, and provide backup retention on threaded fasteners in vibrating equipment. Marine deck hardware uses stainless cotter pins throughout. Agricultural PTO shafts, three-point linkages and implement retention rely on cotter pins for tamper-resistant securing that survives field vibration.
Q: What's the difference between a cotter pin and a split pin?
In Australian and UK usage the two terms are used interchangeably — both describe the standard folded-wire pin with two parallel legs and a rounded eye. Split pin is the more precise engineering term; cotter pin is the more common workshop name in Australia and the US. Historically "cotter" also referred to a solid tapered wedge pin (as on old bicycle cranks) but that meaning has largely dropped out of modern usage. For ordering purposes, split pin and cotter pin describe the same ISO 1234 product. Related but different pin types are available in the pins companion range.
Q: How does a cotter pin differ from a clevis pin?
A cotter pin (split pin) is the small folded-wire retainer — its job is to lock another component in place through a cross-drilled hole. A clevis pin is the primary connecting pin — a cylindrical shank with a head at one end and a cross-drilled hole at the other for a cotter pin to pass through. The two work together: the clevis pin makes the connection, the cotter pin locks it. AIMS stocks both — see clevis pins for the primary connectors, roll pins for the tension-pin alternative, and retaining rings for circlip-style retention. For full selection guidance read our split pin and cotter pin guide.