What is a Carriage Bolt and Other FAQ

What is a Carriage Bolt and Other FAQ

Our experts have put together this comprehensive guide to carriage bolts, an important component in the realm of fastening solutions. Within this guide, we dive into the world of carriage bolts, revealing their uses, installation, and the array of benefits they bring to various projects. Carriage bolts serve as a cornerstone for builders, DIY enthusiasts, and professionals alike, offering essential connections in countless applications. 

All FAQ about Carriage Bolts in this article:

What Are Carriage Bolts Used For?

Carriage bolts are predominantly used in woodworking and metalworking applications. They are designed to fasten metal to wood, wood to wood, or metal to metal with a pre-drilled hole. Their smooth, rounded heads with a square section underneath (the shank) prevent the bolt from turning once installed, making them ideal for situations where access is only possible from one side.

Why Are They Called Carriage Bolts?

The term "carriage bolt" dates back to the 1800s when these bolts were used in the construction of carriages and related hardware. Their design allowed for fastening without the need for a tool to hold the bolt head, making them suitable for assembling complex carriage parts.

What are some other names for Carriage Bolts?

  • Coach Bolts:

    This is one of the most common alternative names, especially in the UK and other parts of Europe. The term tends to be used interchangeably with "carriage bolt."
  • Round Head Square Neck Bolts:

    This name describes the physical characteristics of the bolt, highlighting its round head and square neck, which prevents the bolt from turning once installed.
  • Mushroom Head Square Neck Bolts:

    Similar to the above, this name also describes the shape of the bolt, with "mushroom head" referring to the rounded, dome-like shape of the bolt head.
  • Dome Head Bolts:

    Another descriptive term based on the shape of the bolt head.
  • Cup Head Bolts:

    In some regions, carriage bolts might be referred to by this name, again referring to the shape of the head.
  • Plow Bolts:

    While plow bolts are a distinct type of bolt used primarily in plowing equipment, they are sometimes confused with or referred to as carriage bolts because of their similar head shape. However, plow bolts typically have a flat or slightly countersunk head.

When to Use Carriage Bolts

Ideal for connecting wood to metal, carriage bolts are also suitable for wood-wood or metal-metal connections in specialized versions. Common industries and applications include:

  • Wood Constructions:

    Carriage bolts are extensively used in woodworking and timber construction, including decks, docks, outdoor furniture, and wooden bridges. The smooth, round head is ideal for applications where a flat surface is necessary or desirable for aesthetic and safety reasons.
  • Outdoor Structures:

    They are ideal for outdoor structures such as pergolas, gazebos, and playsets, where the bolt’s corrosion resistance (especially in treated or galvanized variants) and the smooth head design are beneficial.
  • Fencing and Railings:

    Carriage bolts are used in the construction of wooden or metal fencing and railings, providing a secure fastening solution that is also visually appealing.
  • Automotive Applications:

    In automotive construction, carriage bolts are used to assemble frame structures, trailers, and other components where a smooth bolt head is required to reduce the risk of injury or snagging.
  • Marine Construction:

    For marine applications, stainless steel or silicon bronze carriage bolts are used due to their high corrosion resistance, especially in environments exposed to saltwater.
  • Furniture:

    They are often used in the assembly of furniture, offering a secure and aesthetically pleasing fastening solution.
  • Playground Equipment:

    Carriage bolts are a popular choice for securing playground equipment, ensuring that the fastenings are safe and do not present a snagging hazard to children.
  • Agricultural Machinery:

    In agricultural settings, carriage bolts are used in the construction and maintenance of machinery and equipment, benefiting from the bolt’s strength and secure fastening.

The broad use of carriage bolts across different industries and applications is a testament to their versatility, strength, and the safety and aesthetic benefits of their design.

How do Carriage Bolts compare to other types of bolts?

Carriage Bolts vs. Hex Bolts

Head Shape: Carriage bolts have a round, domed head with no drive, and a square shoulder under the head that pulls into the material to prevent the bolt from turning when tightened. Hex bolts have a hexagonal head that is driven with a wrench.

Applications: Carriage bolts are often used for wood applications, where the smooth head is aesthetically pleasing and minimizes catch points. Hex bolts are more versatile, and used in construction, machinery, and automotive applications where higher torque is needed.

Carriage Bolts vs. Lag Bolts (Lag Screws)

Thread and Installation: Carriage bolts are machine-threaded and require a nut for fastening. Lag bolts have a wood screw thread and are screwed directly into wood or a pilot hole.

Use Case: Carriage bolts are better for situations where the bolt may need to be removed or adjusted since they can be easily unscrewed. Lag bolts provide strong holding power in wood but are less convenient to remove.

Carriage Bolts vs. U-Bolts

Shape and Purpose: U-bolts are shaped like the letter "U" with threads on both ends to secure items to poles or other structures. Carriage bolts are straight and used to join two pieces of material.

Applications: U-bolts are commonly used for piping, conduit, and to attach wires or cables. Carriage bolts are used in woodworking, metalworking, and in the assembly of furniture and outdoor structures.

Carriage Bolts vs. Anchor Bolts

Design and Use: Anchor bolts are designed to attach structural elements to concrete. They come in various shapes, including L-shaped, J-shaped, or straight with an expansion mechanism. Carriage bolts are used for wood or metal-to-wood connections and have a consistent straight design.

