Carriage bolts are great fasteners for large outdoor projects like decks as well as shop projects like heavy-duty shelves and other furniture. One of the advantages of using a carriage bolt in a wood project is that its low-profile rounded head tightens flush to the wood surface, but there are times when you need the head to sink below the surface.
If you're a woodworker, your first inclination would be to countersink the hole, and you'd be almost right. What you actually need to do when installing a carriage bolt is counterbore the hole, and that's more than just a semantic difference. Depending on how soft the wood is, you may not even have to do that, because you may be able to get the head to sink just by tightening, but that approach won't work for shop projects using oak, maple or similarly hard woods.
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Working With Carriage Bolts
Carriage bolts differ from conventional bolt/nut types in that they actually have two heads. The head you see has a dome shape that's impossible to grip with a wrench and has no slot for a screwdriver. Hidden underneath that is a conventional square head that sinks into the wood to create an indentation that prevents the bolt from turning as you tighten the nut.
The part of the head that you see is large, and it has a flat underside, so it's very unlikely to penetrate the wood if you over-torque the nut, so you don't need a washer on the head side of the bolt, although you do need one on the nut side. You can use a carriage bolt on any type of wood or plywood, but it isn't recommended for MDF or particleboard, because those materials are too crumbly to prevent the square head from turning.
Counterbore vs. Countersink: What's the Difference?
Most woodworkers are familiar with countersinks, and although you can do one with a drill bit, it's more accurate and efficient to use a countersink bit. When you drill a countersink, you create an angled depression that conforms to the shape of a screw head. It allows the screw to seat firmly against the wood and hold securely.
Because a carriage bolt head has a flat profile, you need a hole with flat bottom, and a countersink bit can't do that. This type of depression is called a counterbore, and you can make one using either a Forstner bit or a spade bit. A spade bit is less expensive and works well for exterior projects like decks, but if you're working in the shop, you'll probably want to use a Forstner bit, because it's less likely to burn or chip the wood.
How to Install a Carriage Bolt
To properly counterbore a carriage bolt, you need two drill bits; one to make the counterbore depression and one to make the pilot hole for the bolt. Choose a Forstner or spade bit with a diameter slightly larger than the bolt head and drill the depression about 1/2 inch deep — or deeper if you're working with thick wood.
After you've drilled the counterbore, center the pilot bit — which should be the same diameter as the bolt shank or slightly larger — on the depression made by the counterbore bit and drill the hole. Insert the bolt into the hole, tapping with a hammer if necessary. Slip a washer onto the other end and tighten the nut with a wrench. Apply enough torque to draw the head into the counterbore and secure the bolt, and you're done.