Why Should You Use a Carriage Bolt?


This is a picture of a green wooden carriage. Large image.

So, your plans call for 3 carriage bolts to hold the top planks to a frame. Why should you use a carriage bolt? Can you use something else?

This is a picture of a carriage bolt. It has a hexagonal shaped neck and also no washer next to the nut. Small image.
Carriage Bolt

Well, the short answer is: You should use carriage bolts in order to fasten wood to metal. Carriage bolts make this process quick and easy. You just need to drill a hole the size of the bolt through the wood. The carriage bolt has a square or hexagonal neck under the rounded head. You should make an opening in the metal with the size and shape of the neck. When you tighten the bolt, the neck gets pushed into this opening. This will prevent the bolt from spinning as it’s tightened. Carriage bolts fasten the pieces very securely. However, there’s really more to the story:

Carriage Bolt Definition

First, it’s good to start with a definition of a Carriage bolt:

A round-headed bolt for timber, threaded along part of its shank, inserted into holes already drilled.

Dictionary.com

As you can see, carriage bolts can also be used to fasten two wood pieces together. After all, this is a woodworking blog. For that reason, let’s confine our discussion to wood.

For example, you might want to make an outdoor fence and attach a wooden fence slat to a wooden support piece. Should you still use a carriage bolt?

In this case, you won’t have the metal piece to hold the neck and prevent the bolt from spinning as you tighten the nut. The neck can then cut the wood up as the bolt spins.  When this happens, it’s difficult to tighten the bolt. This is because the rounded head has no screwdriver slot. That is, you cannot use a screwdriver to stop the bolt from turning.  The only way to tighten the bolt is to tighten the nut. You’re out of luck if the bolt keeps spinning!

Timber Bolt To The Rescue

Actually, a better approach might be to use a variation of the carriage bolt known as a timber bolt. This type has “fins” under the head instead of the square or hexagonal “neck”. As the bolt is tightened in wood, the fins “dig in” to the wood and hold the bolt in place. They will prevent it from spinning.

This picture shows locking wrench pliers. It is best to avoid pliers like these when tightening a carriage bolt. That is becase these wrenches often chew up nuts or put tooth marks on them. They can cut through plating and make the fastner less resistant to the environment. Small image.
Locking Wrench Pliers – Avoid With Carriage Bolt

You should also consider using washers on the bolt.  In particular, you should have one between the nut and the wood. This is because the nut will chew up the wood. This is even more of a problem with a soft wood such as Pine. A washer can prevent the nut from actually contacting the wood and ruining the surface.

Also, you might want to use a wrench rather than pliers to tighten the nut. Pliers often leave tooth marks on the nut, and can easily slip off. The marks are unsightly and can also make the bolt less resistant to rust and moisture.  You can also get marks on the nut if you use locking wrench pliers or an adjustable wrench. However, a wrench the exact size of the nut should work properly, and allow you to tighten the nut without difficulty.

Is A Lag Bolt The Same As A Carriage Bolt?

Another fastener you can use is the lag bolt. This type of bolt does not have a nut. Instead, it has a thread much like a screw. The head has a square or hexagonal shape. Tighten the bolt with a wrench. This should reduce the chance of the wood cracking as the lag bolt is tightened. However, you need to be concerned about the size of the pilot hole. If it’s too large, the threads will not get an adequate bite of the wood as the bolt is screwed in. If this happens, the joint will be weak. It’s often best to try things out with scrap wood first and satisfy yourself that the hole size is correct before proceeding with the actual project.

The lag bolt may hold the wood more securely than the carriage bolt since the screw threads will dig into the wood. However, you have no nut at the end of the bolt to hold the pieces together. So, the joint can actually be weaker if the threads fail to hold the pieces securely. This can happen if there is a sharp enough jolt to the joint, the wood pieces crack, or the pieces are pushed apart with enough force.

However, an advantage of the carriage bolt is that it usually takes less time and effort. Just put the bolt in the pilot hole, tighten the nut, and you’re done. No problem.

Also, the rounded heads of carriage bolts have a finished look and they can be put on the visible surface of a project. For example, picnic tabletops often show rounded carriage bolt heads.

Drill A Counterbore Hole For Your Bolt

If you want to hide the rounded head, you can drill a counterbore hole for the carriage bolt. Use a Forstner bit or Spade bit for this. Use it to drill a hole bigger than the diameter of the rounded head. Set the depth of this hole to be the desired depth of the rounded head. Then, you can use a bit with the diameter of the bolt to finish the hole through the wood piece. Put the center of this hole at the center mark of the large hole.

What About The Outdoors?

What if you want your bolt to be outdoors? Again, I refer to the example of an outdoor picnic table. The carriage bolts on it will be exposed to the elements and moisture. In a short time, they will begin to rust. Is there anything you can do?

This picture shows rust on the rounded head of a hardware fastner. Small image.
Rust On Bolt Head

Actually, you have several options. You can handle this problem by treating the bolts and by using particular materials. Here are some examples:

  • Electroplated Treatment – The bolts get a thin coating of zinc. This makes it more resistant to corrosion. Also, the process isn’t too expensive. However, scratches and marks on the bolt will decrease it’s resistance to rust. In addition, the plating will get thinner and wear off over time.
  • Galvanized Treatment  – These bolts have a thicker coating of Zinc material. Note that scratches or marks on the bolt can wear through this plating, and make the hardware less resistant to weather. Also, the coating will wear off in time with exposure to the elements.
  • Stainless Steel Material –  Bolts and nuts made of stainless steel offer good resistance to corrosion. Another advantage is that the entire piece of hardware is resistant to the environment, not just the surface plating. However, these bolts are often not as hard as other materials. In addition, this alternative is much more expensive.

Source of corrosion resistance information

What If The Carriage Bolts Are Too Long?

Of course, you should try to purchase carriage bolts of the correct length. However, this isn’t always practical or possible. Often you need to use what’s available in your supply of hardware.

If your bolts are really too long, one approach is to tighten the nut, then cut off the excess bolt with a hacksaw. You may need to file the cut edge down because it might be sharp. You certainly don’t want people to cut themselves on it! Please be aware, this process may remove the zinc coating or plating on the bolt, making it less resistant to the environment.

Issues Sanding Wood

How do you sand the wood surface if the rounded head is protruding through it? One approach is to sand the pieces before inserting the bolts. However, you may need to do a final sanding even after the bolts have been put in place.

In cases like this, you might really want to drill counterbore holes for the bolts. These put the head below the surface of the wood. However,  these are not really countersinking holes. They do not have chamfer angle tapers to accommodate wood screws.

When you’re finished, you’ll be able to put the carriage bolt into the counterbore you’ve just made. The head will be below the surface so that it won’t interfere with sanding, either by hand or by machine. If this is not done, you risk tearing up pieces of sandpaper as they rub against the rounded heads of the carriage bolts.

Why Should You Use A Carriage Bolt: Conclusion

Yes, you should use carriage bolts. They are often very convenient and easy to use. Also, they don’t take too much time or require extra effort. We showed how to use carriage bolts with wood pieces: A better alternative is often the variation of carriage bolt known as a timber bolt. We also discussed several issues and considerations regarding the use of carriage bolts.

Well, now it’s up to you. Do you have anything to add? Feel free to make comments in the section below.

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