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What is the Difference Between Carriage Bolts and Coach Bolts?

What are Carriage Bolts?

Carriage bolts are a type of hexagon bolt specialized for use in woodworking, in particular, fastening metal to wood. Carriage bolts are designed with a domed head, which can prevent loosening from one side, an enlarged head shape also prevents the bolt from being pulled through a wooden construction. The presence of a domed head does mean that carriage bolts cannot be installed by methods that push the bolt axially during installation, such as a spanner drill adaptor.

The square head section of a carriage bolt allows it to be installed using a spanner, for improved leverage. A square section also provides some level of locking ability when tightening into soft materials such as wood. An unthreaded section below a carriage bolt’s square section adds strength and resistance to shear forces.

The downside of carriage bolts is that their head shape makes them incompatible with torque wrenches and drill adaptors.

Carriage bolts also provide several secondary benefits, such as the aesthetic appeal of their rounded head, which can give a finished appearance to many applications. Despite being intended for use with wood, carriage bolts have a machine screw thread, and are equally suitable for metal applications. However, carriage bolts may be more difficult to install due to their incompatibility with drill adaptors.

It is important to remember that this definition does vary depending on manufacturer and country - What is considered a carriage bolt may be a coach bolt or lag bolt to another user, so one should always exercise caution to ensure the purchase of the correct bolt.

What are Coach Bolts?

Coach bolts, frequently called coach screws, are self tapping fasteners with the head of a hexagon bolt. Coach bolts are commonly used in fastening wood to metal and wood to wood. The unthreaded section of a coach bolt adds strength to its construction, and makes it more resistant to shear forces when compared to fully thread variants such as self tapping hexagon bolts.

While specialized drives such as the pozi drive are specially designed to resist cam-out, the sturdy hexagon head of a coach bolt is even more resistant to slippage, and is typically more suitable for heavy-duty applications.

Due to their suitability for more heavy-duty applications and their compatibility with larger tooling, coach bolts are generally available in larger sizes than other self tapping screws.

Accu's definition of a carriage bolt, pictured left, and coach bolt, pictured right.

What Is the Difference Between Coach Bolts and Carriage Bolts?

While coach bolts and carriage bolts are distinct screw types, they do share similarities in their general head shape, and the fact that they are suitable for use with wood. The most significant difference is the coach bolt’s presence of a self-tapping thread, which makes it able to create its own threads in timber - by contrast, a carriage bolt features a machine thread, so will always require an adequately sized pilot hole.

The difference in application for these two screw types is also not as simple as boiling down to the difference between a carriage and a coach. Confusingly, carriage and coach are near synonymous terms, and carriage bolts and coach bolts can be found in many designs for both types of vehicles.

While there is little in the way of concrete evidence for the origin of the term ‘carriage bolt’. One theory is that it originates from the Old French ‘cariage’, which does not refer to carriages in a vehicular sense, but is more closely related to the English word ‘carry’, potentially because this type of bolt was meant for load-bearing applications, rather than being specifically designed for use in the manufacture of carriages.

Why Manufacture Coach and Carriage Bolts from Stainless Steel?

Coach bolts and carriage bolts are frequently used in heavy-duty or load-bearing applications, which may make it seem counter-intuitive to use stainless steel in their construction.  But in fact, most coach bolts are made from mild steel because it means they can bend slightly thus accommodating small movements in timbers over time without snapping.  For applications where tensile strength is of absolute importance, stainless steel fasteners may not be preferable to hardened steel alternatives, but the appeal of stainless steel lies in its chemical resistance.

Coach and carriage bolts are frequently used outdoors, which causes unprotected steel to rust rapidly. Certain wood types are also acidic, and may contain corrosive natural chemicals. In these applications, stainless steel coach screws and carriage bolts are an ideal alternative.

Exhibit A - From 'A Treatise On Carriages' 1794

Etymology Of Carriage Bolts

It won't come as a surprise to learn that these bolts were originally used to secure the traces by which a carriage is drawn by horses. The evidence is shown in the extracts from antique books shown here. Special thanks to John Stallard of the Carriage Museum of America for pointing us in the right direction in our search for proof.

Exhibit B - From 'History Of The Bolt And Nut Industry In America' 1905

If you are interested in reading more of these fascinating antique tomes - the links below allow you to do so:

A Treatise on Carriages By William Felton (1794)

History of the Bolt and Nut Industry of America By W.R. Wilbur (1905)

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