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Security screws and other tamper resistant fasteners are a great addition to any installation to provide an extra level of security. However, they won’t do their job correctly if you don’t use the right fastener for the job. It is vital to use the exact size to ensure the fastener fits – too big or small and it will be useless, therefore you will have wasted valuable time and money in ordering the wrong item.

So, when you are measuring up for security screws, how do you know if your measurements are accurate?

Screw heads and screw length measurements

The first thing to consider when measuring up for security screws is the head of the fastener, as this will determine how to measure the screw itself.

Any fastener that has a raised head, i.e. that isn’t flush with the material it is holding in place once it is fitted, is measured from the underside of the head to the tip of the screw. A screw that has a flat or countersunk head is measured from the top of the head to the end. The key factor to remember here is that if the top of the screw sticks out from the material when in place, that extra length shouldn’t be counted in your measurement.

Measuring for machine security screws

Machine security screws have a tamper proof head that usually requires a specific tool to remove them. The shank of these types of screws is threaded from directly underneath the head to the tip, and it is this part that determines the measurement.

Each screw size is described using two numbers – the ‘M’ number that signifies the diameter of the thread, and the second number that stands for the length of the shank. Most sizes are given in metric measurements so the numbers given will refer to millimetres. For example, if you are looking for a machine security screw with a barrel diameter of 5mm and barrel length of 18mm, you would need an M5 x 18mm screw measurement. Here’s an example, our Security Pin Hex cap screw.

Although metric measurements are most common, we do have options available to order in imperial sizes, so don’t be put off if you can’t see the size you require in our product range.

Security nut measurements

Security nuts obviously need to be paired with a machine security screws, so their measurements will depend on the corresponding fastener. To determine the right security nut size, you need to know the diameter of the screw thread. The diameter of the nut, i.e. the ‘M’ number, should therefore match the ‘M’ number of the screw with which it will be paired. It is also important to ensure that you have the correct thread pitch, which is the distance between each thread. For example, an M8 x 1.25mm thread (standard) will not work with M8 x 1mm thread (fine pitch).
If you are using a security barrel nut, you will need to know the length of the screw thread in addition to the diameter. Once you have the screw length, you will need to subtract the thickness of the material that the fastener and nut will be used on to ensure the nut barrel fits the screw securely.

Self-tapping security screw gauges

Whilst millimetres are the common unit of measurement for many types of screw, this is not the case for the self-tapping variety, where the diameter and length are measured using completely different units.
The length of a self-tapping screw is measured in imperial units, with the length given in inches, and the diameter is given using the gauge measurement. This is difficult to measure without the appropriate tools, so we have converted the gauge measurements for our self-tapping fasteners into millimetres to make measuring up for screws as simple as possible. Here’s an example, see our 2-hole pan self-tapping screw.

Need more help measuring up for screws?

Have a look at our security screw catalogue where you’ll see all of the standard sizes listed. If you are still unsure about how to find the right screw size for your requirements, or if you need a special size of screw, please contact us, our team will be happy to advise you.