GAS, UNC, UNF, and NPT Threads: Differences and Applications

GAS Threads: How to Recognize Them

Among the most widespread threading systems, gas threads are mainly used for the joint of pipes and hydraulic fittings. The name "gas" indeed comes from the primary use of these threads in the past, namely in gas pipelines.

Today, these elements are used in connections for piping and equipment that serve to convey fluids.
In fact, the very fine pitch of these threads and thus the closeness between the various threads (with a peak angle of 55°) allows for very strong connections.
These increase in resistance if the element is coupled with gaskets and thus resistant to high pressures. A particularity that distinguishes these threading systems from others is the unit of measurement: the classic metric system is not used, but inches are used instead. To understand how many inches correspond to the same millimeters, you can refer to the gas thread table.

In detail, there are two types of gas thread:

• Cylindrical gas thread
• Conical gas thread.

Difference Between Cylindrical and Conical Gas Thread

The cylindrical gas thread is defined as "not sealing on the thread".
This designation comes from the fact that the seal of this element is not generated by the male and female forms of the threads (both cylindrical) but by the application of a sealing material that ensures greater resistance and, therefore, that the conveyed fluid does not leak.

The conical gas thread is defined as "sealing on the thread". Unlike the former, the male thread is conical while the female thread is cylindrical. The intertwining of the two threads is what guarantees the greater seal of the element, without the application of other materials.

In general, regardless of the type of thread, in gas threading, the use of a sealant is recommended based on the type of fluid to be transported and its temperature.

Special attention deserves the diameter of the gas thread.
Indeed, unlike other threading systems where the nominal diameter coincides with the outer diameter of the screw and nut, here it corresponds to the inner diameter of the pipe.

UNC and UNF Threads: American Standard NPT

As highlighted earlier, the spread of threading systems also depends on geography. Indeed, while gas threads find global application, the situation changes if we move to the United States.

The American standard, in accordance with AISI B1.20.1 regulation, is the NPT (National Pipe Thread), the second most used system in the hydraulic sector. The thread angle of this type is 60°, thus with a larger pitch compared to the gas thread. Another difference with the gas thread concerns the shape of the thread. NPT is a type of thread only conical, therefore with a greater seal, although the use of gaskets is always recommended.

According to current regulations, there are three American threads:

• UNC thread (coarse pitch)
• UNF thread (fine pitch)
• UNEF thread (extra fine pitch).
  
  

Below is a comparison table for American threads.

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