Your Questions about Screws, Nuts and Bolts Answered

Being in the business of selling a carefully selected range of high quality fasteners we are routinely asked some common questions about screw, nuts and bolts. In our latest blog post we endeavour to answer some of these frequently asked questions.

What Types of Screws, Nuts and Bolts are Available?

There are many types of screws, nuts and bolts commonly used these days. Screws are one of the most widely used fasteners, available in a wide variety of types and sizes. They are often categorised by their head types:

• Flat head or countersunk: Designed to sit flush with the surface when tightened. Commonly used in woodworking and other applications where a protruding screw head would not be suitable. The term ‘flathead’ is also sometimes used to refer to screws with a head type that accepts flat bladed screwdrivers.
• Pan head: These screws have a flat, rounded-off head. Typically used in general purpose applications.
• Round head: These screws have a domed head shape. Sometimes used for more aesthetic applications.
• Semi countersunk: These screw heads are countersunk with slightly domed top. Used in a wide range of general purpose applications.
• Hex head: These have a hexagon shaped screw head that requires a spanner to tighten or loosen them.
• Socket head: These screws have cylindrical heads with a specifically shaped orifice for the tool used to tighten or loosen them, such as Allen screws that require Allen keys Torx screws and bolts that require Torx drivers.
• Security Screws: either one way (can only be tightened) or two way (require special tools to tighten and loosen) are increasingly popular.

As noted, the term ‘flat head’ is also sometimes used to described slotted head screws that require flat-bladed screw drivers. Cross head screws, also called cross-point or cruciform, require a cross-head screwdriver. These are sometimes called Phillips screws and screwdrivers. Pozidriv screws and screwdrivers are an enhanced version of the Phillips cross head type.

Bolts, just like screws, are available in a vast range of designs and types including hex bolts, carriage bolts, anchor bolts and more. As for screws, bolts are often categorised by their head types, along with their application. For example anchor bolts, which are commonly used to attach objects to concrete, stone or brickwork.

Most bolts also require suitable nuts and there is also a wide variety of these including hex nuts, lock nuts, wing nuts and cap nuts. Perhaps the most familiar form of nut and bolt combination is a hex bolt with a hex nut. Both the bolt head and the nut are hexagonal in shape, requiring a spanner to tighten and loosen them. These ubiquitous fastenings have been in common use since the 19th century.

A variety of one-way and two-way security fasteners are also available. Clutch head screws, for example, are a one way security screw. They have a special screw head design that enables the screw to be tightened, with a standard, flat bladed screwdriver, but they can’t be loosened.

What Size of Screw, Nut or Bolt Do I Need?

The size of fastener needed for a job is determined by a number of considerations.

If a fastener is needed to replace a damaged, worn or lost screw of bolt then ideally the replacement should be the same size and type as the fastener that was previously used. But if fasteners are needed for a newly constructed object then the size and type will be determined by the design, the loads the fasteners must withstand and the environment in which the fasteners must remain secure.

To measure a screw, nut or bolt, use a caliper to determine the diameter of the shaft, across the widest part of the threaded section. Then measure the length from the underside of the screw or bolt head to the tip. For countersunk screws and bolts the length measurement is taken from the top of the countersunk head. And then determine the thread pitch using a thread gauge.

What Materials are Fasteners Made From?

Screws, nuts and bolts are available in a wide range of materials including:

• Stainless steel. A2 and A4 stainless steel are commonly found. A4 stainless steel offers higher corrosion resistance than A2 and is typically used in marine applications.
• Carbon steel. Often zinc plated for durability.
• Titanium. Highly resistant to corrosion and light weight.
• Brass. Excellent corrosion resistance which is why brass screws have historically been used in marine applications.
• Nylon and plastic. Very light weight, will not corrode and don’t conduct electricity.

What are the Differences Between Metric and Imperial Fasteners?

There is a great deal of confusion over metric and imperial fasteners. The basic difference is that metric fasteners are measured in millimetres while imperial fasteners are measured and defined in inches.

This means the diameters, lengths and threads are defined differently for metric and imperial bolts and screws. Importantly, metric and imperial bolts are not interchangeable. Metric threads will not mate with imperial and vice versa.

So how can you tell if a bolt is metric or imperial? A notable difference between metric and imperial fasteners is that metric fasteners have a coarser thread than imperial. If a bolt has lines on its head then it is likely to be imperial. And if a bolt has numbers on its head then its metric.

In the UK the metric system was introduced in 1965. But there are many old cars and legacy pieces of machinery that continue to use imperial fasteners.

What Standards do UK Fasteners Comply With?

In the UK one of the key standards for fasteners is called “British Standard” or BS. There is a wide range of these standards such as BS1083, the specification for precision hexagon bolts screws and nuts.

