Fastener Frequently Asked Questions

Nyloc nuts can be used where vibration resistance is required

This page lists answers to a few common questions related to the identification and application of fasteners. If you have a question that is not listed, feel free to contact us to suggest a new entry.

Q: How are fasteners measured? What length is used?

A: Bolt and screw lengths are always stated according to the clamping lenth. Therefore, a normal hex-headed bolt or set screw or cap screw or cheese or pan headed screw are all measured from under the head, to the end of the thread. A screw or a bolt with a normal countersunk head is measured overall. Raised or domed countersunk screws are measured from the widest end of the cone, where the dome meets the cone, to the end of the thread (again, this is the clamping length, as for other fasteners).

Q: What's the difference between a bolt and a set screw?

A: A bolt is not fully threaded, whereas a set screw is. Bolts are generally not available in the shortest lengths for each thread size, as there is insufficient room to form a useful length of thread and to still have an unthreaded section below the head. For example, the shortest bolt in an M8 thread will normally be around 35mm - below that length, if you ask for a bolt you will typically be given a set screw!

Q: Why do you stock different finishes? Why aren't all steel fasteners zinc plated?

A: Bright zinc plating is a very effective corrosion inhibitor and is indeed well suited to the majority of applications, but it has some disadvantages: The plating process itself introduces some weakening that can compromise high-grade high tensile fasteners (some manufacturers will refuse to supply grade 10.9 fasteners in anything other than self colour precisely because of this); and, plating can cause issues with other processes such as welding, where a nut needs to be captivated onto an assembly, for instance. Dissimilar metals can also suffer from electrolytic corrosion so the material and finish of the fastener should be chosen with the material of the fixture in mind. The surface of a fastener and any lubrication or contamination also have a significant effect on the tightening torque.

Q: Can I really have as many or as few fasteners as I want?

A: Yes. We can certainly provide full box quantities when they are required, but if you only want a few M24 nuts, for example, you will likely be very pleased to buy just the number you need - in common with many large or specialist fasteners, these can be expensive!

Q: Why are there metric fine (and extra fine) threads?

A: Metric fine fasteners are typically stronger in tensile loads than standard metric (coarse) fasteners, as the thread depth is not so great and therefore there is less likelihood of necking and shearing failures. Note that any higher strength will only be achievable if the appropriate nuts are used (see below). Higher grade fasteners are more expensive to manufacture. Note that there are sometimes multiple fine pitches at the same diameter - if it isn't a standard metric (coarse) thread, you should be careful to identify the precise thread pitch. You can do this by measuring over several threads, if necessary.

Q: Is "metric" the same as "metric coarse"?

A: Yes. If it doesn't say "fine" or "extra fine", it is the standard ISO metric fastener, which is the coarser thread. E.g. "M8" means ISO Metric 8mm x 1.25mm pitch. We tend to spell out the precise size and pitch to be clear.

Q: Do you supply High Tensile bolts/sets and what nuts should be used with them?

A: Yes, High Tensile typically means grade 8.8 and above and we hold a very comprehensive stock of bolts, sets and high grade nuts. It is important to use a grade 10 nut with a grade 10.9 bolt, for instance, in order to achieve the rated strength. Mild steel fasteners are also available and are much weaker but still useful in some minimal load situations (e.g. we supply mild steel allthread threaded rod, so that arbitrary length fixings can be improvised, and we offer high tensile allthread too).

Shear nuts provide an easy anti-tamper fixing

Q: What is a Shear Nut? What about other anti-tamper fixings?

Shear nuts provide a simple, but effective anti-tamper fixing. Some examples are shown in the photo to the right, here. They are threaded nuts but with a reduced hexagonal section that is designed to shear off when the nut is tightened. This leaves the remaining, smooth, conical section that is difficult to remove. We stock shear nuts in a zinc plated finish in M8, M10 and M12 sizes.

We are also able to supply a wide range of other specialist anti-tamper fixings, e.g. hex socket with an internal pin, and various tekscrews with tamper-proof heads. Setting tools are available for all of these. Please contact us with your requirements.

Q: How tight should a fastener be? What torque setting is appropriate?

A: This is not a simple question and many factors will affect the torque that should be used. Tightening a fastener to a given torque will usually result in some small amount of stretch to the fastener itself; if this stretch exceeds the tensile strength of the fastener either during the tightening process or subsequently with operational load added, the fastener will fail. The torque is therefore important and many engineering applications will specify the correct torque settings. If these are not respected, failures can result which could easily have safety implications. If you are servicing or building a piece of equipment, especially if it has any safety-related aspect, you should always try to obtain the tightening torque values that were specified by the designer and to follow them carefully.

Considering the general principles, the type, size, thread and grade of the fastener itself naturally all have a significant effect on acceptable torque values (mild steel fasteners could fail with a torque that would be fine with a grade 10.9 fastener; a small bolt will fail way before a large bolt). The surface finish and any lubrication and/or contamination also make a big difference - the tightening torque must overcome the friction between the threads and any lubrication will obviously allow a fastener to be tightened more for a given torque, so critical torque requirements should normally be set with the fasteners clean and with known lubrication. Another often overlooked aspect is the torque requirement of the application itself - just because a bolt can withstand a high tensile load, it doesn't mean that the fitting will not be damaged. The tightening torque should therefore be appropriate not only to the fasteners but also to the strength and materials of the fitting. If a fastener is being used in a gasketed joint, it is important to close the fitting onto the gasket with sufficient pressure to seal the gasket and to withstand any operating pressures. The tightening torque should generally be sufficient to take up any looseness or play in the joint - fasteners are more likely to suffer from fatigue failures if they are subjected to cyclic loading without being tight enough.

There is a handy guide to maximum torque settings for various common fastener sizes on the Volvo Owners' Club web site (this link will open in a new window). Please note that CW Fasteners is not responsible for the content on any external web site and that you have full responsibility for verifying the accuracy of any information you find, especially in any safety-related situation.

Any critical torque should be set with the aid of a good quality (and preferably recently calibrated) torque wrench. CW Fasteners keep a range of torque wrenches in stock.

Q: I am restoring an old tractor. Do you supply imperial threads?

A: Yes, we can help with some imperial threaded fasteners. There is such a huge range of threads and types that we focus more on metric/metric fine/metric extra fine as many of the imperial types have been deprecated (i.e. non-recommended) by the British Standards Institute for 30+ years. We do still carry a range of UNF and UNC fasteners, for example, and more can be obtained by special order. Please contact us with your requirements and we will try to help.

Q: What size spanner/key do I need?

A: Please refer to our Spanners/Key Sizes summary.

Q: What is the difference between all these threads?!

A: As well as varying in diameter and in pitch (e.g. M10 x 1.5 and M14 x 2.0), different thread types also have different angles and different forms in the way the thread is cut (e.g. ISO metric threads all use a 60o angle, whereas UNF/UNC threads have the same angle but a different shape to the thread so fixings are generally not interchangeable). An American company actually has quite a good thread reference (opens a new window).

Q: I don't see the fastener that I need. Can you help?

A: We will try to help with specialist and obsolete fasteners. We can often suggest an alternative that is cheaper and we are happy to place special orders on your behalf where necessary.