A bolt is a type of threaded hardware fastener that is used to position two workpieces in specific relation to each other. Bolts come in several configurations for their application and specification variances.
Since the terms "bolt" and "screw" were in use before the advent of easily-produced helix fasteners, they are often synonymous. However, several standards bodies have attempted to differentiate the terms, with many concluding that it is not the devices which are different, but how they are used. As provided by Machinery's Handbook and ASME B18.2.1, bolts are externally threaded fasteners that are prevented from being turned during assembly, but are positioned or released by torqueing a nut. Screws are externally threaded fasteners that can be inserted into pretapped holes or can perforate a material and create their own internal threads. Screws are fastened by torque applied to the head. This definition is still somewhat ambiguous and is not all-encompassing, but does provide a basis to begin differentiation.
Image credit: Engineer Explains
The components that form a bolt can be identified in three sections. The head is the part of the bolt with the largest diameter, which provides a mount for tools to either apply or resist torque. It also provides part of the bearing surface for substrates being bolted. The shank of the bolt is the longest part of the bolt and has external, helical threads on its circumference. This piece is responsible for the alignment of the workpieces. Finally, the end opposite of the head is known as the chamfer, which provides a slightly beveled edge to aid the bolt's insertion into holes and nuts.
Successful bolt implementation almost always requires a nut to retain components on the bolt's axis. Nuts are applied via torque while the bolt itself is held in place, or vice versa. Nut position and bolt employment are dependent upon the same physical principles: the friction of the bolt and nut threads against one another, which is comprised of a slight stretch of the bolt from the compression resistance of the components being fastened, and a slight elastic deformation of the parts held together.
Video credit: Ian Collier / CC BY-SA 4.0
The dimensions of a bolt can be expressed in metric or imperial units, both of which are better explained in the Bolt Standards section of this selection guide.
Image credit: UC Davis Engineering
- Pitch is the measurement between the apex of adjacent threads on the bolt's shank.
- Diameter is the width of the screw shank. This should not include the bolt head.
- Length is the measurement of the bolt from the edge of the chamfer to the undercut of the head. Against this measurement should not include the head.
- Aluminum screws are light, resistant to oxidation, thermal and electrical conducive, and easy to manufacture.
- Brass screws are strong, conductive, and corrosion resistant, with low magnetic permeability.
- Copper alloy screws have good load capacity, wear resistance, and are suitable for use near magnets.
- Plastic screws are inexpensive and corrosion resistant for light loads. They are common for applications near water, such as pools.
- Steel screws are produced of strong, carbonated iron. Uncoated steel is vulnerable to corrosion.
- Hardened steel screws are stronger than steel screws, but more brittle. They are made of steel treated by tempering and quenching methods.
- Stainless steel screws are chemical and corrosion resistant with an appealing finish. They cannot be hardened like carbon steel.
- Screws consisting of superalloys exhibit good mechanical strength, surface stability, corrosion resistance, and resistant to creep at high temperatures. Common super alloys include Hastelloy®, Inconel®, Incoloy®, and Monel®.
- Titanium screws are hard and strong, light, and corrosion resistant. When alloyed with other metals, it increases strength and durability.
Bolts are regularly marked to indicate their strength, which is dependent on the material and dimensions of the bolt.
Metric class and marking
Imperial grade and marking
Metric/imperial proof load
|Low or medium carbon steel||
Medium carbon steel, quenched & tempered
|Medium carbon alloy steel, quenched & tempered||
|Alloy steel, quenched and tempered||
Finishes applied to the base metal of the bolt enhance the durability and corrosion resistance of the material.
- Black oxide finish does not enlarge the dimensions of the screw and is a processed black rust. It is mostly used for aesthetic purposes.
- Chrome coating is a bright, reflective finish that is decorative and very durable. It is applied via electropating.
- Zinc plated coatings act as a sacrificial anode, protecting the underlying metal. It is applied as a fine white dust.
