How to Identify a Thread

Threads are an incredibly useful aspect of fastener design. But if you have a collection of unlabelled bolts or pipes, how can you identify the thread to make sure you use the right one for the job?

Fortunately, the team here at The Hosemaster is here to help you, with our handy guide to thread identification and measurement. In this article, we’ll take you through what threads are, and the various methods you can use to identify the threads on your components.

What is a thread?

So first off, what is a thread?

A thread describes the notched profile used to fasten materials together. It runs down in a helix formation down the body of the fastener to create a grooved texture. This is important because it helps to ensure the bolt, screw or pipe stays firmly attached. Basically, the friction created by the notches makes it harder to pull the connection apart – it needs to be twisted to be removed.

Threads can come in ‘male’ or ‘female’ designs. On the male threads, the grooves are raised, whereas on female threads this is the opposite. This allows male threads to be screwed into female ones for a more secure fastening.

How to identify a thread

Understanding the function of a thread is just the beginning. The next step is to learn about the various techniques you can use to identify different threads, and what characteristics will make this easier (or harder).

Thread characteristics

The first step to correctly identifying the thread on your fastener is to examine it for the following characteristics. We’ve also included a labelled diagram below, to help clarify some of the measurements we’ll mention throughout this guide.

Is the thread internal or external?

Firstly, establishing whether the thread is internal or external will tell you whether you have the male or female part of the connection. You can do this by measuring the point called the ‘major diameter’.

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For external, or male, threads, this is the distance from the crests (highest points) on either side of the bolt. For internal, or female, threads, you should measure the distance between the roots – the lowest point of the thread.

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Is the thread straight or does it taper?

Another element to look out for is whether your joint thread is straight or tapers, as this will affect where you measure the diameter from. Unsurprisingly, straight threads will have the same diameter all the way along, so you can take your measurement wherever is easiest for you.

Alternatively, tapered threads get progressively narrower towards the base. So, we recommend measuring from the 4th or 5th thread (around the midpoint) to get an accurate diameter.

What is the thread pitch?

The thread pitch is the measurement between two consecutive thread crests, and is important for identifying different thread types. For example, metric threads use the pitch measurement as part of the naming conventions of their fine threads – so you need to know this figure to correctly identify your fastener.

How many threads per inch are there?

Threads per inch, or TPI, is another measurement used to identify different threads. This is, as the name suggests, the number of threads found along an inch of the fastener. Many UTS (unified thread standard) uses the TPI number to understand pitch, so it’s important you know this value.

Thread types

This leads us nicely through to thread types. These are the several types of connector thread used in the UK, including:

• ISO Metric.
• British Standard Whitworth coarse (BSW).
• British Standard Fine (BSF).
• British Standard Pipe Parallel (BSPP).
• British Standard Pipe Taper (BSPT).

Note: BSP threads are identifiable from other types of fasteners used in other areas of the world – like North America – because they have rounded crests and roots. They also have a 55° angle between these areas (instead of 60°).

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Looking for a more specific guide for BSP threads? Explore our handy BSP measurement page for more information.

Tips to accurately measure threads

During the course of understanding how to identify threads, you’ll need to take several measurements – and the right tools can make a big difference in how easy (and accurate) this is. In this section, we’ve pulled together some of our top tips for measuring threads, and the types of tools you’ll need for the job.

• A Vernier calliper will make your life a lot easier. These have two sets of jaws to clamp around the fastener or pipe to measure internal and external joint threads quickly and accurately.
  
  
• When you’re trying to identify a tapered thread, measure the diameter of the first, fourth, and last thread. If these decrease, your thread is tapered.
  
  
• To measure the threads per inch, rest the top of the thread against a ruler and make a mark at the one-inch mark. Then, count either the number of crests or roots between the start and end of the inch.
  
  
• Investing in a thread gauge will make measuring the pitch much easier, as these can slot into grooves and show you the size.

Once you have your measurements, identifying your thread is a simple job of comparing these to standard reference guides to find which one you have.

Choose The Hosemaster for the best range of fittings

At The Hosemaster, we have a fantastic range of threaded fittings for you to choose from, in a wide range of materials to suit any application. 

Not sure which is best for you? Our team has plenty of experience in the industry, and are always on hand to offer you valuable advice throughout your buying journey. Simply give us a call on 01282 604 002, or fill out our contact form to get in touch today.

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