Carriage bolts are like standard bolts but with a different head. The head, or top, of a carriage bolt is a round, dome shape with a square shank protruding from the flat side of it.
From the square shank the threads extend out like a standard bolt. Carriage bolts are found in many different sizes, both in metric and SAE (Society of Automotive Engineers), and are most commonly made from zinc, galvanized or brass-coated steel.
Ease of Use
Carriage bolts are most commonly used where a flat or a more smooth surface is desired, such as in the bed of a pickup truck. Whereas a standard bolt head protrudes and prevents ease of sliding material across the surface, a carriage bolt sits lower and allows material to slide up and over it.
Carriage bolts are also used for their appearance, such as on wooden picnic tables. The smooth, flat surface may be desired, but it isn't as critical as on a pickup truck.
A carriage bolt adds a “finished” look to the table surfaces.
Carriage bolts offer an advantage when working with wood. The square shank on the base of the head is pulled down into the wood and holds the bolt in place when a nut is tightened onto the carriage bolt--while a standard bolt would require a tool to hold the bolt for tightening the nut.
Carriage bolts can be used wherever a standard bolt is used. However, if the surface material is hard, like metal, the hole the carriage bolt is to go into needs to be the same square shape as the square shank to hold the bolt for tightening.
Working With Wood
When you use a carriage bolt, identify where the bolt is to be placed and mark the spot with a pencil for drilling. Select a bit that is the same size as the threads of the carriage bolt.
Drill as straight as possible through the material so the drill bit protrudes from the wood on the other side. Remove the drill and place the carriage bolt in the hole.
It is OK if it is a tight fit as long as it goes down the hole. Place a peace of scrap wood over the head of the carriage bolt and hit the wood with a hammer to set the bolt.
The wood will hold the square shank for tightening. On the other side, place a washer, then a lock washer and, finally, the nut, and then tighten.
Working With Metal
Mark and drill the metal as you would with wood. Remove the drill and place the carriage bolt in the hole for a test fit.
The square shank should not fit in the hole. Remove the bolt and use a file to make the opening of the hole square and the same size as the square shank of the carriage bolt.
Constantly fit the bolt to check progress on the hole's shape and size. Eventually, the square hole will hold the square shank for tightening.
On the other side, place the washers and nut, and then tighten.