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What Is a Castle Nut? Its Benefits and How to Use One

What Is a Castle Nut? Its Benefits and How to Use One

Whether you’re a mechanic, repair specialist, or DIY enthusiast, you’ve likely come across castle nuts. 

What is a castle nut? 

Castle nuts or castle bolts, as they are sometimes called, are handy locking mechanisms that are commonly found in cars and axles that hold bearings and wheels in place. 

If you’ve been wanting to learn more about them, you’ve come to the right place. 

I’ll cover everything you need to know about castle nuts and break down how they work along with common applications so that you can put them to good use.

Let’s take a closer look at the modest but ever-useful castle nut.

Castle Nut Overview

Wonder why they’re called “castle nuts?”

Castle nuts are believed to have been named after the openings of parapets because of their likeness to the battlements found on top of ancient castles.

These hexagonally shaped hardware nuts are designed most commonly with six slots that have a 60-degree space in between.

These nuts are also called castellated nuts and spindle nuts. 

They are easily interchanged with slotted nuts which are different. 

In contrast, castellated nuts have a higher profile thanks to their cylindrical tops.

Some mistakenly believe that castle nuts can work with lock washers, but this isn’t actually true…

So how do they stay in place then?

Castle nuts rely on cotter pins and protruding triangular sections to create a formidable locking mechanism that can withstand a lot of vibration and movement.

Most castle nuts follow unified inch fine (UNF) or unified inch coarse (UNC) thread standards with diameters from a quarter to 1 ½ inches in different widths and heights.

By design, a castellated nut’s cylindrical top where the notches are found has a smaller diameter. 

This nut’s rounded shape allows the pin to be locked in more securely.

Castle nuts are manufactured in different materials, but most of them are made with steel and stainless steel. 

They also come in various sizes to provide you with a perfect fit.

Typical Applications of Castle Nuts

Castle Nuts

You’ve probably seen castle nuts before without realizing it. 

Where are we most likely to find them?

Castle nuts are often used in automotive vehicles to ensure that the nuts remain tightly in place despite constant and regular vibration.

Here are some other examples of how castle bolts are commonly used:

In Bicycles

One of the most common uses of castle nuts is wheel bearings. 

These nuts are used to hold the wheel to an axle. 

The cotter pin helps ensure a secure fit that helps the wheel bearing stay in place. 

In Cars

Like bikes, castle nuts are also used in the wheel bearing of cars. 

If you’ve ever had to change a flat tire, then you would have come across the castle nut. 

The castle nut is typically removed before releasing the damaged tire and replacing it.

In Trains

Trains have lots of wheels so it is common to find castle screws among the train’s many axles.

Knowing how to use a castle nut is definitely a valuable tool in your arsenal if you work with wheels and other mechanisms that need securing. 

In the next section, we brief you on the tools you’ll need and how to use them.

Tools You Need to Use Castle Nuts

cotter pin with lock nut

Fasteners play an essential role in ensuring that castle nuts are securely in place. 

A wire, cotter pin, and an additional nut are all tools that you can use with them.

  • Cotter Pin – thread this through the gaps of the nut and lock it in place
  • Wire – you can also thread this through the castle nut, attach it to the bolt and tie it in place
  • Another nut – if you do not have a cotter pin or wire, you can use a second nut to secure your first castle nut. The friction produced by the two nuts can be an effective way to reduce movement. Use this only as a temporary solution.

Installing a Castle Nut

nut and cotter pin

Is it hard to install a castle nut?

Using a castle nut requires some knowledge and tools but it may be easier than you think.

To install a castle nut properly, you need to have a bolt with a hole and a castle nut cotter pin. 

The entire installation process is relatively simple, especially since bolts already come pre-drilled with holes intended for castle nuts.

Before buying your bolt, you need to know the length you need. 

If you don’t have access to a bolt with a hole, you have to drill the hole on your own.

Here are the steps for installing a castle nut:

“X” Marks the Spot: Make a Mark Before You Drill

Slide the bolt through the installation material and screw on the castle nut on the other side. 

If your bolt doesn’t have a hole yet, mark the area where you need to make one. 

Remove the nut and bolt and set the bolt into a vice.

Get the Nut Into the Bolt

Screw the nut into the bolt. 

Make sure to align it based on the mark you made in the first step. 

Prepare the Drilling Area

Create an indent into your bolt with a punch so your drill bit has a preparatory point. 

Drill perpendicular to the hole so it comes out sandwiched between the mounts on the other side.

Get Your Cotter Pin In

After drilling, remove the bolt and place it where you need to install it. 

Place the cotter pin through the newly drilled hole.

Lock It In with Pliers

Use a pair of pliers to bend the cotter pin away from the hole and around the nut. 

This will lock the pin in place so it doesn’t fall out.

If you want to see how it’s done, this quick video shows how to install castle nuts on ball joints:

Frequently Asked Questions

Can castle nuts be used again?

Yes, you can use castle nuts for a second or third time. 

Castle nuts are good locking mechanisms when you don’t want to secure something permanently. 

As long as the castle nut is held with a cotter pin, you can rely on it not to move despite vibration. 

You can easily remove the castle nut with a wrench. 

Unless thoroughly damaged or deformed, you can reuse it for your next repair job.

How tight should castle nuts be?

You’ll know that it’s tight enough when you can no longer feel any movement on the spindle. 

What is the main advantage of castle nuts over conventional hexagonal nuts?

As a positive locking mechanism, castle nuts provide a tighter lock than hexagonal nuts.

 They’re especially useful for wheel bearings and other low-torque applications that are susceptible to constant movement and vibration washing machine. 

Hexagonal nuts can quickly become loose, but since castle nuts are secured with a pin, joints remain secure and do not allow the slightest movement or vibration.

Castle Nuts: Secure Vibrating and Moving Parts Together

If you need a locking device for parts that undergo constant vibration and movement, use castle nuts. 

When paired with a cotter pin, this small yet highly reliable fastener is a great choice for properly securing parts in place.

Now that you know how to use them, tell us about your experience. 

Did you find it easy or hard to install? What advice would you give other people who might use them?

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