Nuts

What is a nut?

A nut is a fastener with an internal thread that screws onto a bolt, stud, or threaded rod to clamp parts together. Working as a pair with the bolt, the nut creates the clamping force that holds a joint tight, whether that is a machine frame, a bearing housing, a guard, a bracket, or a steel structure. Nuts are among the most common engineering components, and while they look simple, the right choice of type, size, grade, and finish makes a big difference to how securely and reliably a joint stays together.

Most nuts are hexagonal, with six flat sides that let them be tightened with a spanner or socket. Others are designed for special jobs, such as resisting vibration, sitting flush, being tightened by hand, or protecting the end of a bolt.

A typical nut has:

• Internal thread - matches the thread of the bolt or stud, such as metric coarse, metric fine, UNC, or UNF
• Body shape - usually hexagonal, but also square, round, flanged, winged, or domed, depending on the design
• Bearing face - the flat face that presses against the joint or washer when the nut is tightened
• Locking feature - on some nuts, a nylon insert, distorted thread, serrated flange, or slot for a split pin to stop the nut loosening

Key features:

• Secure clamping - creates and holds the clamping force in a bolted joint
• Easy assembly and removal - joints can be taken apart and reassembled for maintenance
• Wide range of types - standard, thin, nyloc, flange, dome, wing, castle, and many specialist forms
• Standardised sizes - made to international standards, so nuts and bolts from different suppliers fit together
• Strength grades - available in different property classes to match the strength of the bolt
• Choice of materials and finishes - steel, stainless steel, brass, and nylon, with zinc-plated, galvanised, black, and self-colour finishes
• Left-hand and right-hand threads - right-hand threads are standard, with left-hand nuts available for special applications

In simple terms, a nut is the threaded partner to a bolt that turns two loose parts into a secure, tight joint.

Why are nuts essential?

Nuts are essential because almost every machine, structure, and assembly relies on bolted joints. A bolted joint is only as good as its nut. If the nut is the wrong size, too weak, the wrong thread, or unsuitable for vibration, the joint can loosen, strip, or fail, leading to noise, damage, downtime, or even safety risks. Choosing the correct nut keeps joints tight and dependable.

Their main benefits are:

• Strong, reliable joints - correctly matched nuts and bolts hold parts together under load
• Removable fastening - unlike welding or riveting, bolted joints can be taken apart for maintenance and repairs
• Vibration resistance - locking nuts, such as nyloc and all-metal lock nuts, help stop joints from loosening on vibrating machinery
• Adjustability - nuts allow joints to be tightened, adjusted, or repositioned, such as when setting belt tension or aligning parts
• Versatility - different nut types suit different jobs, from hand-tightened wing nuts to heavy structural nuts
• Standardisation - standard sizes make replacements easy to find and fit
• Corrosion protection - plated, galvanised, and stainless steel nuts resist rust in harsh environments
• Cost-effective - nuts are inexpensive, simple to fit, and widely available

These qualities make nuts important in a wide range of industries and applications, including:

• Engineering and manufacturing - machine frames, guards, brackets, motors, and gearboxes
• Bearing and power transmission - bearing housings, plummer blocks, pulleys, couplings, and chain and belt drives
• Construction and steelwork - structural connections, frames, and fixings
• Agricultural machinery - tractors, implements, trailers, and attachments
• Automotive and commercial vehicles - chassis, suspension, engine, and body components
• Hydraulic and pneumatic systems - mounting brackets, clamps, and equipment frames
• Marine and offshore - deck equipment and fixings exposed to seawater
• Maintenance and repairs - general fixing and replacement work across all industries

The main types of nuts

Nuts are usually grouped by their shape, locking method, thread, material, and finish.

Standard hexagon nuts:

• Full hex nuts - the most common type, used with bolts and studs for general fastening. Made to standards such as ISO 4032 and DIN 934
• Thin nuts (half nuts or jam nuts) - about half the height of a full nut, used where space is limited or as a lock nut tightened against a full nut. Made to standards such as ISO 4035 and DIN 439
• Heavy hex nuts - larger and stronger nuts used in structural steelwork and heavy-duty joints

Locking nuts:

• Nyloc nuts (nylon insert lock nuts) - have a nylon ring inside that grips the bolt thread, resisting loosening caused by vibration. Available in standard and thin versions, such as DIN 982 and DIN 985
• All-metal lock nuts - use a slightly distorted thread to grip the bolt, suitable for higher temperatures where nylon cannot be used
• Serrated flange nuts - have a built-in flange with serrations that bite into the surface, spreading the load and resisting loosening without a separate washer
• Castle and slotted nuts - have slots that line up with a hole in the bolt, so a split pin can be fitted to stop the nut turning. Common on axles, steering, and safety-critical joints

Specialist nuts:

• Flange nuts - have a wide built-in flange that spreads the load, useful on thin or soft materials
• Dome nuts (acorn or cap nuts) - have a closed, rounded top that covers the bolt end, giving a neat finish and protecting people and threads
• Wing nuts - have two wings so they can be tightened and loosened by hand, ideal for parts that are removed often
• Square nuts - have four sides and a larger bearing area, often used in slots and channels
• Coupling nuts (connector nuts) - long nuts used to join two threaded rods or studs
• Weld nuts - designed to be welded onto a surface to provide a fixed thread
• T-slot nuts - fit into the T-slots of machine tables and fixtures
• Bearing lock nuts - slotted nuts used with lock washers to secure bearings on shafts and adapter sleeves

