Bolts & Screws

What are bolts and screws?

Bolts and screws are threaded fasteners used to join parts together securely and allow them to be taken apart again when needed. They are among the most widely used engineering components, holding together everything from machinery, vehicles, and steel structures to bearing housings, guards, brackets, and electrical equipment. A bolt or screw has a head at one end and a thread along all or part of its length, which screws into a nut or a threaded hole to create a strong clamping force.

Bolts and screws are made in a huge range of head styles, thread sizes, lengths, materials, and strength grades, in both metric and imperial sizes. Choosing the correct one is important, as the wrong size, thread, or strength can lead to loose joints, stripped threads, or failure.

Bolts and screws are usually described by:

• Thread size - the nominal diameter of the thread, such as M8, M12, 1/4 inch, or 1/2 inch
• Thread pitch - the distance between threads in metric sizes, such as 1.75 mm on an M12 coarse thread, or the number of threads per inch on imperial sizes
• Length - usually measured from under the head to the end of the bolt, except on countersunk screws, which are measured overall
• Head style - such as hexagon, socket cap, button, or countersunk
• Strength grade - such as grade 8.8 for metric high tensile steel, or grade R for some imperial fasteners
• Material and finish - such as zinc plated steel, self-colour steel, or stainless steel
• Standard - such as DIN 933 or BS 1083, which set the dimensions and specification

For example, an M12 × 20 grade 8.8 hexagon setscrew to DIN 933 has a 12 mm thread diameter, a 20 mm length, a 1.75 mm coarse pitch, and is made from high tensile steel.

Key features:

• Strong, secure joints - creates high clamping force to hold parts together
• Removable - joints can be taken apart and reassembled for maintenance
• Wide range of sizes - metric and imperial sizes for new and older equipment
• Choice of head styles - to suit different tools, spaces, and appearances
• Strength grades - to match the loads in the application
• Corrosion protection - plated, galvanised, and stainless options
• Standardised - made to recognised standards for consistent sizes and quality

In simple terms, bolts and screws are threaded fasteners that clamp parts together securely, while allowing them to be removed when needed.

Why are bolts and screws essential?

Bolts and screws are essential because almost every machine, vehicle, and structure depends on threaded fasteners to hold it together. The strength and reliability of an assembly is often only as good as the fasteners holding it. Using the wrong size, thread, grade, or material can lead to loosening, stripped threads, corrosion, or broken bolts, causing breakdowns or safety risks. Choosing the right bolts and screws keeps assemblies strong, safe, and easy to maintain.

Their main benefits are:

• Reliable clamping - holds parts tightly together under load and vibration
• Easy maintenance - parts can be dismantled and reassembled for repairs and servicing
• Adjustability - allows parts to be positioned and adjusted during assembly
• Strength options - high tensile grades carry heavier loads
• Versatility - suits metal, plastic, and wood, in light and heavy-duty applications
• Compatibility - metric and imperial sizes allow repairs to modern and older equipment
• Corrosion resistance - the right material and finish protect fasteners in harsh environments
• Cost-effective - an economical and reliable way to join parts

Bolts and screws are used across a wide range of industries and applications, including:

• Engineering and maintenance - machinery, guards, frames, and bearing housings
• Construction and steelwork - structural connections, frames, and brackets
• Automotive and transport - engines, chassis, suspension, and bodywork
• Agricultural machinery - tractors, implements, trailers, and equipment
• Manufacturing and assembly - production machinery and assembled products
• Electrical and electronics - enclosures, panels, and terminals
• Marine - deck fittings and equipment exposed to seawater
• Classic and older machinery - imperial bolts and screws for older equipment and vehicles
• DIY and home projects - general repairs, furniture, and fabrication

The main types of bolts and screws

Bolts and screws are usually grouped by head style, thread length, thread type, strength, and material.

