Screws Bolts

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Carriage Bolts

Carriage bolt, coach bolt or round head square neck bolt is a form of bolt.

It is distinguished from other bolts by its head: the shank of the bolt is circular for most of its length, the portion immediately beneath the head is formed into a square section. This makes the bolt self-locking when placed through a square hole, 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.

Plough bolts are a flush-fitting carriage bolt, where the head is countersunk beneath the surface. They were first developed to hold replaceable ploughshares onto the mouldboard of iron ploughs. The share is the most quickly wearing part of the plough and would be replaced several times over the life of the plough.


Pieces of timber are often secured together using one or more carriage bolts during construction of homes, outdoor decks, and pilings. These bolts are commonly used in chain link fences, patio and ready-to-assemble furniture, machinery assembly, and even in automobiles. Since the square fitting keeps the bolt from turning, carriage bolts are a good choice for situations in which the head of the bolt cannot be easily accessed.

When using carriage bolts, it is important to select a bolt of the right width and length for an application. A bolt that is too narrow may lead to the fastening failing under strain, causing an object to break. If the carriage bolt is too big, it can stress the surrounding wood or metal, causing cracks. Length is also important, as the bolt must be long enough to go through the material being fastened, but not so long that the protruding end becomes a nuisance.