What is a carriage bolt and when to use it?

What is a carriage bolt?

Carriage bolt is a fastener with round head and square neck. Unlike hexagon bolt, it cannot be tightened directly with a wrench, and needs to insert a special groove with the washer and nut to achieve the purpose of fastening.

Why is it called a carriage bolt?

In the early days of the 19th century, the carriage wheels often loosened in muddy potholes, the designer designed the round head square neck bolt to install the wheels, this design can reduce the loosening problem caused by vibration, not easy to lead the wheel damage.

What are some other names for Carriage Bolts?

Carriage bolts are also called coach bolt, round head square neck bolt, carriage screw, mushroom head square neck bolt, pan head bolt, Dome Head Bolts, Cup Head Bolts, plow bolt (Countersunk head carriage bolt), fin neck bolt.

Try these other names when you want to search for the carriage bolts.

Types of carriage bolts

Common Types of Carriage Bolts

  • Ribbed Neck Carriage Bolts

Ribbed Neck Carriage Bolts

Ribbed neck carriage bolts are a type of carriage bolt that differs from the standard square neck design. Instead of a square neck, these bolts feature a ribbed neck, which includes a series of ribs or ridges around the neck. This ribbed design provides additional grip and prevents the bolt from rotating when inserted into a fastening material.

  • Short Neck Carriage Bolts

Short Neck Carriage Bolts

These are square neck carriage bolts specifically designed for connecting two thin pieces. They are smaller in size and suitable for medium to low strength connections.

  • Long Neck Carriage Bolts

Long Neck Carriage Bolts

These are elongated round head square neck bolts, generally featuring a partial thread. They are suitable for connecting larger components.

  • Square Neck Flat Head Carriage Bolts

Square Neck Flat Head Carriage Bolts

Unlike regular carriage bolts, these have a countersunk head to save space. They are primarily used where a flush connection surface is required.

  • Plastic Head Fin Neck Carriage Bolts

Plastic Head Fin Neck Carriage Bolts

Also known as silo bolts, these have four fins on the neck and a plastic-coated head, making them suitable for outdoor connections.

Depending on your needs, you can also customize the carriage bolts with cross slots or straight slots on the head.

Carriage bolt sizes

The standard sizes of carriage bolts range from M5 to M20, and from 3/16 inch to 3/4 inch in diameter. The lengths available are from 15mm to 300mm, 20mm to 200mm, and from 1/2 inch to 16 inches. Custom lengths and sizes can also be made to order.

Material and grade of the carriage bolts

Stainless Steel Carriage Bolts

Stainless steel carriage bolts excel in appearance and corrosion resistance, making them suitable for exposed components. Their surface is polished and chrome-plated. Common materials include 304, 316, and 316L stainless steel, with grades such as A2-50, A2-70, A4-70, and A4-80. Additionally, duplex stainless steel such as S32750, S32760, S2205, and S2507 can also be used to manufacture carriage bolts, although they are more costly.

Carbon Steel Carriage Bolts

Carbon steel carriage bolts have higher strength levels compared to stainless steel but are less resistant to corrosion unless surface-treated. To enhance corrosion resistance, these bolts are typically zinc-plated or hot-dip galvanized, forming a protective layer that allows them to withstand extreme conditions, making them suitable for outdoor use. The strength grades of zinc-plated carriage bolts range from 3.6, 4.6, 4.8, 5.6, 6.8, to 8.8.

Hot-dip galvanized carriage bolts have strength grades of 8.8, 9.8, 10.9, and 12.9. Black oxide carriage bolts, similar to hot-dip galvanized ones, feature a black coating that prevents corrosion and loosening in harsh environments. These bolts are made from high-carbon steel, with strength grades of 8.8, 10.9, and 12.9.

Aluminum Carriage Bolts

Aluminum carriage bolts are lightweight, suitable for components that do not require high strength. The strength grades for aluminum carriage bolts are 2024, 6061, and 7075.

Brass Carriage Bolts

Brass carriage bolts naturally resist corrosion without needing surface treatment, with a strength typically ranging between 200-300 MPa.

brass carriage bolt

About the standard for carriage bolts

The Chinese standard of carriage bolts is GB12, GB14, GB801. German standard DIN 603 carriage bolts American standard ASME / ANSI B18.5 carriage bolts, Japanese standard JIS B1171 carriage bolts, ISO 8678 is the international standard of carriage bolts

What are carriage bolts used for?

Do you know When to Use Carriage Bolts? The earliest use was only for the installation of wagon wheels. Now you can do it with use a carriage bolt in wood, use a carriage bolt in metal, and use a carriage bolts for deck.

Practical applications such as the installation of doors, the connection of furniture, the fixation of outdoor railings, cars, aircraft, trains, and construction, and electrical equipment all use carriage bolts.

How to instal a carriage bolt?


STEP 1. Prepare the necessary tools, including an electric drill, hammer, wrench, and several common sizes of drill bits.


STEP 2. Mark the location where the carriage bolt will be installed and clear the surrounding debris.


STEP 3. Prepare to drill a pilot hole that is slightly smaller than the diameter of the bolt’s neck. This will help ensure that the bolt remains secure once installed.

STEP 4. Insert the carriage bolt into the pilot hole, align the two connecting pieces, and then screw on the nut.


STEP 5. Use a wrench or socket to tighten the bolt. If you want to check the torque precisely, use a torque wrench to prevent over-tightening and damaging the threads.


The above is a comprehensive explanation of the carriage bolts. When you choose and use the carriage bolts, you need to understand its structure, materials, specifications and the correct installation method, so as to know which kind of fasteners are what you need most.


How to remove carriage bolts?

The removal method for a carriage bolt is similar to that of a regular bolt. First, gather the necessary tools for removing the bolt, such as a hammer, socket, or wrench.


Align the tool with the nut and turn it counterclockwise. It might be a bit difficult at first, so you can tap the socket with a hammer a few times. Once you feel it slightly loosen, you should be able to remove it easily.

Where to buy carriage bolts?

There are many ways to buy carriage bolts, a few of which can be bought in a hardware store. But if you want to buy a large number of carriage bolts can consider us to buy, we can give you a reasonable price, at the same time the size and material can be customized according to your needs, a salesman is responsible for following up your order. If necessary, contact us now and look forward to your consultation.

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