Have you ever heard of a cotter pin? It’s a small but super useful piece of metal that helps keep machinery and parts together. Whether you’re fixing a bike, working on a car, or dealing with farm equipment, a cotter pin might be exactly what you need.
In this article, we’ll explore what a cotter pin is, how it works, where you can use it, and even how to install and remove one safely. Let’s dive in!
Table of Contents
What Is a Cotter Pin?
A Simple Yet Smart Tool
A cotter pin (sometimes called a split pin) is a metal fastener used to hold parts together. It’s usually made of steel or stainless steel and shaped like a long, folded piece of wire. Imagine a hairpin or a bobby pin—that’s kind of what it looks like!
How It Looks
A cotter pin has two tines (legs) joined at the top. These legs can be bent outward after you insert the pin into a hole. That bending locks the pin in place and prevents it from falling out.
What Does a Cotter Pin Do?
Locks Things in Place
The main job of a cotter pin is to keep nuts, bolts, or other fasteners from slipping off. It works by passing through a hole in a bolt or shaft and then bending the ends to secure it.
Safety First
Cotter pins are used in places where safety is important. If a nut came loose in a car’s suspension, for example, it could be dangerous. A cotter pin helps stop that from happening.
Where Are Cotter Pins Used?
You’ll be surprised by how many machines and tools use cotter pins! Let’s look at some common places where you might find them.
Cars and Trucks
- In the steering system
- In the brake assembly
- On the axles
Cotter pins are used with castle nuts to make sure they don’t loosen due to vibration.
Bicycles
Older bikes or heavy-duty ones may have cotter pins holding the pedal crank arms in place.
Farm and Garden Equipment
Tractors, lawnmowers, and plows often use cotter pins because they’re easy to remove and replace when doing repairs.
Construction and Industrial Equipment
From forklifts to scaffolding, cotter pins are everywhere in heavy-duty equipment.
Types of Cotter Pins
Not all cotter pins are exactly the same. Let’s take a quick look at a few different types you might come across.
Standard Cotter Pin
This is the most common type. It has two tines that can be bent outward once it’s inserted.
Hairpin Cotter (R-Clip)
Looks like a bent wire or the letter “R.” These are often used on tractor hitches and trailers.
Tapered Cotter Pin
These are shaped like a wedge and fit tightly. You’ll find them in some older bike cranks or tools.
Split Ring Cotter
More like a key ring, these are used in areas where a loop is helpful for pulling the pin out.
How to Use a Cotter Pin
Let’s walk through how to install and remove a cotter pin step by step.
Tools You’ll Need
- Pliers
- Wrench (for removing the nut or bolt)
- Safety glasses (just to be safe)
Installing a Cotter Pin
- Insert the Bolt or Nut
First, tighten the nut onto the bolt or shaft. Make sure it has a hole drilled through it for the cotter pin. - Line Up the Hole
Align the hole in the bolt or shaft with the slots in the nut. - Insert the Cotter Pin
Push the pin through the hole until it’s all the way in. - Bend the Legs
Use your pliers to bend the two tines of the pin in opposite directions. This locks it in place.
Removing a Cotter Pin
- Straighten the Tines
Use pliers to bend the legs of the pin straight again. - Pull It Out
Once straight, grab the looped head and pull the pin out. - Dispose of the Old Pin
Cotter pins are usually not reusable. They get bent out of shape and may not hold properly a second time. Always use a new one.
Cotter Pin Sizes
Cotter pins come in many sizes to match the bolt or shaft they’ll be used with. Common diameters include:
- 1/16 inch
- 1/8 inch
- 3/16 inch
- 1/4 inch
Lengths vary too. You’ll want to choose a pin that’s long enough to pass through the hole with room to bend the legs.
If you’re not sure which size to use, you can buy cotter pin assortment kits that include a mix of sizes.
What Materials Are Cotter Pins Made Of?
Most cotter pins are made of steel, but you’ll also find ones made of:
- Stainless steel – For wet or outdoor environments
- Brass – Less common but good for electrical uses
- Aluminum – Lightweight but not as strong
For most jobs, zinc-plated steel cotter pins are a great all-around choice.
Cotter Pin vs. Other Fasteners
Wondering how a cotter pin compares to other types of pins or fasteners? Here’s a quick comparison:
| Type | Use | Can Be Reused? |
|---|---|---|
| Cotter Pin | Secures nuts and bolts | Usually not |
| Split Pin | Same as cotter pin | |
| R-Clip | Quick-release connections | Yes |
| Clevis Pin | Works with cotter pins | Yes |
| Hitch Pin | Trailer hitches | Yes |
Safety Tips When Using Cotter Pins
- Wear safety glasses – The ends can snap or poke when bending.
- Don’t reuse bent pins – Always replace with a new one.
- Check for tight fit – A loose cotter pin may fall out.
- Keep extra pins on hand – It’s always good to have backups.
Common Problems and Fixes
Problem: Pin Won’t Fit
Fix: Make sure you have the right size. You may need a thinner pin.
Problem: Can’t Bend the Legs
Fix: Use needle-nose pliers. If the pin is too thick, try a smaller one.
Problem: Pin Keeps Falling Out
Fix: Make sure the legs are bent firmly. Also check that the hole isn’t too big.
Where to Buy Cotter Pins
You can find cotter pins almost anywhere:
- Hardware stores (Home Depot, Lowe’s)
- Auto parts stores
- Online stores (Amazon, eBay)
- Farm and tractor supply shops
They’re cheap and sold in packs, so it’s easy to stock up.
Fun Fact!
Did you know the cotter pin was invented in the 1800s and is still used today? It’s one of the simplest yet most reliable inventions in mechanical history!
Final Thoughts
A cotter pin may look like just a little piece of metal, but it plays a big role in keeping things safe and secure. From cars to tractors to bikes, this tiny part helps stop nuts and bolts from coming loose—and that can make a huge difference.
