A bolt is a mechanical fastener with an external helical thread that typically engages an internal thread in a nut or a tapped hole. Bolts form a principal class of threaded fasteners used to clamp assemblies together by converting turning torque into axial force (preload). They range from tiny machine screws to large structural studs and are manufactured in many sizes, materials and finishes to match strength, corrosion resistance and appearance requirements.

Design, parts and common types

Fundamental bolt elements include the head, the shank and the threaded portion. The head provides a surface for tools to apply torque; heads come in many shapes designed for specific tools or aesthetics. Typical head styles include:

  • Hex head (standard for wrenches)
  • Socket head (internal hex)
  • Carriage bolt (rounded head with a square neck)
  • Lag screw or lag bolt (large coarse thread for wood)
  • T-bolt (a rectangular head for insertion into slots)

Threads can be coarse or fine, straight or partially rolled, and follow different standards depending on region and application. A threaded bolt engages a nut or a tapped element to produce clamping force; when driven directly into a softer material the fastener is more often called a screw, although the distinction is not absolute. Thread form and pitch affect strength, ease of assembly and vibration resistance; the term helical thread describes the spiral geometry common to bolts and screws.

Materials, finishes and mechanical properties

Bolt materials include carbon steels, alloy steels, stainless steels, titanium and nonferrous metals. Mechanical grades indicate tensile strength and are often represented by markings on the head. Surface finishes such as zinc plating, hot-dip galvanizing, black oxide or specialized coatings improve corrosion resistance and reduce galling. Proper selection of grade and finish is important for safety-critical uses like structural connections or automotive assemblies.

History and standards

The standardization of threads and fastener production emerged alongside the Industrial Revolution. Thread standards created predictable mating parts and simplified manufacturing and maintenance. Today bolts are specified using international and national systems (for example ISO metric threads, UNC/UNF, DIN and ANSI standards) that define dimensions, tolerances and identification marks. Thread standards and grade systems help engineers communicate strength and compatibility requirements.

Uses, installation and safety

Bolts are ubiquitous in construction, machinery, vehicles, appliances and consumer goods. Proper installation requires matching bolt, nut and washer types, applying recommended torque to achieve the designed preload, and using locking methods where needed (locknuts, thread-locking adhesives, prevailing torque features). Incorrect torque or improper lubrication can lead to under‑preload, loosening, or bolt fracture. For joining wood, builders commonly use lag bolts that are driven into a pilot hole or directly into timber (wood). T‑bolts are frequently used in slotting systems and clamping fixtures.

Distinctions and notable facts

Although the terms bolt and screw are sometimes used interchangeably, a practical distinction is that bolts are intended for use with a nut or a tapped hole and rely on clamping force across components, while screws often form their own mating thread within the material. Many hybrid forms exist and standards bodies avoid rigid one-word rules; selection should be guided by application, load type (tension versus shear), environmental exposure and service demands.

For engineers and technicians, understanding bolt geometry, material properties and installation practice is essential for reliable joints. Further technical references and specification tables are available from standards organizations and fastening manufacturers for detailed selection and testing criteria.