By thread length:

• Hexagon bolts - partially threaded, with a plain shank under the head. The plain shank gives extra strength where shear loads act across the joint. Made to standards such as DIN 931 and ISO 4014
• Hexagon setscrews - fully threaded right up to the head, ideal where maximum thread length is needed or for screwing into tapped holes. Made to standards such as DIN 933 and ISO 4017

By head style:

• Hexagon head - the most common head style, tightened with a spanner or socket
• Socket cap screws - round heads with an internal hex socket, tightened with an Allen key. Compact and strong, ideal for tight spaces and recessed holes. Made to standards such as DIN 912 and ISO 4762
• Button head socket screws - low, rounded heads with a hex socket, giving a neat finish. Made to standards such as ISO 7380
• Countersunk socket screws - sit flush with the surface in a countersunk hole. Made to standards such as DIN 7991 and ISO 10642
• Flange bolts - have a built-in flange under the head that spreads the load, removing the need for a separate washer
• Pan head and countersunk machine screws - smaller screws with slotted, cross, or Pozi drives, used for light assemblies and electrical equipment
• Grub screws - headless screws with a hex socket, used to lock parts onto shafts, such as pulleys and collars. Available with cup, flat, cone, and dog points

Other common types:

• Carriage bolts - have a domed head and a square neck that grips the material, stopping the bolt turning as the nut is tightened. Commonly used in wood and fencing
• Coach screws - large screws with a hexagon head and a wood thread, used for heavy timber fixings
• Eye bolts - have a ring or eye for lifting or attaching
• U-bolts - U-shaped bolts used to clamp pipes, tubes, and round sections
• Threaded rod and studs - threaded along their length, used with nuts at both ends

By thread type:

• Metric coarse - the standard thread for most engineering applications, such as M6 × 1, M8 × 1.25, M10 × 1.5, M12 × 1.75, and M16 × 2
• Metric fine - finer pitch threads that give greater thread strength and finer adjustment, common in automotive and precision applications
• BSW (British Standard Whitworth) - an older British coarse thread, still found on older machinery and equipment
• BSF (British Standard Fine) - an older British fine thread, found on older vehicles and machinery
• UNC and UNF - American Unified coarse and fine threads, used on American and some older British equipment

By strength:

• Grade 4.6 - lower strength steel for light-duty applications
• Grade 8.8 - high tensile steel, the most widely used grade for engineering and machinery
• Grade 10.9 - higher strength steel for heavily loaded joints
• Grade 12.9 - very high strength steel, often used for socket cap screws in demanding applications
• Imperial grades - such as grade R and other high tensile grades used on British imperial fasteners, and SAE grades on American fasteners

Understanding metric strength grades:

• The first number multiplied by 100 gives the approximate tensile strength in megapascals. For example, a grade 8.8 bolt has a tensile strength of about 800 MPa
• The second number multiplied by 10 gives the percentage of tensile strength at which the bolt starts to stretch permanently. For example, a grade 8.8 bolt starts to yield at about 80% of its tensile strength

By material:

• Carbon steel - strong and economical, used in various strength grades
• Alloy steel - used for high-strength grades such as 10.9 and 12.9
• Stainless steel - usually A2 for general corrosion resistance, or A4 for marine and chemical environments
• Brass - corrosion resistant and non-magnetic, used for electrical and decorative applications
• Nylon - lightweight, non-conductive, and corrosion resistant, for light-duty applications

By finish:

• Bright zinc plated - a common silver finish with good corrosion protection for general use
• Self-colour - plain steel, usually lightly oiled, often used on high tensile fasteners
• Black - a dark oxide or phosphate finish with light corrosion protection
• Hot dip galvanised - a thick zinc coating for outdoor and structural use
• Stainless steel - naturally corrosion resistant without a coating

How to choose the right bolts and screws?

