Search for these two terms and you will find pages insisting they are different fasteners. Here is the truth from the standards themselves: they are the same part. What differs is where you are standing when you say the name.
The Short Answer
A split pin - equally, a cotter pin - is a fastener folded from semi-circular wire so it forms a round shank with two legs of unequal length. It passes through a drilled hole in a bolt, clevis pin, or shaft; the legs are then bent apart around the part, mechanically preventing the assembly from backing out. It is the classic lock for a castle nut.
Who Says What, Where
- United States: cotter pin (also cotter key)
- United Kingdom: split pin - because in older British usage a cotter is the solid tapered pin that fixed bicycle cranks to the bottom-bracket axle, an entirely different part
- India: both names are used interchangeably; purchase orders commonly say cotter pin while IS 549, the governing Indian Standard, titles them split pins
Ordering tip: to avoid any ambiguity on drawings and POs, cite the standard and size - e.g. “Split pin IS 549 / ISO 1234, 3/16″ × 2″, SS 304”. Nobody can misread that.
Standards & Sizes
| Standard | Region | Notes |
|---|---|---|
| ISO 1234 | International | Metric split pins |
| DIN 94 | Germany / EU | Dimensionally aligned with ISO 1234 |
| IS 549 | India | “Split Pins - Specification”; mild steel half-round wire per IS 10794 |
| BS 1574 | UK | Legacy British standard |
The designated size corresponds to the hole diameter the pin fits. We stock diameters from 1/16″ to 1/2″ (≈ 1.6 - 13 mm) in lengths from 1/2″ to 6″, priced per 100 pieces - see the Cotter Pins tabs in our pricelist.
Fitting One Correctly
- Insert the pin through the hole until the head (the folded eye) seats against the surface.
- Bend the longer leg back over the nut flat or around the shaft with pliers; bend the shorter leg the opposite way.
- Trim legs if they could snag, and ensure neither leg can contact rotating or moving parts.
- On castle nuts: tighten to torque first, then advance (never back off) to align the next slot with the hole.
Fit a new pin on every reassembly. Re-bending work-hardened legs invites fatigue failure - an unacceptable risk in steering, brake, and rigging applications for a part that costs a rupee or two.
MS or Stainless?
- Mild steel (self-colour): the economical default for dry, indoor, general engineering use.
- SS 304: humid environments, food machinery, washdown areas.
- SS 316: marine, coastal, and chemical exposure - see our SS 304 vs SS 316 guide.
Need Split / Cotter Pins?
Full imperial range 1/16″ - 1/2″ in MS and SS 304/316, priced per 100, stocked ex-Mumbai with pan-India delivery.
View Fastener RangeFrequently Asked Questions
Is a cotter pin the same as a split pin?
Yes. Cotter pin (common in the US and India) and split pin (common in the UK) are two names for the same fastener: a pin folded from semi-circular wire, inserted through a drilled hole and locked by bending the two legs apart. The relevant standards are DIN 94, ISO 1234, and IS 549.
Is there any case where cotter means something different?
Yes - in traditional British usage, a "cotter" is the solid tapered pin that fixed bicycle pedal cranks to their shaft, a completely different component. That is why UK engineering documents prefer the unambiguous term split pin. In Indian and American practice, cotter pin almost always means the split-leg wire pin.
Can I reuse a split pin?
Best practice is no. Bending the legs work-hardens the wire, and re-straightening then re-bending it risks fatigue cracking at the fold. Split pins are inexpensive - fit a new one every time the joint is opened, especially on castle nuts in steering, brake, and suspension assemblies.
What size split pin do I need?
The designated size corresponds to the hole diameter it fits: a 1/8″ split pin is used in a 1/8″ (approx. 3.2 mm) hole. Choose a length that lets both legs be fully bent around the nut or shaft. We stock 1/16″ to 1/2″ diameters in lengths from 1/2″ to 6″, in mild steel and stainless.