What Size Drill Bit Do I Use To Drill A Hole For A Carriage Bolt?
A common question we get when discussing carriage bolts is “What size drill bit do I use to drill a hole for a carriage bolt?” The answer is simple. Use the same size drill bit as the diameter of the bolt. If you are using a 1/2″ diameter bolt use a 1/2″ drill bit.
Installing A Carriage Bolt
Carriage bolts are very misunderstood and yet very simple to install. Made primarily for wood, carriage bolts have a square shoulder right below the head. This shoulder is designed to catch on wood and be pulled into it (by tightening the nut). Often you will find the domed head of a carriage bolt countersunk into the wood. To learn how to make a countersunk hole you can check out this video we made: How To Drill A Countersunk Pilot Hole.
The first thing to do when installing a carriage bolt is to determine the diameter of your carriage bolt. After you figure out the diameter with a thread gauge or other measuring tool, find a wood drill bit with the same diameter and then get your drill ready.
Now that you have your drill bit, mark the wood and drill out the hole. Depending on the type of wood drill bit you use to make the hole, the carriage bolt may slide right into the hole or be very tight. If it is very tight grab a hammer and tap the rounded head of the bolt so it slides into the hole. *Pro Tip: Since you already have the hammer out, once the square shoulder gets down to the wood give it a couple solid hits, so the square portion begins to sink into the wood.
Take a washer and nut and fasten them to the carriage bolt against the wood. As you tighten the nut against the washer, it will pull the square shoulder into the wood preventing it from spinning. Once the domed head of the carriage bolt is tight against the wood the carriage bolt is installed.
Carriage bolts are surprisingly simple to understand and yet many people still struggle with them. Hopefully this post will help you to understand which size drill bit you will need and how to install a carriage bolt into wood.