hex set screw

The Time and Place for using Long Coach Bolts

Long coach bolts are usually called carriage bolts or coach carriage bolts are used mainly in the construction and wooden applications. Usually confused with Coach Screws, Bolts vary in size and shape, number of threads and shape of head, etc. Let us look into detail how they can be used and what are the best practices to follow.

Coach Bolts: A Basic Definition

The easiest way to distinguish a coach bolt from others is by its head. They feature a shallow domed head, which is enlarged for a good reason. This large head prevents it from being pulled into the wood when you are tightening the nut on the other end. But the domed head prevents installation methods that push the bolts axially. Tools like spanners and wrenches come in handy during the installation process.

Coach bolts usually have a square underside below the head, giving them the name ‘cup square hex bolts’. The nuts will be hexagonal as well. These bolts have metric coarse threads. This means the threads are wide apart, which is considered being more durable.

The advantage of coarse thread is that there is more material between each of the threads. They are also known to have fewer chances of stripping a cross threading, which are likely to reduce the strength of the structure.


The installation of a coach bolt has to be done carefully like a hex set screw. You must know the size of drill bits and other important calculations to make sure the bolts stay well within the construction. Here is a brief installation guide on how to install a coach bolt.

  • Measure the Coach Bolt

Coach bolts are measured using the ISO International standard metric thread size. The size of the bolts translated to the outer diameter of the thread. We prefix each bolt size with the letter “M”. That is for a bolt with thread size 6mm is called an M6 bolt. There are other sizes like the M8, M10 and M12.

Another prominent part of bolt measurement deals with the length of the bolt. There are 40mm, 50mm, 100mm, 130mm and 180mm coach bolts, of which the latter ones are used for heavy-duty applications.

  • Get the suitable drill bit and make a hole

Once you know the size of the long coach bolts, it is time to dig a hole in the material. For this, you need to find the right drill bit size, unless you want a loose bolt after installation. It is quite easy to find the right drill bit size. For an M10 diameter, you will need a 10mm drill bit and for an M12 bolt you must use a 12mm bit. So you must use the same drill bit size as the coach bolt diameter. This ensures a perfect fastening for a long period.

  • Install the bolt into the hole

Since the hard part is done, now is time to install the bolt. The bolt can easily be slid down the hole and you can also employ a hammer to make the process easy. But use minimal force the bolt does not have any structural damage.

  • Secure the bolt with the nut

The last step of the process is to tighten the bolt from the opposite side. If the bolt comes with a washer, then the washer must be on the opposite side as that of the head of the bolt. This is a common error made by inexperienced personnel.


One might wonder what the problem is with using normal small nuts and bolts to fasten. But with sturdy wood and metal, small nuts and bolts will not be strong enough to hold the weight of the entire structure. Instead, most construction workers and woodworkers use coach bolts to fasten two large pieces of wood or metal and wood.

Some Common Alternatives

Another common way to fasten two materials in heavy duty situations is using a threaded rod. These can be used with wood, metal and even concrete to provide stability to the structure either for temporary use or as a permanent solution. Cutting threaded rods can be used, but they break more easily compared to long coach bolts under pressure in some situations. Needless to say, the fastening of rods takes more time and manual effort compared to that of a coach bolt.

Set screws are another popular option to secure two materials. But the distinguishing factor is the absence of a head. On the other hand, there are hex set screws which have a hexagon head. It stays inside the material it is installed by exerting the clamping force through the tip of the screw. This can be a problem in case of thin and weak wood types. It is hard to conceal a screw, as they are easily visible apart from the material destruction discussed above.

Wrapping Up

One common way in which coach bolts were used in earlier days was with a washer. This was because washers used to prevent damage to the surface during the process of installation. This way, the material will stay intact and the structure will be sturdy as well.

Using a bolt with a washer can reduce the load on it evenly. The stress on the material will be removed as the pressure of the load gets dispersed evenly. The threads themselves will not move or damage under pressure too.