An Introduction to Carriage Bolts

Huyett Marketing Department
10/17/2024

Introductory Guide to Carriage Bolts Blog Hero Image

Carriage bolts, also referred to as DIN 603 bolts, are a type of bolt widely used in wood-to-wood, wood-to-metal, and even metal-to-metal applications on some occasions. Their unique design, featuring a smooth, domed head shape and a square or knurled neck, sets them apart from other threaded fasteners by allowing for secure fastening without the need to twist the bolt into place.

Finding use in applications from construction, and woodworking to security applications like signage and playground equipment, these versatile fasteners provide an array of benefits. Here, we discuss what they are, how they are used, how to measure them, and how they are different from other similar bolts.

What Is a Carriage Bolt?

Carriage bolts (also known as cup head square neck bolts, cup although it is also used to connect wood to metal and sometimes metal to metal. It features a shallow, smooth dome head; a square, knurled, or finned under head; and a threaded shank with a blunt end.

Unlike most other types of bolts, carriage bolts are not screwed or twisted into position, even though they have threads. The shallow head is hammered to secure the fastener in place, while the typically square neck locks it into the mating material and prevents spinning. The threaded shank grips into the material and accepts washers and nuts to secure the bolt from the other end.

Carriage Bolt Anatomy

These bolts have three distinct anatomical elements: the head, the under head, and the shank.

The Head: The head features a mushroom-shaped domed head and a flat underside; it is similar to a timber bolt, but timber bolts have wider, flat heads to eliminate the need for a washer.
The Under Head: Located just beneath the head, the under head (or neck) is typically square-shaped, although it can be knurled or finned. All styles help prevent spinning as the nut is tightened onto the shank, but the square provides the strongest hold (hence why it is the standard style).
The Shank: The shank is either partially threaded or fully threaded up to the point where threads meet the neck; the end of the shank is blunt.

What Is a Carriage Bolt Used For?

Carriage bolts are used in countless applications, like outdoor applications for structural and/or aesthetic wood-to-wood connection needs, although they are found in metal-to-wood and metal-to-metal settings as well. They are also used in similar applications that require some degree of tamper-resistant capabilities and may be sold as an assembly with anti-theft nuts; for example, these bolts often secure stop signs onto wooden support beams and require anti-theft nuts that partially snap off the shank, preventing removal from the shank end.

Other common applications include:

Docks and wooden walkways
Decking
General
Wood construction
Signage
Railroads
Woodworking
Furniture
Construction projects
Playground equipment
Mining industry
Heavy-duty wood applications that require a stronger hold than timber bolts or wood screws
Secure applications that require tamper-resistant capabilities

While these are common use cases, carriage bolts are used in a wide range of applications and may be found in settings beyond those noted here.

How to Measure a Carriage Bolt

There are four main dimensions involved when measuring a carriage bolt:

Head Diameter (A): The distance across the head profile.
Head Height (B): The distance from the flat underside of the head to the top of the dome.
Length (C): The distance from the bottom of the shank to the flat underside of the head.
Thread Size (D): Notated as: (outside thread diameter x thread pitch/TPI).

Of these four dimensions, thread size and length are most often used to determine the carriage bolt size.

To determine the thread size, it is important to know what "thread pitch" and "threads per inch (PTI)" mean and how they inform thread dimensions. Check out our "Quick Guide to Reading Thread Sizes" for a short, practical explanation.

Quick Guide to Reading Thread Size Chart

How to Install and Remove a Carriage Bolt

The installation process for carriage bolts is very simple and only requires typical hand tools and fasteners, but you will need access to both sides of the application. You will need a drill and drill bit (to create pre-drilled holes), a nut, a washer, and a hammer.

Pre-drill the hole to the same diameter as the bolt.
Insert the bolt into the hole.
Tap the head of the bolt with the hammer until it fits snug in the hole (the threaded shank does not aid installation at this point - the bolt is not tightened into position from the top).
Attach the washer (if needed) to the protruding threaded shank; then attach the nut to follow it.
To tighten the carriage bolt, hand-tighten the nut until you can no longer tighten it, then use a wrench or rachet to continue until the square neck is pulled into the mating surface.
When finished, the bolt's flat underside should sit flush against the mating surface.

To remove the bolt, simply reverse the installation process:

Loosen the nut until it is flush with the end of the blunt shank.
If you want to save the bolt, keep the nut on the end of the shank and tap the shank with the hammer to loosen the square neck. If you do not plan to keep the bolt intact, you can remove the nut entirely and just hammer the shank.
Continue either hammering the shank end or loosening and hammering the nut until the bolt protrudes from the top enough to pry it out with the hammer claw.

Carriage Bolts Compared to Other Bolts

There are all kinds of bolts available, and many have overlapping functions and similar designs. Here, we look at what makes a carriage bolt unique when compared to three similar fasteners: step bolts, plow bolts, and lag bolts.

Step Bolts

Perhaps the most closely related to carriage bolts, step bolts look and function almost identically, right down to the square neck. The main difference is the head; step bolts have rounded heads, but they are wider and shallower than the head of carriage bolts. These fasteners are used where a flush surface is desirable, like decking or steps (hence the name).

Plow Bolts

Plow bolts are also often confused with carriage bolts because of their similar design. However, the head is again the distinguishing factor; plow bolts have a flat, countersunk head to provide a flush, flat surface. Plow bolts are also used more for metal applications.

Elevator Bolts

Elevator bolts have the same threaded bolt shank and square or knurled neck, but they have a flat, smooth head as opposed to the carriage bolt's domed head. They are most often used in applications like grain elevators or other moving assemblies because the flat head is unobstructive.