NPS, NPT, NPTF, and FNPT thread are all standards used to describe pipe threads. However, they differ in their application and design.

1. NPS: NPS stands for “National Pipe Straight.” It refers to a standard straight thread used for pipes that have a nominal size of 1/8 inch to 12 inches. The threads on NPS pipes are designed to create a tight seal when the pipes are joined together, but they are not intended for use in high-pressure applications.
2. NPT: NPT stands for “National Pipe Taper.” It refers to a tapered thread used for pipes with a nominal size of 1/16 inch to 24 inches. The threads on NPT pipes are designed to create a tight seal when the pipes are joined together, and they are commonly used in plumbing and hydraulic systems. NPT threads are also used in low-pressure applications, such as air compressors and pneumatic tools.
3. NPTF: NPTF stands for “National Pipe Taper Fuel.” It is a variation of the NPT thread that is designed to provide a better seal in high-pressure applications. The threads on NPTF pipes are cut at a steeper angle than the standard NPT thread, which allows for a tighter seal. NPTF threads are commonly used in hydraulic systems, air brakes, and fuel lines.

NPT vs. NPTF
ANSI B1.20.1 specifies the NPT (National Pipe Thread) requirements.
ANSI B1.20.3 specifies the requirements for NPTF (National Pipe Thread Fuel).

Threads per inch, pitch diameters, and taper per inch are all the same for NPT and NPTF. The major and minor diameters, as well as the root and crest of the threads, differ.

The NPTF thread crests are within the parameters of the NPT requirements but have a narrower range.

The NPTF thread roots are not the same as the NPT thread roots. NPTF thread roots are designed to interfere with the crest of the mating thread in order to create a mechanical seal during assembly via thread form deformation. There are two types of NPTF threads: Class 1 and Class 2. On assembly, NPT thread roots are designed to allow clearance with the mating thread crests.

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