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A carriage bolt, coach bolt or round head square neck bolt is a form of bolt used to fasten metal to wood.
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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.
Timber bolts
Closely related to carriage bolts are timber bolts, meant for use with large wood planks and structures. They have a domed head that is proportionally wider than that of a carriage bolt, and instead of a square section of shank under the head, they have four sharp-cornered fillets that grip the edge of the hole in the wood to prevent rotation. They are also known as mushroom head bolts or dome head bolts. They are used to fasten wood to wood, instead of metal to wood.
Plough bolts
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.
Such bolts continue to be used to hold shovels onto cultivators.