How to Identify PT/NPT/G/Metric thread?

One day, when the cylinder or solenoid valve is broken, you want to buy a new one on the net, but you find that in addition to the thread size you need to choose, you also need to confirm thread type, such as PT thread, NPT thread, G thread (see figure 1), make you feel very confused and difficult to choose.

(Figure 1)

By the way, thread sizes like 1/4, 1/2, 1/8 which are marks in the thread, refer to the diameter of the thread size in inches.

If you purchase the accessories that do not match the original thread, and if you install them on the equipment, it will not only damage the equipment but also affect the pressure retention ability and sealing reliability of Air fittings, Solenoid valves, etc. So, it’s important to know which thread you want.

Next, we will divide it into 3 parts to explain how to determine the thread.

• 1. What are PT Thread, NPT thread, G thread, and Metric thread?
• 2. How to choose the right one?
• 3. How to measure and identify the thread?

What are PT thread, NPT thread, G thread, and Metric thread?

• PT thread (Figure 2). PT is the abbreviation of Pipe Thread, 55 degrees sealed tapered Pipe Thread, the taper regulation is 1:16, it is popular mostly in Europe and the Commonwealth of Nations also some Asian countries, like China, Japan, Korea.  And they are more commonly used in the water and gas Pipe industry. This connector is identical to and interchangeable with the BSPT connector, which is also tapered.  So usually, PT=BSPT. It is better to use sealant to ensure a leak-free seal.                                                                               (Figure 2)
• NPT thread (Figure 3).  NPT is the abbreviation of National Pipe Thread, 60 degrees sealed tapered Pipe Thread, the taper regulation is also 1:16, it is mainly used in U.S. and Canada. Usually, thread sealant is needed to achieve a leak-free seal but only 2 turns of thread sealant are required. NPT is similar to BSPT, there are two differences between them, one is degrees difference, 55 for BSPT and 60 for NPT. Another is thread pitch. So, fittings that are popular in China and Japan are rarely used in North America.                                                    (Figure 3)
• G thread (Figure 4). G thread also called BSPP, is a British standard pipe parallel, is most popular in the UK, Europe, Asia, Australia, New Zealand, and South Africa. No thread sealant is needed to form a seal. An appropriate sealing compound can be used in the thread to ensure a leakproof joint.                                                                                                                                        (Figure 4)
• Metric thread (Figure 5). Metric thread is a globally standardized thread, denoted by M, like M10. It’s an ISO thread. Compared to standard threads (coarse threads), the fine thread has a smaller pitch. Male and female threads are both parallel. It is very common in Europe. Metric threads have two kinds of pitch: coarse pitch and fine pitch. Coarse threads have the default pitch size whereas fine threads have smaller pitch sizes and are used less often.                                                                                                    (Figure 5)

After Knowing the above threads, how to choose the right one?

• Emergency selection.

If the time is very tight, you can also make a preliminary judgment like this: if the equipment was bought from China and Japan,   PT thread is generally used, and if the equipment was bought from the U.S. and Canada,  NPT thread is suitable for it. And the G thread is always with an O ring.

• Determine your thread carefully

The tools you need:

1. Caliper

Caliper is a measuring instrument commonly used for inspection in manufacturing to provide precise measurements of an object.

2. Screw Pitch Gauge

It is used to measure the number of threads per inch or the distance between threads on the metric thread.

3. Thread Identification Table

The tables are as below:

PT thread: 

(Table 1)

NPT thread:

(Table 2)

G thread: 

(Table 3)

Metric thread: 

(Table 4)

*O.D. = Outside Diameter I.D. = Inside Diameter

How to measure the thread?

First, inspect the thread.

Check it is a tapered thread or parallel thread. See Figure 6.                                                             (Figure 6)

Use the caliper to measure the first, fourth, and last full threads, if they are the same, it is a parallel thread, or else it is a tapered thread.

PT, NPT threads and Metric threads are tapered threads, while the G thread is the parallel thread (Straight thread).

Second, Measure the Thread diameter.

Use the caliper or easily with a straight ruler to measure the outside diameter on a male thread and the inside diameter on a female thread. The data measured by yourself may not be particularly accurate, but it doesn’t matter, you can find the closest data in the tables.

Sure, it is very easy for you to see whether it is a male thread or a female thread, shown below.

Third, Measure the Pitch size.

Pitch size is the distance between threads on metric thread types of the number of threads per inch. Use a screw pitch gauge to measure the distance between threads. If the gauge sits neatly within the threaded form, then you have a match.

Last, Determine the thread type standard.

Compare your measurements to the size tables above. See table 1- table 4,

Then finally determine which thread you need.

There are many other types of threads, here are just a few types related to pneumatic products,

hope it can help you to identify the PT thread, NPT thread, G thread, and Metric thread.