Bolt

(design engineering) A rod, usually of metal, with a square, round, or hexagonal head at one end and a screw thread on the other, used to fasten objects together.
(forestry) A short section of tree trunk.
(materials) In veneer production, a short log of a length suitable for peeling on a lathe.
(mining engineering) bolthole
(ordnance) The sliding part in a breechloading weapon that pushes a cartridge into position and holds it there as the gun is fired.
(textiles) The entire length of cloth from a loom.

A cylindrical fastener with an integral head on one end and an external screw thread on the other end designed to be inserted through holes in assembled parts and to mate with an internally threaded block, called a nut, which is turned to tighten or loosen the bolt. Tensioning the fastener by turning the nut differentiates a bolt from a screw, which is tightened by turning its head. See also Bolted joint; Nut (engineering).

Bolts are generally manufactured from metals, but bolts made of other materials, such as nylon, are commercially available. The properties of some bolting metals are modified by heat treatment and other means to increase yield strength. Bolt heads have various shapes to suit different applications (see illustration). Hexagon-headed bolts (hex bolts) are available in two head widths for the same body diameter—regular hex bolts and heavy hex bolts, which have wider heads. The heavy-series bolts are normally supplied with heavy nuts that are wider and thicker than regular nuts.

countersunk bolt; and (e) round-head bolt.">
Examples of bolts inserted through clearance holes: (a) square bolt; (b) hex bolt; (c) round-head square-neck bolt for connecting wood to metal; (d) countersunk bolt; and (e) round-head bolt.

Various types of bolts may be used for automobile, machinery, appliance, farm implement, and structural connections. For example, low-carbon-steel unfinished bolts with hex heads are used in machinery, and with square heads for structural steel connections. Heat-treated medium-carbon-steel finished hex-head bolts are high-strength bolts used for connections in structures as well as machinery. However, there are two kinds of high-strength bolts made specifically for structural steel connections, both kinds are heavy hex structural bolts (dimensions differ slightly from those of heavy hex screws). Other kinds of bolts are medium-carbon-steel or atmospheric-corrosion-resistant-alloy-steel, quenched and tempered bolts; and alloy-steel or atmospheric-corrosion-resistant-alloy-steel, quenched and tempered bolts. See also Screw fastener; Washer.