What is "carriage bolt"

carriage bolt \carriage bolt\ n. a type of bolt threaded only at the end opposite the head, used mostly for fastening pieces of timber together, and inserted into pre-drilled holes.

n. A type of bolt with a rounded head and a square shank which will not turn.

n. a round-headed bolt for timber; threaded along part of the shank; inserted into holes already drilled

A carriage bolt, coach bolt or round head square neck bolt is a form of bolt used to fasten metal to wood.

It is distinguished from other bolts by its shallow mushroom head and that the shank cross-section of the bolt is circular for most of its length, as usual, but the portion immediately beneath the head is formed into a square section. This makes the bolt self-locking when placed through a square hole in a metal strap, or a round hole in most wood. This allows the fastener to be installed with only a single tool, a spanner or wrench, working from one side. The head of a carriage bolt is usually a shallow dome. The squared section is of the same size as the diameter of the bolt shank, with a plain unthreaded shank.

Carriage bolts were developed for use through iron strengthening plates on either side of a wooden beam. It is commonplace though to use them to bare timber, the squared section giving enough grip to prevent rotation.

Carriage bolts are extensively used in security fixings, such as the Brenton Bolt where the bolt must only be removable from one side. The smooth domed head and square nut below prevent the carriage bolt from being unlocked from the insecure side.