How to Install a Cotter Pin: Sizes, Bend Angle & Reuse Rules
To install a cotter pin, push it fully through the aligned hole so the closed loop seats flush, then bend both prongs outward in opposite directions — never straight down. A pin sized wrong for its ASME B18.8.1 hole, or reused after removal, can back out under vibration and loosen a castle nut or clevis pin. This guide covers correct pin sizing, castle-nut alignment, bend angle, and why a used pin should never go back in.
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What Is a Cotter Pin?
A cotter pin (also called a split pin) is a metal fastener with two bendable prongs that locks a nut, clevis pin, or bolt in place by passing through a drilled hole and splaying outward on the far side. It is a one-time-use fastener — not a reusable clip — and its diameter is matched to the hole size defined by ASME B18.8.1 (inch) or ISO 1234 (metric). You will find cotter pins securing trailer hitches, wheel-bearing spindles, tie-rod ends, and even ceiling fan downrods.
Tools and Materials You’ll Need
- Correctly sized cotter pin (matched to the hole diameter — see the sizing chart below)
- Needle-nose or diagonal pliers, for bending the prongs
- Diagonal wire cutters, for trimming excess length
- Ruler or calipers, for measuring hole diameter and depth
- Flashlight, for locating the hole in tight spaces
Best Cotter Pin Kit Pick
NEIKO 50454A Cotter Pin Assortment, 555-Piece, Zinc Plated – $14.21
Covers 13 common inch sizes so you can match the hole diameter from the ASME B18.8.1 chart below without a hardware-store trip mid-repair.
- Best for: keeping a full size range on hand for wheel bearings, trailer hitches, and general repairs
- Why we picked it: zinc-plated steel resists rust better than bare steel pins in outdoor/automotive use
- Main drawback: zinc plating is not stainless — swap to a stainless kit for marine or wet environments
Compare more cotter pin installation tools
 Option 1 CRAFTSMAN 6-in. Diagonal Pliers (CMHT81646) – $10.99
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 Option 2 Amazon Basics Extra Long Reach Needle-Nose Pliers – $16.37
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 Option 3 WORKPRO 7-Inch Diagonal Pliers, CRV Steel – $12.99
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Choosing the Right Cotter Pin Size
Cotter pins are one of several fastener types, and picking the wrong size is the most common reason an installation fails. Using an undersized pin lets the assembly rattle loose; forcing an oversized pin can crack the hole edge or make bending impossible.
Types of Cotter Pins
- Extended prong: longer prongs that bend more easily and resist vibration better — good for high-movement applications.
- Standard prong: two equal-length prongs, the most common type for general-purpose fastening.
- Clinch pin (hairpin cotter): has a pre-formed loop at one end, so only the straight leg needs bending — common for quick, temporary fastening.
Cotter Pin Sizing Chart (ASME B18.8.1)
Match the pin’s nominal size to your hole diameter using the inch-series chart below. Note that the recommended hole is intentionally larger than the pin — forcing a pin into an exact-diameter hole is a common installation mistake.
| Nominal Pin Size | Max Pin Diameter | Recommended Hole Size |
|---|---|---|
| 1/8 in | 0.120 in | 9/64 in (0.141 in) |
| 3/16 in | 0.176 in | 13/64 in (0.203 in) |
| 1/4 in | 0.225 in | 17/64 in (0.266 in) |
| 5/16 in | 0.280 in | 5/16 in (0.312 in) |
| 3/8 in | 0.335 in | 3/8 in (0.375 in) |
📊 Per ASME B18.8.1, inch-series cotter pin holes are sized 1/64 in larger than the pin’s nominal diameter up through 1/4 in, then equal to nominal from 5/16 in up. — Source: ASME B18.8.1, Clevis Pins and Cotter Pins (Inch Series)
How to Install a Cotter Pin (Step-by-Step)
Once you have the right pin, installation is five steps from measuring the hole to trimming the finished bend.
- Identify and measure the hole: Locate the drilled hole in the shaft, bolt, or nut slot, and measure its diameter with a ruler or calipers so you can match it to the sizing chart above.
- Choose the correct pin: Select a cotter pin sized per the chart, with prongs long enough to extend past the shaft on the far side once inserted.
- Insert the pin: Push the straight end through the aligned hole until the closed loop seats flush against the surface; do not force or bend the pin during insertion.
- Bend the prongs: Using pliers, bend both prongs outward in opposite directions to roughly a 90-degree angle — horizontally away from each other, never straight down along the shaft.
- Trim and test: Cut off any excess prong length with wire cutters to avoid sharp edges, then grip the pin’s loop and gently pull to confirm it will not slip out.
Installing a Cotter Pin on a Castle Nut
Castle nuts (castellated nuts) are the most common place cotter pins show up in automotive and trailer repair — on tie rods, ball joints, and spindle nuts. The nut must be torqued to the manufacturer’s spec first, then the cotter pin hole in the bolt or spindle must line up with one of the nut’s slots before the pin goes in.
“When correctly installed, the cotter pin hole should align with the slots (castellations) in the castle nut… the nut may be tightened slightly to improve the alignment, but never back the nut off.”
If the holes still won’t align after tightening within spec, that usually signals the wrong nut or an improperly seated stud — not a problem the cotter pin can solve. Never loosen a torqued castle nut to force alignment.
Common Mistakes When Installing a Cotter Pin
- Using an undersized or oversized pin instead of matching the hole per the ASME chart above.
- Bending both prongs straight down along the shaft instead of splaying them outward — this can catch on moving parts and doesn’t lock as securely.
- Reusing a removed cotter pin — the bend point is already weakened and can snap under load (see FAQ below).
- Backing off a torqued castle nut to force hole alignment instead of tightening slightly within spec.
- Leaving sharp, untrimmed prong ends that can cut skin or snag on nearby components.
FAQ
Can you reuse a cotter pin after removing it?
No. Removing a cotter pin means straightening its bent prongs, which work-hardens and weakens the metal at that bend point. Reinstalling the same pin risks a prong snapping off during the next bend or under vibration in service. Always install a fresh pin.
Which way do you bend a cotter pin?
Bend the two prongs in opposite directions, angling them horizontally away from each other at roughly 90 degrees. Avoid bending them straight down along the shaft — that orientation is more likely to catch on nearby parts and can work itself loose.
What size cotter pin do I need for a 1/4 inch hole?
A 1/4 inch nominal cotter pin fits a 17/64 inch (0.266 inch) recommended hole per ASME B18.8.1. If you only know the hole size, pick the pin one size class down, since inch-series holes run slightly larger than the pin’s nominal diameter.
Do you tighten or loosen a castle nut to align the cotter pin hole?
Tighten slightly, never loosen. Per Moog’s technical guidance, a castle nut may be tightened a bit further within torque spec to line up the hole with a slot, but backing the nut off to align it can leave the connection under-torqued and unsafe.
What tools do you need to install a cotter pin?
Needle-nose or diagonal pliers to bend the prongs, and wire cutters to trim excess length once the pin is bent. A ruler or calipers helps confirm hole and pin size beforehand, and a flashlight helps locate recessed holes on axles or hitches.
Conclusion
Installing a cotter pin correctly comes down to three things: matching the pin to the hole per the ASME B18.8.1 chart, bending both prongs outward rather than straight down, and never reinstalling a pin you’ve already removed. Get those three right and the fastener will hold for the life of the assembly.
