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A bolt is a form of threaded fastener with an external male thread requiring a matching pre-formed female thread such as a nut. Bolts are very closely related to screws. Bolts are designed to pass through an unthreaded hole in a component and be fastened with the aid of a nut, although such a fastener can be used without a nut to tighten into a threaded component such as a nut-plate or tapped housing. They are often used to make a bolted joint. This is a combination of the nut applying an axial clamping force and also the shank of the bolt acting as a dowel, pinning the joint against sideways shear forces. For this reason, many bolts have a plain unthreaded shank (called the grip length) as this makes for a better, stronger dowel. The presence of the unthreaded shank has often been given as characteristic of bolts vs. screws, but this is incidental to its use, rather than defining.
Bolts use a wide variety of head designs, as do screws. These are designed to engage with the tool used to tighten them. Some bolt heads instead lock the bolt in place, so that it does not move and a tool is only needed for the nut end. Many bolts use a screwdriver head fitting, rather than an external wrench. Screwdrivers are applied in-line with the fastener, rather than from the side. These are smaller than most wrench heads and cannot usually apply the same amount of torque. It is sometimes assumed that screwdriver heads imply a screw and wrenches imply a bolt, although this is incorrect. Coach screws are large square-headed screws with a tapered wood screw thread, used for attaching ironwork to timber. Head designs that overlap both bolts and screws are the Allen or Torx heads; hexagonal or splined sockets. These modern designs span a large range of sizes and can carry a considerable torque. Threaded fasteners with screwdriver-style heads are often referred to as machine screws whether they are being used with a nut or not. There are many types of bolts, such as hex bolt - Bolt with a hexagonal head and threaded body. Section immediately under head may or may not be threaded; carriage bolt - Bolt with a smooth rounded head and a square section to prevent turning followed with a threaded section for a nut; anchor bolt - Bolt designed to allow objects to be attached to concrete. The bolt head is usually placed in concrete before it has cured or placed before the concrete is poured, leaving the threaded end exposed; U-bolt - Bolt shaped like the letter U where the two straight sections are threaded. A straight metal plate with two bolt holes is used with nuts to hold pipes or other round objects to the U-bolt.