Applications: Anchor bolts are essential for securing sills and frames to foundations in construction, while carriage bolts are more suited for non-structural, decorative, or light structural applications.

Carriage Bolts vs. Flange Bolts

Head Design: Flange bolts have a flange under the head that acts as a built-in washer to distribute the load. Carriage bolts have a smooth, round head without a flange but with a square section beneath that embeds into the material to prevent rotation.

Applications: Flange bolts are often used in automotive and machinery applications where the flange provides extra surface area for load distribution. Carriage bolts are preferred for wooden constructions and decorative purposes.

Understanding Carriage Bolt Sizes

While sizes vary, carriage bolts range in diameter and length to accommodate different project needs, as well as the number of threads per inch. Reading the specs of a carriage bolt can be confusing, so here’s an example of our 1/2"-13 x 2" Conquest Carriage Bolt: reading this from left to right you would get a carriage bolt with 1/2" diameter, 13 threads per inch, and 2 inches long. When choosing carriage bolts, be sure to select both the correct diameter and length of bolt necessary for your project.

Carriage Bolt ‘Pitch’ or ‘Threads per Inch’

Threads per inch (TPI) is a measure used to describe the pitch of a threaded fastener, such as a bolt or screw. It specifically refers to the number of thread peaks along a one-inch length of the fastener. This measure is crucial for identifying the thread density of the bolt, which in turn affects how well the bolt can grip and hold materials together, as well as its suitability for different materials and applications.

For imperial carriage bolts (as opposed to metric carriage bolts), there are two thread types: coarse and fine. See the charts below for the different pitches of threaded coach bolts.

Bolt Diameter Threads per Inch (Coarse) Threads per Inch (Fine)
1/4″ 20 28
5/16″ 18 24
3/8″ 16 24
7/16″ 14 20
1/2″ 13 20
9/16″ 12 18
5/8″ 11 18
3/4″ 10 16
7/8″ 9 14
1″ 8 14

Materials Used to Make Carriage Bolts

Carriage bolts are manufactured from a variety of materials to suit different environmental conditions, strength requirements, and application needs. Here are some of the common materials used to make carriage bolts:

  • Stainless Steel

    Stainless steel carriage bolts are widely used due to their corrosion resistance and strength. They are ideal for outdoor applications, especially in marine or coastal environments where saltwater corrosion is a concern. Common grades of stainless steel used for carriage bolts include 18-8 and 316, with 316 offering higher corrosion resistance.

  • Carbon Steel

    Carbon steel bolts are strong and durable, making them suitable for construction and machinery. They can be further protected through coatings like zinc plating to enhance corrosion resistance.

  • Hot-Dip Galvanized Steel

    Hot-dip galvanized carriage bolts are carbon steel bolts coated with a thick layer of zinc. The hot-dip galvanization process provides excellent corrosion resistance, making these bolts suitable for outdoor use, particularly in wet or corrosive environments.

  • Zinc-Plated

    Zinc-plated carriage bolts are coated with a thin layer of zinc to provide corrosion resistance. While not as durable in harsh environments as hot-dip galvanized bolts, they are suitable for a wide range of indoor and less corrosive outdoor applications.

  • Silicon Bronze

    Silicon bronze bolts are known for their strength and excellent corrosion resistance, especially against saltwater. They are often used in marine construction and in applications requiring resistance to weathering and corrosion.

  • Aluminum

    Aluminum carriage bolts are lightweight and resistant to corrosion, making them suitable for applications where weight is a concern and in environments that are not highly corrosive.

  • Brass

    Brass carriage bolts are used for their appearance, corrosion resistance, and antimicrobial properties. They are commonly found in decorative applications and in environments where low friction is required.

The choice of material depends on the specific requirements of the application, including the need for strength, corrosion resistance, and aesthetics. Each material brings its own set of advantages and considerations, allowing for a wide range of applications across different industries.

Installation Guide for Carriage Bolts

To correctly use carriage bolts, start by drilling a hole of the appropriate diameter. Insert the bolt into the hole, using a hammer, if necessary. Position a washer on the opposite side, followed by a nut, to evenly distribute the force, protecting the material and ensuring the bolt's secure placement. Tightening the nut draws the square shoulder into the wood or material, locking the bolt in place.

Removing Carriage Bolts

The removal process is straightforward. Loosen the nut with a wrench without completely removing it. Strike the nut with a hammer or mallet to gradually withdraw the bolt. If the bolt becomes stuck, employ tools like a nail remover for extraction.

Advantages of Carriage Bolts

Opting for carriage bolts can enhance security, stability, and resistance to tampering in your projects, thanks to their unique head shape and locking mechanism.

Torque and Tools: Carriage bolts typically require less torque and are tightened from the nut side, often needing no tool for the bolt head due to its square shoulder. In contrast, most other bolts are designed to be driven from the head side and may require specific tools or higher torque levels.

Aesthetic and Safety: The smooth, rounded head of carriage bolts makes them ideal for applications where a smooth finish is desired or where safety is a concern, reducing the risk of injury from sharp edges.


Any Further Questions About Carriage Bolts

Our team of experts has a depth of knowledge in the fastener industry. If you have any additional questions, be sure to reach out and we'll be glad to help.

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