Another important collection of standards come from the International Organisation for Standardisation or ISO. This is a globally recognised standards authority that applies to a wide range of industries. ISO 898-1, for example, is the standard for bolts, screws and studs made from carbon steel or alloy steel.

Another important standards authority is Deutsches Institut fur Normung or DIN. This is the German institute for standardisation, responsible for a wide number of standards used throughout Europe. DIN 134 is a standard for nuts and bolts.

How Do I Choose the Right Fastener for My Project?

Deciding on the right fastener for a project can be difficult. Often, the best option is consult with an expert. The factors that need to be considered are:

• The materials being joined together. E.g. Metal to wood.
• The environment in which the fastening needs to remain secure. E.g. Outdoor or in a salty, marine atmosphere.
• The load or stress the fasteners must withstand.

There are often additional considerations such as the need for security fasteners, that can’t be undone without specialist tools, along with aesthetic requirements.

If you need help deciding what form of fastener is right for your project why not give our friendly, helpful team a call: 01273 974754.

How do I Stop Screws, Nuts and Bolts from Loosening?

Questions about how to prevent screws or bolts from loosening over time often arise. In some environments, such as on machinery, fastenings are subjected to a lot of vibration, which can shake them lose. Options to prevent fasteners from loosening include:

• Lock washers: such as spring washers, that press against the underside of a screw or bolt head.
• Lock nuts: also called self-locking nuts. These are special nuts that incorporate material that makes them difficult to shake lose.
• Threadlocking adhesive: this is a special form of adhesive that hardens after being applied to fastener threads. Some fasteners are available with pre-applied threadlocking adhesive.
• Using a double nut: adding a second nut is commonly used to prevent nuts from coming lose.

Its always important to use the right size fastener and to tighten as required.

How Can I Remove a Stuck Screw or Bolt?

Screws and bolts can become stuck for a variety of reasons. Corrosion is a common cause of bolts and screws becoming very difficult to remove. This problem can usually be effectively overcome by applying some penetrating oil or WD40 and allowing this sit for a while. Gently tapping the bolt head can help loosen the threads and if that doesn’t work, applying a little heat, to expand the metal, might be cautiously attempted.

Stripping the thread from a bolt is another common problem. When the thread is stripped the bolt might rotate, but doesn’t undo. What can work is to apply extraction force to pull the bolt, using grips or pliers, while rotating the bolt with a spanner. If the bolt head can be raised a little then a flat bladed screwdriver or chisel can be used to exert extraction pressure, under the bolt head.

Another option is to use a bolt or screw extractor. These tools generally require a small pilot hole to be drilled into the head of the offending fastener, to insert the extractor tool. This same technique and tool can be used to remove the shaft of fasteners when the head has broken off.

What are the Best Fasteners for Outdoor or Corrosive Environments?

When selecting fasteners for outdoor use corrosion resistance is a primary consideration. If the outdoor environment is anywhere near the sea then there is the additionally corrosive effect of the salty atmosphere that must be considered.

A4 Stainless steel fasteners are the best option for marine, outdoor environments. A2 Stainless steel would be OK in most situations well away from salt water. Brass or nylon fasteners might be used for low load applications.

Where Can I Buy Screws Nuts and Bolts?

There are a wide variety of excellent sources for high quality fasteners in the UK. These vary from the well known, huge DIY stores, located in most towns and cities, to smaller local hardware stores that still exist in some towns. There are also many excellent trade outlets that sell to the general public.

Specialist suppliers, such as Insight Security, offer carefully selected ranges of high quality fasteners for specific applications, along with valuable guidance on fastener use.

When shopping for fasteners its important to remember that product quality can vary. That’s why its essential to select a reputable supplier that has a verifiable background in supplying reliable fasteners. Another important consideration is whether the supplier provides excellent help and guidance.

What Tools are Needed for Screws, Nuts and Bolts?

Working with screws, nuts and bolts requires a variety of tools. For screws, a range of both flat-bladed and cross-head screwdrivers, to suit various screw sizes, should be in the tool kit.

For bolts, spanners, including socket and adjustable spanners, are a valuable toolbox addition. Also, mole grips and pliers can be helpful.

As well as standard screws and hex bolts, Allen bolts are increasingly common these days. It therefore makes sense to include a set of Allen keys in a toolkit. And if there is a need to deal with more specialised fastenings, such as Torx head, then appropriate tools are needed.

Powered driver tools, especially cordless, can be enormously helpful, especially when assembling or disassembling equipment that has many fastenings. Another potentially useful tool is a torque wrench which is used to tighten bolts or nuts to a specific torque value, avoiding both over and under tightening.

Various security fastener require special tools. Snake Eye screws, also known as two hole screws, pig nose screws and pig nose bolts, have a distinctive head type that presents two holes for the required tool. These screws and bolts are popular for their aesthetic appearance and commonly used to secure objects in public environments.

