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Best Castle Nuts

Castle nuts, also known as slotted nuts or castellated nuts, hold a pivotal role in various industries, especially in automotive and machinery. The unique design, resembling the top of a castle, allows the nut to be secured with a cotter pin, ensuring a tight, secure fit. In 2023, several top-notch companies have brought out exceptional castle nuts, each with unique features and benefits. Here, we explore the best castle nuts of 2023, focusing on their key features, benefits, and potential applications.

What Are Castle Nuts?

Castle nuts are a type of locknut. They are designed with slots cut into one end that resembles the top of a castle, hence the name. These slots are not just for aesthetic purposes; they serve a crucial function in the application of the nut. A cotter pin or safety wire is inserted through the slots and a drilled hole in the bolt for safety, which prevents the nut from loosening or falling off due to vibration or rotational force.

Uses of Castle Nuts

Castle nuts are used in applications where safety is paramount. They are often used in automotive suspension systems, bicycles, and aerospace equipment where they secure front wheel bearings on cars and trailers, hold propellers on planes, and fasten other high-vibration machinery.

Key Considerations

  • Size and Threads: The size of a castle nut is determined by the diameter and thread pitch of the bolt it will be used on. The size and thread pitch of a castle nut are usually measured in either metric (millimeters) or standard (inches) units. Make sure you choose a castle nut that is the correct size for your bolt.
  • Material: Castle nuts are typically made from steel, stainless steel, or brass. The material you choose should depend on the environmental conditions where the nut will be used. For example, stainless steel and brass are more resistant to corrosion and are suitable for outdoor use or in wet conditions.
  • Coating: Castle nuts can come with different types of coatings, including zinc, cadmium, or phosphate. These coatings can provide additional corrosion resistance. Zinc coatings are common and provide good protection against rust, while cadmium coatings offer even better protection but are more expensive and less environmentally friendly.
  • Load Bearing Capability: The load-bearing capacity of a castle nut is determined by its grade. Higher grade castle nuts can bear more load and are more suitable for heavy-duty applications. For most general applications, a lower grade castle nut is sufficient.
  • Vibration Resistance: Castle nuts are designed to resist loosening due to vibration, but some designs are better at this than others. Nuts with more slots generally provide better vibration resistance, but they may also be more difficult to install and remove.
  • Ease of Installation: Some castle nuts are easier to install than others. Nuts with fewer slots are generally easier to install, but they may not provide as much vibration resistance. If ease of installation is a priority for you, consider choosing a nut with fewer slots.

Our Top Picks of 2023:

1. Caplugs ZCNS Series Castle Nuts

Caplugs ZCNS Series Castle Nuts

Caplugs ZCNS series are crafted with high-quality stainless steel, ensuring maximum durability and resistance to rust. They come in various sizes, making them ideal for a wide range of applications. Their excellent threading offers a secure fit, and the use of a cotter pin ensures that the nuts won't loosen due to vibrations or torque.


  • Resistant to rust and corrosion
  • Durable stainless steel construction
  • Available in different sizes
  • Excellent threading for a secure fit


  • Cotter pins not included
  • Pricey for larger sizes

2. Hard-to-Find Fastener Castle Nuts

Hard-to-Find Fastener Castle Nuts

The Hard-to-Find Fastener brand offers castle nuts known for their strength and durability. Their nuts are made from hardened steel, ensuring that they can withstand high levels of torque and stress. The nuts are zinc-plated, which increases their resistance to corrosion and rust.


  • High strength and durability
  • Zinc-plated for rust resistance
  • Affordable


  • Limited size options
  • Cotter pins not included

3. The Hillman Group Castle Nuts

The Hillman Group Castle Nuts

The Hillman Group's castle nuts are made from high-quality steel. They are available in both metric and SAE sizes, making them versatile for different applications. These nuts also feature a zinc finish to resist corrosion, ensuring a long lifespan in various environments.


  • High-quality steel construction
  • Available in both metric and SAE sizes
  • Zinc finish for corrosion resistance


  • The zinc finish may wear off over time
  • Cotter pins not included

4. Prime-Line Castle Nuts

Prime-Line Castle Nuts

Prime-Line castle nuts are made from grade 2 low carbon steel, ensuring a balance between strength and ductility. These nuts are designed with coarse threading, making them suitable for quick, easy installations in areas with a limited range of motion.


  • Made from grade 2 low carbon steel
  • Coarse threading for easy installation
  • Versatile use


  • Not as strong as higher-grade steel nuts
  • Cotter pins not included

5. Stanley Hardware Castle Nuts

Stanley Hardware Castle Nuts

Stanley Hardware's castle nuts are made from rust-resistant stainless steel. They come in various sizes and feature fine threading, ensuring a secure fit in most applications. These castle nuts also come with a limited lifetime warranty, giving users peace of mind.


  • Rust-resistant stainless steel
  • Comes in various sizes
  • Limited lifetime warranty


  • More expensive than other options
  • Cotter pins not included


Castle nuts are an essential component in many applications, providing a secure and tight fit. When choosing the best castle nut, consider the material, size, threading, and additional features like corrosion resistance and included cotter pins.

The Caplugs ZCNS series are an excellent choice for their rust resistance and various sizes. The Hard-to-Find Fastener castle nuts are known for their strength and durability, while The Hillman Group offers a wide size range. Prime-Line's castle nuts are great for quick installations, and Stanley Hardware's offer a limited lifetime warranty.

Remember, the best castle nut depends on your specific needs and applications. Always consider the specific requirements of your project when selecting a castle nut.