Coach Bolts, Carriage Bolts & Lag Bolts - What Makes Them Different?

In precision engineering, distinguishing between components like carriage bolts, coach bolts and coach screws is crucial. These fasteners, while similar in naming conventions, have unique features and applications that are essential for project integrity, as well as compatibility with other parts of an assembly

Grasping the distinctiveness of each component is crucial, especially when considering the structural benefits and specific uses. This article aims to demystify these fasteners, exploring what they are used for, how they look, how they compare against each other and even looking at even more names they go by.

Contents:

• Overview Of Carriage Bolts
• Overview Of Coach Bolts
• Overview Of Coach Screws
• Overview Of Lag Bolts
• Overview Of Lag Screws
• Pros & Cons

What Is a Carriage Bolt?

The carriage bolt, a historical staple in the toolkit of engineering, is a type of fastener distinguished by its unique head design. It is specifically crafted to provide a smooth and low profile fastening with little protrusion and fastened with a nut on just one side.

What Does a Carriage Bolt Look Like?

The carriage bolt definition is characterized by its round, dome-shaped head and a square section under the head. It has a long partially machine threaded shaft which is uniform in diameter.

Designed to sit flush against surfaces, they offer a sleek and clean appearance. The square section under the head is developed to bite into softer materials such as wood to stop the bolt from spinning when being installed.

What Are Carriage Bolts Used For?

In this day and age, these bolts are used in a wide spectrum of engineering projects, particularly in situations where the square part underneath the head can be taken advantage of. Additionally, the smooth head can prevent snagging which is sometimes a crucial element. They are also popular in outdoor applications, such as building decks and fences, thanks to their aesthetic appeal.

Despite being intended for use with wood, carriage bolts have a machine screw thread and are equally suitable for metal applications provided a square slot is provided for the head to sit in. For example, in flatpack metal constructions like industrial racking or furniture, where a square recess is cut to stop the bolt from spinning, allowing it to be fastened or loosened with just one tool.

When Were Carriage Bolts Invented?

Carriage bolt usage dates back to the early 1800s, initially designed for fastening metal to wood in cart and carriage construction, hence their name. This is evidenced in extracts from the History Of The Bolt And Nut Industry Of America and A Treatise Of Carriages. Special thanks to John Stallard of the Carriage Museum of America for pointing us in the right direction in our search for proof.

Carriage Bolt vs. Hex Bolt

Unlike the domed head and square underneath of carriage bolts, hex bolts feature a hexagonal head, making them suitable for applications where torque can be applied from both sides of the fastener, but less ideal for instances where a smooth exterior surface is needed. Hex bolts also usually require additional washers under their head for a precise installation.

What Is a Coach Bolt?

A coach bolt, also known as a carriage bolt, is known for being a heavy-duty fastener used primarily in construction and joinery.

Coach bolts have a domed toolless head, with a square undercut designed to slot into or create its own square indentation to stop spinning once it is being tightened. With machined threads, they are fixed into place using washers and nuts on the threaded end as the head end is designed to remain static during tightening.

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

They are identical, carriage and coach in the bolt variant refer to the same type of fastener. This common misconception arises thanks to their similar appearance and functionality. We could aim to guess that this confusion came about because of the naming conventions between horse-drawn carriages and coaches which are also one and the same. Perhaps just as people may think a coach and a carriage are different, so too may their fixings.

What Are Coach Bolts Used For?

Traditionally, coach bolts were used to connect the chassis of a cart to the upper seated frame forming a stagecoach. Nowadays, they are used for wood-to-wood or wood-to-metal connections where the advantages of that square undercut in the head can be taken advantage of like in the assembly of heavy wooden structures like decks and timber frames.

Coach Bolt vs Coach Screws?

Bridging into the next section of this article, this is where the naming convention does begin to differ thanks to the differentiation between bolt and screw. Coach screws are large self-tapping wood screws designed for heavy-duty wood joinery. They differ from coach bolts in their design, featuring a hexagon-shaped head, deep self-tapping threads and a pointed tip. In contrast to a coach bolt which has a domed head with a square part underneath, machine threads and a flat uniform tip.

What Is a Coach Screw?

Here's where the difference begins! A coach screw is a robust, heavy-duty self-tapping screw typically used in large wood construction projects that require fixings to be driven in from a single side.

What Does a Coach Screw Look Like?

Featuring a hexagonal head, pointed tip and a coarsely pitched threaded shaft, their primary design is for secure fastening in wood or other composite materials similar to wood. The hex head makes installation with a socket adapter quick and efficient while the self-tapping threads allow a coach screw to form its own threads as it is driven into a pilot hole.

What Are Coach Screws Used For?

Primarily to be used as heavy-duty wood screws, coach screws are ideal for heavy timber construction, such as in framing, landscaping, and in securing heavy components. In woodworking, there are an infinite amount of possible use cases, however, in most cases, they are preferred because of their coarse threads and hexagon drive head which offer excellent grabbing power and torque transmission.

