Pipe threads – types, chart, dimensions

What is a pipe thread?

A pipe thread is a type of pipe connection that is used wherever we are dealing with plumbing or any flow of liquids (or other substances). It is also often used in pneumatics.

In other words: it connects one part of the installation to another.

How did pipe threads standards originate?

Pipe threads standards originated in the 19th century, when the hydraulic and pneumatic circuits required many different types of threads and fasteners. This caused a lot of difficulties, so over time manufacturers developed their own fastening systems.

Then another problem appeared - and this time with compatibility.

Therefore, Sir Joseph Whitworth, an English engineer and inventor, developed uniform thread types in 1841. It was based on a 55-degree angle with rounded indentations and backs.

In America, however, William Sellers established his own standard for nuts, bolts and screws, which became the national standard (National Pipe Thread - NPT) in 1864. His 60-degree pipe thread, widely used by early American watchmakers, paved the way for the American industrial revolution.

The Whitworth form was chosen as the connecting thread for pipes, which gained additional self-sealing properties when it was given a conical shape. It became the British standard (British Standard Pipe thread). Today, though, it is used worldwide as the basic thread for connecting pipes.

It is available in two versions: BSPT (tapered thread) and BSPP (parallel thread).

However, the most well-known and widely used connection, where the pipe thread provides both a mechanical connection and a hydraulic seal, is the American National Pipe Tapered Thread (NPT). NPT has tapered external and internal threads.

What are the types of pipe threads?

Pipe threads used in hydraulic systems are divided into two main types:

• Connecting threads - these are sealed tapered external threads and parallel or tapered internal threads. The sealing effect is improved by using a bonding compound. 
• Clamping threads - these are pipe threads without a sealed joint. Both male and female threads are parallel (cylindrical), so that the joint requires additional sealing.

What are the designations of pipe threads?

The following list includes the most important designations of pipe threads:

• NPT (American Standard Pipe Taper Thread) - also referred to as a “Briggs pipe thread” is a tapered thread that has a 1:16 taper; 
• NPSC (American Standard Straight Coupling Pipe Thread) - an American cylindrical pipe thread; 
• NPTR (American Standard Taper Railing Pipe Thread) - an American tapered pipe thread with additional protection;
• NPSM (American Standard Straight Mechanical Pipe Thread) - also referred to as “NPS” is another version of the cylindrical pipe thread; 
• NPSL (American Standard Straight Locknut Pipe Thread) - American standard cylindrical pipe thread with a locknut;
• NPTF (American Standard Pipe Thread Tapered Dryseal) - American tapered pipe thread designed primarily for fuel systems; 
• BSPP (British Standard Pipe Thread Parallel) - also referred to as a “Whitworth pipe thread” cylindrical version; 
• BSPT (British Standard Pipe Thread Tapered) - the same variety as above, only as a tapered pipe thread.

As all of the above threads are of Anglo-Saxon origin, they occur in the inch unit of measurement. However, bear in mind that American and British threads are not compatible with each other!

Even if you screw an inch NPS pipe thread into a BSPP, the joint will have backlash, which will cause a lack of tightness.

What are the dimensions of pipe threads?

Below you will find charts with the parameters of British and American threads. These will help you easily check their dimensions.

What are the dimensions of cylindrical pipe threads?

Cylindrical pipe threads are distinguished by the following parameters:

What are the dimensions of tapered pipe threads?

Below you will find the dimensions with which tapered pipe threads, i.e. American pipe threads, are characterised:

How inch pipe threads are translated to millimetres?

As we rarely use the inch system at our latitude, pipe threads can cause some problems in terms of dimensions - especially if someone does not use them on a daily basis.

It is true that 1 inch is equal to 25.4 millimetres, but in this case this rule does not apply. If you apply this conversion factor, you will choose your threads wrongly.

This is because we measure inch pipe threads according to standards. This is clarified in the charts below:

NPT / NPTF tapered thread chart

BSPP cylindrical / BSPT tapered thread chart

Therefore, if someone asks: "I have a ½ inch pipe thread, how many mm is that?" The standard answer would be: 12.7 mm. But in reality, it is about 18-21 mm, depending on whether you mean the internal or external dimension.