By thread:

• Metric coarse - the standard thread for most engineering applications, such as M6, M8, M10, M12, M16, and M20
• Metric fine - finer pitch threads that give greater thread strength and finer adjustment, common in automotive and precision applications
• UNC and UNF - inch threads used on American and older imperial equipment
• BSW and BSF - older British inch threads, still found on some older machinery
• Left-hand thread - tightens anticlockwise, used on rotating parts where a standard thread could loosen, and on turnbuckles and special assemblies

By material and strength:

• Carbon steel - the most common material, made in property classes such as 8, 10, and 12 to match bolt strength grades
• Stainless steel - usually A2 (304) or A4 (316), for corrosion resistance in wet, marine, and food-processing environments
• Brass - corrosion resistant and non-sparking, used for electrical and decorative applications
• Nylon - lightweight, non-conductive, and corrosion resistant, for light-duty applications

By finish:

• Bright zinc-plated - a common, low-cost finish giving good corrosion protection indoors and in mild conditions
• Hot dip galvanised - a thick zinc coating for outdoor and structural use, with nuts tapped oversize to fit galvanised bolts
• Self-colour (plain) - an uncoated finish, usually lightly oiled, for indoor use or parts that will be painted
• Black or phosphated - a dark finish with light corrosion protection, often used on high-tensile fasteners

How to choose the right nut?

Choosing the right nut means matching it to the bolt, the joint, and the working conditions. Work through these points in order:

• Thread size - match the nut to the bolt diameter, such as M8, M10, or M12, or the inch size for imperial bolts
• Thread pitch - check whether the bolt has a coarse, fine, or inch thread, as nuts with the wrong pitch will not fit
• Thread direction - confirm whether the bolt has a standard right-hand thread or a left-hand thread
• Strength grade - choose a nut with a property class that matches the bolt. As a general guide, use a class 8 nut with a grade 8.8 bolt, a class 10 nut with a grade 10.9 bolt, and a class 12 nut with a grade 12.9 bolt
• Nut type - use standard hex nuts for general joints, nyloc or all-metal lock nuts where there is vibration, thin nuts where space is tight, and flange, dome, wing, or castle nuts for specific needs
• Vibration and loosening - choose locking nuts, serrated flange nuts, or castle nuts with split pins on machinery that vibrates or moves
• Temperature - avoid nyloc nuts above the temperature limit of the nylon insert, usually around 120°C, and use all-metal lock nuts instead
• Material and environment - use zinc-plated steel for general indoor use, galvanised steel for outdoor and structural use, and stainless steel for wet, marine, or food-processing environments
• Compatibility with the bolt finish - use galvanised nuts with galvanised bolts, and stainless steel nuts with stainless steel bolts, to avoid poor fit and galvanic corrosion
• Space and access - check that there is room for the nut and a spanner or socket, and choose thin, flange, or socket-friendly designs where space is limited
• Washers - consider using a washer to spread the load, protect the surface, or add extra locking
• Standards and quality - choose nuts made to recognised standards such as ISO or DIN, from reputable suppliers, especially for safety-critical joints

FAQ - all you need to know about nuts

What is the difference between a nut and a bolt?

A bolt has an external thread and a head, while a nut has an internal thread. The nut screws onto the bolt to clamp parts together.

What is a nyloc nut?

A nyloc nut is a lock nut with a nylon insert inside the top of the nut. The insert grips the bolt thread, helping to stop the nut loosening due to vibration.

Can nyloc nuts be reused?

They can sometimes be reused a few times, but the nylon insert loses some grip each time. For safety-critical or high-vibration joints, it is best to fit a new nyloc nut.

What is a thin nut used for?

A thin nut, also called a half nut or jam nut, is used where space is limited, or tightened against a full nut to lock it in place.

How do I measure a nut?

Measure the inside thread diameter and check the thread pitch with a thread gauge, or test it on a known bolt. The spanner size, measured across the flats, can also help identify the nut, but this can vary between standards. For example, an M10 nut is 16 mm across flats to ISO standards and 17 mm to the older DIN standard.

What strength nut should I use?

Match the nut property class to the bolt grade. For example, use a class 8 nut with a grade 8.8 bolt and a class 10 nut with a grade 10.9 bolt. Using a weaker nut can cause the thread to strip before the bolt reaches full strength.

What is a left-hand thread nut?

A left-hand thread nut tightens anticlockwise instead of clockwise. They are used on rotating parts where a standard thread could work loose, and on assemblies such as turnbuckles that need opposite threads.

How far should the bolt stick out past the nut?

As a general rule, at least one to two full threads of the bolt should protrude past the nut, so the nut is fully engaged. For nyloc nuts, the bolt must pass completely through the nylon insert.

Why do galvanised nuts need galvanised bolts?

Hot dip galvanised bolts have a thick zinc coating, so galvanised nuts are tapped slightly oversize to fit them. A standard nut will usually not fit a galvanised bolt, and a galvanised nut will be loose on a standard bolt.

Why did my nut come loose?

Common causes include vibration, insufficient tightening, the wrong nut type, a soft or uneven joint surface, worn threads, or a reused locking nut that has lost its grip. Using the correct torque, a locking nut, or a suitable washer can help prevent loosening.