When Were Coach Screws Invented?

While there is no exact timeline for this, judging from the excerpts earlier shown we can theorise that the idea of the coach screw was around in the 18th century evolving from earlier screw designs used in coachbuilding, however, it may not have had that exact name.

What Is the Difference Between Coach Screws and Coach Bolts?

While coach screws and coach bolts are distinct screw types, they do share similarities in the fact that they are suitable for use with wood. The most significant difference is the coach screws’ presence of a self-tapping thread, which makes it able to create its threads in timber - by contrast, a coach bolt features a machine thread, so will always require an additional fastener or threaded hole for installation. The other differentiating factor is that coach screws have a hexagon head and coach bolts have a domed head with a square underneath.

What Is the Difference Between Coach Screws and Wood Screws?

Technically speaking, a coach screw is a wood screw, however, coach screws are larger, heavier, and stronger than standard wood screws that you may find at your local hardware store, making them suitable for heavier applications. If you had to spot on the difference, it's that traditional wood screws often are equipped with a Phillips or Pozi drive type, however, coach screws have a hexagonal head.

What Is the Difference Between a Coach Screw and a Bolt?

Unlike traditional bolts, coach screws do not require nuts as they are designed to be screwed directly into a pilot hole. They also have a pointed tip which helps them to tap their own threads as opposed to bolts which tend to have a blunt tip in a similar diameter as the threads. Lastly, unlike fully threaded bolts, coach screws have an unthreaded section near the head of the screw allowing for greater shearing resistance.

Coach Screw vs Lag Bolt

While this article discusses two distinct fastener profiles, here's where we begin to stray from usual naming conventions as both a coach screw and a lag bolt are the same component.

We know this is confusing due to their names ending with either screw or bolt, which earlier in this article is where we have pinpointed the difference, however, language is always full of funny yet misleading nuances like this. Rest assured, when comparing coach screws vs lag bolts, they are the same component.

What Is a Lag Bolt?

As we've eluded, a lag bolt perfectly mirrors the lag screw in functionality and design, serving as a robust fastener for connecting heavy materials. Identically, it has a hexagonal head and a pointed tip with self-tapping threads along the shaft. They're integral in construction where a secure and enduring bond is essential, particularly in timber framing, decking, and building supports.

What Is the Difference Between Lag Bolts and Lag Screws?

When we compare lag bolts vs lag screws, we're once again met with the confusing naming convention between bolt and screw. In this specific circumstance when comparing lag bolts vs lag screws, they are exactly the same fixing often used synonymously in the hardware and construction industries.

Lag Bolt vs. Carriage Bolt

Looking at the differences between a carriage bolt vs lag bolt, the lag variant has a sharp tip, coarse self-tapping thread designed for pre-drilled pilot holes in wood and a hexagonal head designed to be driven with a socket set offering superior grip. In contrast, the carriage variant has a smooth rounded head design with a square underneath which prevents the bolt from spinning during installation and uniform machine threads designed to be matched with a nut and washer.

What Is a Lag Screw?

A lag screw, also recognized as a lag bolt or coach screw, is a substantial and sturdy wood screw often utilized in wooden structures. It has a hexagonal head similar to a bolt, a partially threaded shaft with deep self-tapping threads and a pointed tip making it perfect for installation into a pilot hole.

Its design is optimized for creating strong connections in heavy timber and other robust materials. Known for its size and durability, it plays a crucial role in ensuring structural integrity in various building projects.

Is a Lag Screw the Same as a Coach Screw?

Yes, a lag screw is the same as a coach screw in every sense. The "lag" terminology is much more common in the United States than it is in Europe and the United Kingdom.

The term lag screw originates from their original use being to secure barrel staves, also known as lags. These lags were arranged cylindrically, then metal hoops tapped on to form the barrel shape, lastly, lag screws were used to secure everything in place.

Lag Screw vs Carriage Bolt

Lag screws are industrial self-tapping wood screws with a sharp tip and a hexagonal head offering strong gripping power in wooden structures, making them a preferred choice for heavy-duty construction. In contrast, carriage bolts, with their smooth domed heads with a square undercut and machined threads, are designed for applications where a snag-free surface is desired and where the square underneath can bite into the material to stop it from spinning.

Which Should You Use? Pros & Cons

Now that we know what each of the two fasteners goes by, what they look like and what sets them apart, we can round off this article by looking at the pros and cons of each component to offer you a simple way to know in what scenarios you should use each one.

So there we have it, everything you could ever want to know about carriage bolts, coach bolts, carriage screws, lag screws and lag bolts.

We hope this article has helped you to understand the naming conventions of these two unique fasteners and provided you with insight on which one you should use in your next woodworking project.