- Other coatings like galvanization and phosphating are common for particular types of hardware, like screws meant for fence or window applications.
Manufacturers produce bolts in both imperial and metric units, and due to their explicit correspondence with nuts and pretapped holes, the units cannot be mixed. Furthermore, screws are produced with fine or coarse threads, which are a designation of the screw's thread pitch--not the quality of the product. Coarse-thread bolts are less susceptible to galling, thread crossing, and seizing, while fine-thread bolts are less likely to loosen from jostling, and are more easily tapped and adjusted.
For imperial sizes, bolt producers will provide a designated number or fractional inch for the dimension of the screw. North American countries follow this system, called the Unified Thread Standard. Also included will be a "threads per inch" (TPI) count which will indicate a course or fine thread.
ISO, JIS, and DIN standards are based upon the metric system and are closely related (ISO guideline 261). Most hardware measured in metric units is subject to Society of Automotive Engineers (SAE) edict as well. These standards are common in the majority of the world, and speculation exists that this may become a global hardware standard. The threads on metric bolts can be available in both fine and course pitch configurations.
Bolt Head Styles
Table credit: Engineering Parts Sourcing Inc.
Image credit: Portland Bolt
|Anchor bolts are embedded in concrete or masonry for structural applications.|
Image credit: Bolts 'n Nuts Plus
|Carriage bolts are used to fasten metal to wood. The undercut of the bolt head is square to hold the bolt in place once tightened.|
Image credit: TOPS Inc
|A flat, plain or countersunk head with a squared undercut holds the bolt in place when a nut is tightened. These are common in conveyor systems.|
Image credit: Packer Fastener & Supply
|Flange bolts have an integral washer on the undercut of the bolt head that distributes the bearing load. These are also known as frame bolts.|
Image credit: Monster Fastener
|A hanger bolt does not contain a bolt head. Both ends are threaded, but with one end containing a wood screw.|
|Hexagon bolt/Tap bolt||
Images credits: Portland Bolt; Fastwell Ind. Co.
|Hex bolts have six-sided heads. Tap bolts are threaded the entire length of the shank.|
Image credit: MoWiNet
|Lag bolts are also called lag screws. They create their own mating thread in wood and other soft materials when tightened.|
Image credit: Infly Industry Inc.
|Machine bolts are meant for assembling metal components through predrilled holes with an accompanying screw. These are more commonly called machine screws.|
Image credit: K & R Supply, LLC
|With a countersunk flat head, a square shank neck, and a unified thread pitch, plow bolts are durable and common in construction devices.|
Image credit: Wilson Glass
|Sex bolts do not require a nut. They come with a mating female component that covers the bolt shank. They are useful for fastening items that cannot be exposed to the abrasive threads.|
Image credit: Unified Supply
|More commonly know as shoulder screws. Visit GlobalSpec's Shoulder Screw Selection Guide and Specification Filter for tutorial information.|
|Square head bolt||
Image credit: Melfast
|Square head bolts a four-sided bolt head with a machine screw thread.|
Image credit: Xiamen Landee Ind. Co.
|Stud bolts utilize hex nuts on both ends of the bolt. The workpieces are held between the two bolts.|
Image credit: Jiaxing
|Timber bolts are explicitly meant for use with large wood planks and structures. They are common in lumber and marine industries.|
Image credit: Shanghai Mountain Co.
|A T-head bolt has a T-shaped head which can fit into a slot or can be gripped easily by a wrench.|
Image credit: Johson Industrial
|With an expanding wing-like nut, toggle bolts are used to mount objects to walls. The nut inserts through a hole before it expands to lay flat against the surface.|
Image credit: Portland Bolt
|U-bolts are partially threaded on both ends and are bent in the shape of the letter 'U', with either a radius or two right angles.|
MSC; CFC Distributors; Admiral Ship Supply
Read user Insights about Bolts