Carriage bolt


A carriage bolt, is square immediately beneath the head. This makes the bolt self-locking when it is placed through a square hole in a metal strap. This allows the fastener to be installed with only one tool, a spanner or wrench, working from one side. The head of a carriage bolt usually is a shallow dome. The shank has no threads; and its diameter equals the side of the square cross-section.
The carriage bolt was devised for use through an iron strengthening plate on either side of a wooden beam, the squared part of the bolt fitting into a square hole in the ironwork. It is common to use a carriage bolt to bare timber, the square section giving enough grip to prevent rotation.
The carriage bolt is used extensively in security fixings, such as locks and hinges, where the bolt must be removable from one side only. The smooth, domed head and square nut below prevent the carriage bolt from being unlocked from the insecure side.

Timber bolt

Closely related to the carriage bolt is the timber bolt, meant to fasten wood to wood, for use with large wood planks and structures. It has a domed head that is proportionally wider than that of a carriage bolt. Instead of the carriage bolt's square part of the shank immediately under the head, the timber bolt has four fillets, whose sharp corners grip the edge of the hole in the wood to prevent rotation.

Plough bolt

The plough bolt is a flush-fitting carriage bolt, whose head is countersunk beneath the surface of the wood. The plough bolt was devised to hold replaceable ploughshares to the mouldboards of iron ploughs. The share, the fastest-wearing part of the plough, would be replaced several times over the life of the plough. Such bolts continue to be used to hold shovels onto cultivators.