Choosing the right bolt or screw means matching it to the thread, the load, the material, and the environment. Work through these points in order:

• Thread size - measure the diameter of the hole, nut, or existing bolt to find the correct thread size
• Thread type - check whether the thread is metric, BSW, BSF, UNC, or UNF. These look similar but are not interchangeable
• Thread pitch - check the pitch with a thread gauge, as coarse and fine threads of the same diameter will not fit each other
• Length - choose a length that gives enough thread engagement. As a guide, a bolt in a nut should protrude by at least one or two threads, and a screw in a tapped steel hole should engage by at least its own diameter, with more in softer materials such as aluminium
• Fully or partially threaded - choose a setscrew for full thread length or tapped holes, and a bolt with a plain shank where shear loads act across the joint
• Head style - choose hexagon heads for spanner access, socket caps for tight spaces, button heads for a neat finish, and countersunk screws to sit flush
• Strength grade - choose a grade suitable for the load. Grade 8.8 suits most engineering applications, while 10.9 and 12.9 are used for higher loads
• Matching nuts and washers - use nuts and washers of a matching strength grade, material, and finish
• Material - choose steel for general use, stainless steel for wet or corrosive environments, and A4 stainless steel for marine and chemical conditions
• Finish - choose zinc plated for indoor use, galvanised for outdoor structures, and stainless steel for long-term corrosion resistance
• Standard - choose fasteners made to recognised standards, such as DIN, ISO, or BS, for consistent dimensions and quality
• Quality and brand - choose bolts and screws from reputable suppliers, especially for safety-critical or high-load joints

FAQ - All you need to know about bolts and screws

What is the difference between a bolt and a setscrew?

In the UK, a bolt is partially threaded with a plain shank under the head, while a setscrew is fully threaded right up to the head. Bolts are often used where shear loads act across the joint, and setscrews are often used in tapped holes or where full thread length is needed.

What does M12 × 20 mean?

M12 means the bolt has a 12 mm metric thread diameter, and 20 means it is 20 mm long, measured from under the head.

What does grade 8.8 mean?

Grade 8.8 is a metric strength grade for high tensile steel bolts. It has a tensile strength of about 800 MPa and starts to yield at about 80% of that strength. It is the most common grade for engineering use.

What is the difference between coarse and fine threads?

Coarse threads have a larger pitch, are quicker to assemble, and are less likely to cross-thread. Fine threads have a smaller pitch, giving more thread strength and finer adjustment, and are common on automotive and precision parts.

What is the pitch of a standard M12 bolt?

The standard coarse pitch for an M12 bolt is 1.75 mm. Fine pitch M12 bolts are also available, commonly in 1.25 mm and 1.5 mm pitches.

What is BSW?

BSW stands for British Standard Whitworth, an older British coarse thread standard. It is still found on older machinery, vehicles, and equipment, and BSW bolts are needed for replacements on these machines.

Can I use a metric bolt in an imperial thread?

No. Metric and imperial threads have different diameters and thread forms, so they will not fit properly. Using the wrong thread can damage the hole and give a weak joint.

How do I measure a bolt?

Measure the thread diameter with calipers, check the pitch with a thread gauge, and measure the length from under the head to the end of the thread. For countersunk screws, measure the overall length including the head.

Which bolts should I use outdoors?

Use stainless steel or hot dip galvanised bolts for outdoor use. A2 stainless steel suits most outdoor applications, while A4 stainless steel is better for marine and coastal environments.

Can I mix stainless steel bolts with other metals?

Mixing metals can cause galvanic corrosion in wet conditions. Where possible, use matching materials for bolts, nuts, and washers, or use insulating washers between different metals.

Why do stainless steel bolts sometimes seize?

Stainless steel threads can gall, or cold weld, when tightened quickly or under high load. Tightening steadily and using an anti-seize compound on the threads helps prevent this.

How tight should I tighten a bolt?

Tighten bolts to the torque recommended for their size, grade, and application. Overtightening can stretch or break bolts and strip threads, while undertightening can allow joints to loosen.

Can I reuse bolts?

Standard bolts can often be reused if they are undamaged and have not been stretched. Bolts that are stretched, corroded, bent, or have damaged threads should be replaced, as should bolts designed for single use, such as some torque-to-yield bolts.