Security Hex bolts and screws have a Allen style socket head type, but there is a pin in the centre of the recess which means a standard Allen key will not work. These fastenings require a Security Hex Driver tool or driver bit.

Standard Torx screws and bolts have a head type much like Allen screws and bolts except the recess is a six pointed star shape that requires a Torx driver. But tamper proof Security Torx fasteners have an additional security pin in the centre of the star shaped recess which means conventional Torx drivers will not work.

Another form of Torx security fastener that’s growing in popularity is the 5 Lobe Torx Screw. Like their 6 Lobe Pin Torx counterparts, these tamper proof Security Torx fasteners require a matching security driver tool for installation and removal.

Shear Nuts are a clever one-way security fastener. The unique design of these nuts means that, once tightened to a predetermined torque force, the breakaway section of the nut shears off leaving a highly secure, tamper resistant fixing.

Security Scroll Nuts are another form of clever two-way security fastening. These attractive nuts are also known as wave nuts due to the wavy nut design that requires a special tool to tighten and loosen them. They are conical in shape making it virtually impossible to remove them without the required tool.

What is the Difference between a Screw and a Bolt?

People are often understandably confused by the distinction between screws and bolts. Broadly speaking, screws are intended to be used in either preformed or threaded holes, or are capable of forming their own threaded hole (self-tapping screws). Screws are tightened by using an appropriate driver tool to apply torque to the screw head.

Another key distinction is that screws tend to be threaded over the full length of the shaft, whereas bolts are often only partially threaded. And bolts are generally intended to mate with matching nuts, which can require appropriate tools (spanners) to be used for both the nut and bolt, to tighten them.

But there are exceptions. For example, Atlas Bolts. While these excellent fasteners are called bolts, strictly speaking they should be called self-tapping concrete screws, as the thread extends over the full length of the fastener shaft and they cut their own thread into a substrate when screwed in.

How Can I Identify the Thread Type of a Fastener?

Fastener threads have a number of key attributes. The gender terms, male and female, are used to refer to holes and what goes into them, for obvious reasons. Bolts and screws have a male thread while the hole in nuts has a female thread.

Threads can also be both right-handed and left-handed. Right-handed threads are the standard form of conventional thread that’s tightened by turning clockwise.

Thread pitch and threads per inch are also important thread characteristics. Thread pitch is the distanced between the crest of one thread to the next, in millimetres, for metric fasteners. And threads per inch (TPI) refers to the number of threads in an inch, generally applied to imperial fasteners.

The major and minor diameters refer to the fastener shaft diameter. The major diameter is measured from the peaks or crest of the threads and the minor diameter is measured from the dips or root between the threads.

The flank angle is another thread characteristic. This is a measure of the angle of the threads, against a perpendicular line.

There are a confusing variety of threads including:

• M - ISO threads (metric)
• NPT - Pipe threads
• G/R/RP - Whitworth threads
• UNC/UNF - Unified national threads.

ISO Metric threads are the best known and most commonly used in Europe. Thread measurements are in millimetres. As well as the ‘M’ metric thread there is also the ‘MF’ thread, which refers to metric fine. This is a finer gauge of metric thread, commonly used in watches and high precision applications.

NPT pipe threads are a standard used for connecting pipes and plumbing. NPT stands for National Pipe Tapered. These threads are more commonly used in the USA.

Whitworth threads are named after British inventor Joseph Whitworth. He is responsible for the introduction of the world’s first thread standardisation in 1841. British Standard Whitworth (BSW) is a standardised imperial thread that’s still used on some older vehicles and machinery.

UNC stands for Unified National Coarse thread and UNF refers to Unified National Fine thread. Mainly used in the USA, this is an American equivalent to the Metric ISO thread.

To determine the thread type of a bolt you need to measure the bolt shaft major diameter, determine the thread pitch and compare these metrics with a thread chart. There are also some useful thread checking tools that can be used to accurately determine the thread of a nut or bolt.

Are There Differences between Fasteners for DIY and Professional Use?

People sometimes ask if there are differences between the fasteners used by professionals compared with those available for domestic DIY users. The answer is no, both professional and DIY users all have access to the same ranges of fasteners.

But, there are some forms of specialist fasteners, such as certain security fasteners, that require special tools, as previously noted, which most people would not have in their tool kits.

What is the Best Way to Store Screws, Nuts and Bolts?

Storing fasteners in an organised way that ensures they are protected against dirt and moisture is always sensible. Using organised, labelled containers is often the best option.

Fasteners might be organised by size and type in a collection of repurposed coffee jars, for example. Importantly, they should be kept in a dry environment and sachets of silica gel might be included to absorb moisture.

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