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?
A carriage bolt (also called a coach bolt and round-head square-neck bolt) is a type of bolt. It is also known as a cup head bolt in Australia and New Zealand.
It is distinguished from other bolts by its shallow mushroom head and the fact that the cross-section of the shank, though circular for most of its length (as in other kinds of bolt), is square immediately beneath the head.
This makes the bolt self-locking when it is placed through a square hole in a metal strap. This allows the fastener to be installed with only one wrench, working from the opposite end. The head of a carriage bolt is usually a shallow dome. The shank has no threads and its diameter equals the size of the square cross-section.
The carriage bolt was devised for use through an iron strengthening plate on either side of a wooden beam, the squared part of the bolt fitting into a square hole in the ironwork.
It is also not uncommon to use a carriage bolt on bare timber, the square section being allowed to sink into the soft wood Fibers when tightened, giving enough grip to prevent rotation. The principle is the same as in purpose-built timber bolts.
The carriage bolt is used extensively in security applications, such as locks and hinges, where the bolt must be removable from one side only. The smooth, domed head and square nut below prevent the carriage bolt from being gripped and rotated from the insecure side.
Types of carriage bolts
Common Types of Carriage Bolts
- Big Round Head Carriage Bolt. The DIN603 standard carriage bolt is the most common type. The large, round head design provides a larger contact area, distributing the fastener’s pressure across the connected surface.
- Small Round Head Carriage Bolt. Manufactured to ISO 8678 standards, featuring a compact, delicate semi-round head with a square neck. Designed for connecting precision equipment or small structural components where strict head size requirements are essential.
- 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. 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. These are elongated, round-head head square-neck bolts, generally featuring a partial thread. They are suitable for connecting larger components.
- 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. 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.
What are Carriage Bolts Used for?
Carriage bolts are ideal for fastening wood to metal. Alternatively, carriage bolts can also be used to fasten two pieces of wood together. Some specialty versions of carriage bolts allow for the effective fastening of two separate metal components. Furthermore, they can be used in a variety of different industries, including the following:
- Water conservation and treatment industry,
- Railroad industry,
- Farming industry, and
- Mining industry, to name a few.
In order to use carriage bolts, you will need a few additional tools on hand, including a hammer, a drill, a drill bit, a nut, and a washer. Remember, carriage bolts should only ever be inserted into pre-drilled holes.
This is due to the specific shape of a carriage bolt’s head. It is always smooth and round, which means that it is not possible to utilize a drilling device in order to screw it into the material.
Begin by drilling a hole into the material of an appropriate size, considering the size of the carriage bolt itself. Slide the carriage bolt into the hole that you have created. If it is a tight fit, you can use a hammer to gently move it into position. You will then need to attach your washer and your nut.
Place the washer on the rear side of the bolt, and follow it with the nut. A washer is important because it helps to reduce any damage when you use the nut to pull the carriage bolt into the material.
Tighten the nut to move the carriage bolt into its rightful place. The goal is to get the underside of the head of the carriage bolt to sit snugly against the material.
Removing the carriage bolt is also a relatively straightforward process and can be done by twisting the nut, but ensuring that it remains on the bolt. Hit the nut with a hammer to start easing the bolt out of the material, and continue this process until you can remove the bolt by hand.
What’s The Difference Between Carriage Bolts And Lag Bolts?
Although similar, a lag bolt or screw differs from a carriage bolt. Lag bolts typically feature a hexagonal head and pointed tip for being driven into place, whereas carriage bolts have rounded button heads and are flat-tipped for use with nuts and washers.
What’s The Difference Between Carriage Bolts and Coach Bolts?
There is no difference between carriage bolts and coach bolts; they are two names for the same bolt. The names are used interchangeably due to the historic use of these bolts in both carriages and coaches.
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.