When it comes to fastening mechanical parts, ensuring safety, and preventing nuts or bolts from loosening, cotter pins (sometimes called split pins or cotter keys) are among the simplest yet most reliable solutions. They may look like just a small piece of bent metal, but without them, many machines, vehicles, and equipment could fail under vibration or stress.
If you have ever worked with engines, bicycles, machinery, or even basic DIY furniture assembly, chances are you have come across cotter pins. But the real challenge comes when choosing the right cotter pin size for your project. Selecting the wrong size could lead to improper fitting, loss of tension, or even mechanical failure.
That’s where a cotter pin size chart becomes incredibly useful. It helps you pick the right length and diameter based on your bolt, shaft, or hole size. In this guide, I will explain everything about cotter pins, their uses, types, and provide a comprehensive size chart to make your job easier.
What is a Cotter Pin?
A cotter pin is a simple, folded metal fastener shaped like a hairpin, typically made from mild steel, stainless steel, or brass. One end is rounded in a loop, while the other end is split into two tines (legs) that can be bent apart to secure it in place.
Here’s how it works:
Insert the pin through a hole in a bolt, shaft, or other component.
Once it’s in, bend the two split ends outward.
The bent legs prevent the pin from slipping back out.
This straightforward design makes cotter pins a reliable locking device that’s easy to install and remove.
Common Uses of Cotter Pins
Cotter pins are widely used across industries and even in everyday products. Some common applications include:
Automotive industry – securing castle nuts in wheel hubs, axles, and suspension systems.
Bicycles and motorcycles – holding pedals, crank arms, or wheel parts in place.
Industrial machinery – fastening moving parts under vibration.
Marine applications – securing propeller shafts and rudder assemblies.
DIY and home repair – used in furniture, lawnmowers, and small machines.
Essentially, wherever you need a backup safety lock on a bolt or shaft, cotter pins are the go-to option.
Types of Cotter Pins
Though they all follow the same basic concept, cotter pins come in different styles and materials:
Standard Split Pin – the most common, shaped like a bent wire with two prongs.
Hairpin Cotter Pin (R-clip) – shaped like an “R,” used for quick removal without bending.
Circular Cotter Pin – used for securing round shafts.
Hammerlock Cotter Pin – heavy-duty option for industrial applications.
Materials:
Steel (Zinc-plated): General purpose, rust-resistant.
Stainless Steel: Corrosion-resistant, ideal for marine use.
Brass/Bronze: Non-magnetic, softer applications.
Cotter Pin Size Chart
Choosing the right size depends on two key measurements:
Pin diameter (thickness)
Pin length (measured from loop to tip, not including bent ends)
Below is a standard cotter pin size chart (in inches and millimeters) that you can use as a reference.
📊 Cotter Pin Size Chart (Standard Sizes)
| Cotter Pin Diameter (inches) | Cotter Pin Diameter (mm) | Length (inches) | Length (mm) | Typical Bolt Hole Size | Common Applications |
|---|---|---|---|---|---|
| 1/32″ | 0.8 mm | 1/2″ | 12 mm | #4 – #6 screws | Small appliances, light duty |
| 1/16″ | 1.6 mm | 3/4″ – 1″ | 19 – 25 mm | #8 – #10 screws | Bicycles, light machinery |
| 3/32″ | 2.4 mm | 1″ – 1-1/2″ | 25 – 38 mm | 1/4″ bolts | Automotive, general use |
| 1/8″ | 3.2 mm | 1-1/2″ – 2″ | 38 – 50 mm | 5/16″ bolts | Automotive, farm equipment |
| 5/32″ | 4.0 mm | 2″ – 2-1/2″ | 50 – 64 mm | 3/8″ bolts | Trucks, tractors |
| 3/16″ | 4.8 mm | 2-1/2″ – 3″ | 64 – 76 mm | 7/16″ bolts | Heavy machinery |
| 1/4″ | 6.4 mm | 3″ – 4″ | 76 – 100 mm | 1/2″ – 5/8″ bolts | Marine, large machinery |
| 5/16″ | 8.0 mm | 4″ – 5″ | 100 – 127 mm | 3/4″ bolts | Industrial equipment |
| 3/8″ | 9.5 mm | 5″ – 6″ | 127 – 152 mm | 1″ bolts | Construction, heavy-duty use |
Cotter Pin Size Chart PDF
The Cotter Pin Size Chart PDF provides a quick reference for selecting the right pin diameter and length for various applications. From automotive and machinery to marine and DIY projects, this chart ensures proper fit and safety. Download it to simplify your work, avoid guesswork, and choose the ideal cotter pin every time.
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How to Choose the Right Cotter Pin Size
When selecting a cotter pin, follow these guidelines:
Match the diameter: The cotter pin’s diameter should closely match the hole diameter of your bolt, shaft, or nut. Too small, and it may rattle loose; too large, and it won’t fit.
Length matters: Pick a pin long enough to pass through and leave enough legs to bend outward securely.
Material selection: For outdoor or marine applications, go with stainless steel to prevent rust. For general use, zinc-plated steel works well.
Load and vibration: For high-vibration environments (automobiles, tractors), choose stronger materials and slightly thicker pins.
Step-by-Step: Installing a Cotter Pin
Insert the pin through the bolt, nut, or shaft hole.
Make sure the loop end rests securely against the surface.
Spread the two split legs outward with pliers.
One leg is usually bent over the bolt end.
The other is bent down along the side for a secure lock.
Trim excess length if necessary.
Removal is just as simple, straighten the legs and pull the pin out with pliers.
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Advantages of Cotter Pins
Low cost – very affordable and widely available.
Easy to use – no complex tools needed for installation or removal.
Reliable safety lock – prevents nuts from loosening under vibration.
Reusable (in some cases) – though often replaced after use, heavier stainless steel versions can be reused.
Versatile – fits everything from bicycles to aircraft machinery.
Limitations of Cotter Pins
Not suitable for very high-torque applications alone (they’re more of a locking aid than a primary fastener).
Must be replaced after bending too many times (they can weaken).
Limited size range for very large machinery.
Practical Examples
Car wheel hub: The castle nut is tightened to spec, and then a cotter pin is inserted through the nut’s slots and axle hole to prevent rotation.
Tractor hitch pin: Cotter pins lock large hitch pins in place so they don’t slide out while towing.
Marine rudder system: Stainless steel cotter pins keep the rudder shaft assembly secure despite water exposure.
Maintenance Tips
Always inspect cotter pins for wear, bending fatigue, or corrosion before reuse.
Use the correct removal method, do not yank or twist too hard, as it can damage the hole.
Keep spares in your toolbox, they are small but essential.
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Final Thoughts
Cotter pins might be one of the simplest mechanical components, but they play a critical role in safety and durability. Whether you’re repairing a bicycle, maintaining your car, or working with industrial equipment, choosing the right cotter pin size ensures your assembly remains secure.
By using the cotter pin size chart above, you can easily select the proper fit for your application. Always consider the hole size, material, and length when making your choice. A small part like this may seem insignificant but when it fails, the consequences can be serious.
So next time you reach for a nut and bolt, do not forget the humble cotter pin, it might just be the little hero holding everything together.