Carriage Bolts

A carriage bolt - often referred to as a coach bolt, although they are technically different components - is a threaded fastener designed for woodworking application that largely connect wood to metal, but they also work to connect wood to wood. Carriage bolts feature a driveless, domed head with a square neck just under the head and a shank with partial machine threads. Typically, the bolt is installed with a washer and a nut, but the washer is not always necessary. The threads provide extra grip in soft wood or metal materials, while the square neck locks the fastener in to the mating surface at the top.

Application Considerations

Carriage bolts facilitate the connection of wood or metal. Before putting these fasteners to use, it is helpful to consider a few key factors to evaluate how the bolt will perform in the application setting.

Accessibility: You need to have access to both sides of the application to install or remove carriage bolts. Think about how your surface is set up and if you will be able to reach both sides.
Surface Material: Whether you are connecting wood to wood or wood to metal, the wood should have enough density to keep the bolt in place. Any metal should be able to support the bolt without deforming under its installed pressure.
Type of Force: Carriage bolts are better suited to withstand shear force (sideways force). If your application will apply significant tension (force that pulls in opposite directions like tug-of-war), you may want to consider a different fastener. Either way, think about how your application will apply force to your bolt.
Environmental Elements: As with any application, it's important to know if your application will be exposed to corrosive elements like rain, temperature fluctuation, sun damage, chemical, etc. This will influence the material and finish required for your bolt.

Benefits

There are several advantages that carriage bolts offer through their time-tested design. They are:

Aesthetic Appeal: Carriage bolts feature a smooth head that offers a clean appearance to finished applications. The wide head also helps prevent the head from digging in and deforming the mating material.
Easy Installation: Carriage bolts require little support for installation - a washer (sometimes), a nut, a hammer, and a predrilled hole are all it takes for a quick, simple process.
Little Risk to Over-Tightening: The wide bolt head helps distribute the installation force to keep the head from digging into the mating material, while the square neck helps prevent the risk of hammering the bolt too deeply into the wood.
Extra Holding Power: The machine threads on these bolts offer better holding power in wood than fasteners with nonthreaded shanks.

Where Did Carriage Bolts Get Their Name?

In short, carriage bolts are named such because they were a key component in the wheels of carriages as far back as the early 1800s. Since carriages did not typically travel on smooth surfaces, the wheels were often jostled by potholes, uneven roads, etc. They needed fortified fasteners that could withstand the performance pressure and resist dislodgement. Hence, the threads, square neck, and head design of the carriage bolt emerged.

What is the Difference Between a Carriage Bolt and a Coach Bolt?

This is a common question surrounding these bolts. In many cases, the terms are used interchangeably - this isn't far off the mark, since they are very similar. They serve the same purpose and they both feature a driveless head and shank. However, there is one key difference: the threads.

Carriage Bolt: Features machine threads that require a predrilled hole in the wood for installation
Coach Bolt: Features self-tapping threads that allow the bolt to be installed without a predrilled hole