Coach Bolts – A Complete Guide

What are coach bolts and what are they used for? Understand the different types, materials and sizes in our guide.

What are Coach Bolts?

Coach bolts, also known as carriage bolts, are a type of bolt designed specifically to be used in woodworking. In particular, they are intended to be used for fastening metal to wood.

Carriage bolts feature a shallow domed head, typically either square, circular, or hexagonal in shape. The enlarged head shape is specialised to prevent loosening from one side and it also helps to prevent the bolt from being pulled through a wooden construction.

However, the domed head also means that coach bolts cannot be installed using methods which push the bolt axially during installation – such as using a spanner drill adaptor, for example. Instead, the use of tools such as wrenches, spanners, or pliers is required to turn the bolt.

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What are Coach Bolts Used for?

Carriage bolts are intended primarily for use with materials such as wood and metal. These bolts can be used to fasten wood to metal, or for joining different types of timber together. They are ideal for tougher situations where a more heavy-duty fastening solution is required, hence the fact that they are often employed at critical structural joins.

Coach bolts were originally designed to be used alongside square plate washers, but nowadays, many modern carriage bolts can be used alone. These components are regularly used in construction and are suitable for uses ranging from DIY projects to large-scale industrial applications.

Coach Bolt Types

Multiple different coach bolt types are available, each with different specifications and best suited to a variety of purposes and applications. Popular types include countersunk coach bolts and flat head carriage bolts, but some of the most common carriage bolt types you may encounter are detailed below:

DIN 603 Coach Bolts

Deutsches Institut für Normung (DIN) standards are used with a wide range of components and fasteners. DIN standards are used across Europe to ensure quality and easily identify parts for different applications. They can also be used either for components that do not have an ISO (International Organisation for Standardisation) equivalent, or in conjunction with ISO standard parts. Coach bolts are DIN 603 – the standard for mushroom-head square neck bolts. This indicates that the bolt conforms to the DIN metric standard for fasteners.

Square Headed Coach Bolts

The square head section of a coach bolt enables it to be installed using a spanner. This affords the user improved leverage and makes for an easier installation process. The presence of the square head also provides a certain degree of locking ability, particularly notable when the bolt is being tightened into softer materials like wood. Many square-headed coach bolts also feature an unthreaded section below the carriage bolt’s square head. This is useful as it provides additional strength and resistance to shear forces.

What is the Difference Between Coach Bolts and Coach Screws?

Although coach bolts and coach screws are distinct components in their own right, the two do share a number of similarities. The most notable similarity is the head shape, which is domed, or mushroom-shaped, in both instances. Both coach screws and bolts are also best suited for use with wood.

However, a key difference is that coach screws feature a self-tapping thread. This means that these screws can create their own threads in timber, ensuring a more seamless installation. On the flip side, carriage bolts have a machine thread, meaning that they always need an adequately-sized pilot hole to be drilled into the material before installation.

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Coach Bolt Materials

Coach bolts are typically made from steel, mild steel, or stainless steel to ensure a durable, long-lasting fastening solution. Many stainless steel coach bolts are made from A2 304 stainless steel. This steel is widely used in a variety of industries and boasts properties including high levels of strength and strong oxidation resistance.

Steel carriage bolts are also available with a range of different coatings and finishes, suitable for varying applications. Plain coach bolts are the standard, but bright zinc-plated (BZP) carriage bolts are a popular alternative, thanks to the added protection against corrosion that they can offer. Ideal for indoor environments, bright zinc-plated coach bolts are typically used for applications such as fastening timber joists.

Other coach bolt materials and finishes you may encounter include:

Brass coach bolts
Black iron coach bolts
Galvanised coach bolts (ideal for exterior use and weather-proof to a certain degree)

Lastly, specialist coach bolts intended for outdoor environments and use with products like pressure-treated timber are also available. These parts feature additional corrosion-resistant characteristics to suit their intended install environment.

Coach Bolt Sizes

Carriage bolts are available in a wide range of sizes, ideal for many different applications. Coach bolt sizes can be recorded in either metric or imperial measurements, so it is important to select the most suitable carriage bolt size for your specific requirements. Additionally, the sizes can refer to several different measurements with these parts, including bolt length and thread size, so you must carefully select the best coach bolt for the task at hand.

Carriage Bolt Thread Size

Coach bolts are typically measured in metric threads using the international standard ISO metric thread size system. Each bolt is measured and given a size which is denoted by a number preceded by the letter ‘M’. The outer diameter of the thread (measured in millimetres) is indicated by the number in the system. For instance, a coach bolt with a thread size of M6 would have a 6mm thread diameter.

Once again, care should be taken not to confuse carriage bolts with coach screws as the two are different parts despite their similar name and head shape. As a rough guide, coach screws are typically measured in millimetres rather than simply using the ISO metric system alone.

Browse some of the most popular coach bolt thread sizes below:

Carriage Bolt Length

Coach bolts are available in an extensive variety of different lengths, ranging from small and compact parts to much larger, heavy-duty carriage bolts.

Top Tip!

If the carriage bolt you are using is longer than the width of the material, consider using a washer alongside the bolt to prevent damaging the joint or reducing load capacity.

Shop some of the most common coach bolt lengths below: