Without thread fittings, hydraulic systems, pipes, and hoses will not function. These thread fittings seal the pipes, allowing the safe transfer of fluid and gas without leakages and other accidents.
There are different types of threads in the market, and among them is the G thread. G threads are relatively popular. You may know them already, albeit with a different name.
In this post, we go into detail on what G threads are, their different sizes, and how they compare with other popular thread types.
What is a G Thread?
G Threads refer to the British Standard Pipe Parallel (BSPP) threads. The name “G” stands for “Gas,” as G Threads are prevalent in gas systems.
Initially, the name “G thread” referred to the 55-degree apex angle pipe threads used on inch bed pipes. But now, the threads are identical and interchangeable with the BSPP threads. So, G Threads and BSPP threads are the same.
As you probably guessed, these threads conform to British standards. The threads are popular in the UK and a few European countries. It’s also popular in Australia, New Zealand, South Africa, and some Asian countries.
G threads seal with a bonded seal ring, which is sandwiched in between the face of the female fitting and the shoulder on the male fitting. They don’t require a thread sealant to seal but yet seal relatively tightly.
In particular, G threads come with a copper crush washer and longer male threads. The copper crush water goes in between the face of the female fitting and the shoulder of the male fitting to create a tight seal.
There are also G thread fittings that seal with an O ring. For such fittings, the O-ring male connector comes with a straight and metal retaining ring. Meanwhile, the female connector has a flat external surface.
G Threads come in different sizes and are capable of withstanding high temperatures and strong pressure levels. This is why they are prevalent in gas systems.
G Thread Size Dimension
Below are the different thread size dimensions of G threads and their inner diameter, outer diameter, and threads per inch (TPI):
| S/N | Thread Size | Exterior Diameter (Major) | Inner Diameter (Minor) | TPI |
| 1 | ⅛ inches | 9.728 | 8.566 | 28 |
| 2 | ¼ inches | 13.157 | 11.445 | 19 |
| 3 | ⅜ inches | 16.662 | 14.950 | 19 |
| 4 | ½ inches | 20.955 | 18.631 | 14 |
| 5 | ¾ inches | 26.441 | 24.117 | 14 |
| 6 | ⅞ inches | 30.201 | 27.877 | 14 |
| 7 | 1 inch | 33.249 | 31.770 | 11 |
| 8 | 11/8 inches | 37.897 | 34.939 | 11 |
| 9 | 13/8 inches | 44.323 | 41.365 | 11 |
| 10 | 2 inches | 59.614 | 56.656 | 11 |
| 11 | 21/4 inches | 65.710 | 62.752 | 11 |
| 12 | 23/4 inches | 81.534 | 78.576 | 11 |
| 13 | 3 inches | 87.884 | 84.926 | 11 |
| 14 | 4 inches | 113.030 | 110.072 | 11 |
| 15 | 5 inches | 138.430 | 135.472 | 11 |
How to Measure G Thread Size
How you measure G thread size is the same as how you measure all BSP fittings. Knowing how to carry out this measurement is essential, especially when replacing your couplings. Fortunately, doing this is easy.
You first need to measure the outer diameter of the thread. For G Threads, this should be in inches. You can get the measurement from the technical details of the thread.
Next, subtract ¼ inch from the outer diameter size. This is about 25% of the total size. For example, suppose the outer diameter of the G thread is 2”. In that case, the thread size will be 3/2.
Afterward, crosscheck the size you got with the G thread size dimension table above to confirm the dimensions.
G Threads vs. NPT
G threads and NPT threads are often compared with one another. Both are the most popular thread types in the market. In truth, they share many similarities, but they are different. And the primary difference involves the industry standard they follow.
As stated earlier, G threads, also known as BSPP threads, follow the British Standard. On the other hand, NPT threads follow the American National Standard Pipe Thread standards.
NPT threads are the norm in the United States. This contrasts with G threads that are the norm in the UK, along with other BSP threads. NPT threads are also common in Canada, just as G threads are common in a few other European countries.
G threads and NPT threads have notable differences in thread sizes. As a result, they are not interchangeable thread fittings. G threads come with a 55-degree angle thread, as mentioned earlier. But NPT threads usually come with a 60-degree angle thread.
Despite this, many people still attempt to use NPT threads instead of G threads and vice versa. Such is not recommended, as it often results in fluid leakage and other significant problems. It is worse if it is a high-pressure system.
G Threads vs. BSP
Simply put, G threads are BSP threads, but not all BSP threads are G threads. G threads particularly refer to the British Standard Pipe Parallel (BSPP) threads. Meanwhile, BSP threads include the BSPP and the British Standard Pipe Taper thread (BSPT).
The G thread (BSPP) and BSPT both follow British standards. But the BSPP has straight threads, while the BSPT has tapered threads. Also, while the BSPP threads are stylized as “G,” the BSPT threads are stylized as “R.” You’ll find out more about the difference between G and R threads in the next section.
Since G threads are BSP threads, it shares many similarities with the BSPT threads. Notably, both come with a 55-degree angle.
Aside from BSPP and BSPT, BSP threads are classified as jointing and long screw threads. This classification refers to how they form a joint.
Jointing threads create joints by mating two threads with a taper male head. So, BSPT threads are common in such applications, although the joint female can be tapered or parallel.
Long screw threads, on the other hand, create joints by compressing a washer or O ring seal. So, BSPP threads are the go-to in such applications.
What Is the Difference Between G Thread & R Thread?
It is easy to confuse G and R threads since they both conform to British standards. Also, both threads have a 55-degree angle and pitch. But as you’ll see below, they have many differences.
The primary difference between G threads and R threads is the thread type. G threads, which are BSPP threads, come with parallel threads. Meanwhile, R threads, which are BSPT threads, come with tapered threads.
As earlier explained, G threads seal with a washer or O ring. R threads, however, seal by mating. You need a parallel male and female need to seal a G thread. But you only need a tapered male head to seal an R thread. The male tapered head of R threads seals into parallel and tapered female heads.
Therefore, whether G and R threads are compatible is a partial yes. You can use an R thread male head with a G thread female head. However, the reverse won’t work.
If you opt to mate an R thread male head with a G thread female head, it is advisable to use a sealing material. This is to ensure the joint is tight.
Are G Threads Compatible With NPT?
G threads are incompatible with NPT, even though they look incredibly similar. Both thread types have differing angles — 55-degree for G threads and 60-degree for NPT threads.
It’s possible to get 60-degree cone G threads. But then, other differences come into play, including shape and thread pitch differences. Another significant difference is that G threads are parallel, while NPT threads are slightly tapered.
Some manufacturers sell G thread to NPT conversion adapters. When installed, the adapter will make a G thread interchangeable with an NPT thread and vice versa. People who use such conversion adapters confirm that they are safe.
However, getting an actual G thread or NPT thread for your hydraulic system is always safer.
Conclusion
In conclusion, G threads are simply BSPP — British Standard Pipe Parallel — threads. They differ from NPT threads and the British Standard Pipe Taper thread (BSPT), also known as R thread.
G threads are prevalent in high-pressure applications, which makes them more popular than their R counterpart. If you need quality G thread fittings, contact us at StrongFlex. We also provide other thread and fitting types.
