Understanding NPT, NPTF & NPS Thread Connections

What is the Difference between NPS, NPT & NPTF?

NPS = National Pipe Straight
NPT = National Pipe Thread
NPTF = National Pipe Thread Fuel

Both NPT and NPS have the same thread angle, shape, and pitch (threads per inch). However, NPT threads are tapered and NPS threads are straight (parallel). Both threads have a 60° included angle and have flat peaks and valleys. Tapered Pipe Thread is commonly found on ends of pipe, nipples and fittings (ex: couplings, elbows, tees, etc.).

Sharply angled threads are very critical to joint being tight with no leaks. Thread sealant or tape is typically required to complete the seal. Straight Pipe Threads need a gasket or O-ring to create a seal. While NPT and NPS threads will engage, they do not seal properly with each other.

NPT vs. NPTF

The requirements for NPT (National Pipe Thread) are spelled out in ANSI B1.20.1
The requirements for NPTF (National Pipe Thread Fuel) are spelled out in ANSI B1.20.3

Both NPT and NPTF have the same threads-per-inch, pitch diameters, and taper-per-inch. The differences come in the major and minor diameters, the root and crest of the threads.

The NPTF thread crests fall with in the parameters of the NPT requirements, but have a smaller range than the NPT.

The NPTF thread roots are different than the NPT thread roots. NPTF thread roots are designed to interfere with the crest of the mating thread with the intention of creating a mechanical seal through thread form deformation at assembly. NPTF threads have the same basic shape but with crest and root heights adjusted for an interference fit, eliminating the spiral leakage path. NPTF threads have two classes identified: Class 1 and Class 2. NPT thread roots are designed to allow clearance with the mating thread crests on assembly.

NPT Threads - Creating a leak free NPT connection involves the use of thread sealant tapes.

National Pipe Taper Fuel (NPTF), also called Dryseal American National Standard Taper Pipe Thread, defined by ASME B1.20.3, is designed to provide a more leak-free seal without the use of PTFE tape (aka "Teflon") or other sealant compound. NPTF threads are the same basic shape as NPT but with crest and root heights adjusted for an interference fit, eliminating the spiral leakage path.

Can you connect NPTF to NPT threads?

Yes, you can connect an NPFT (National Pipe Fuel Taper) thread to an NPT (National Pipe Taper) thread and achieve a leak-free connection, but it requires careful consideration due to differences in the thread standards. While they can be mated, the connection may not be perfect due to slight differences in thread form and tolerances.

To achieve a leak free connection:

Use a Thread Sealant: Apply a high quality PTFE thread tape or pipe dope compatible with the fluid or medium. Wrap the PTFE 2-3 turns in the direction of the thread to avoid unraveling when you assemble the mating fittings.
Ensure both threads are clean, undamaged and free of debris to maximize the sealing effectiveness.
Use an Adapter if Needed: If the connection is critical or tolerances are a concern, consider using a properly designed adapter or fitting specifically rated for NPFT-to-NPT connections. The NPFT’s tighter tolerances mean it may not seal as reliably with NPT in high-pressure or critical fuel systems without proper sealing measures. Always verify compatibility with the specific application (e.g., pressure, fluid type).
4. Testing: After assembly, pressure-test the connection to confirm it’s leak-free, especially for fuel or high-pressure systems.

NPT NPTF Thread Specifications
Thread Size	Nominal Size (in)	Number of Threads per Inch	Nominal Male Thread Outside Diameter (OD) (in)	Nominal Male Thread Outside Diameter (OD) (in)
1	1/16	27	5/16	0.313
2	1/8	27	13/32	0.405
4	1/4	18	35/64	0.540
6	3/8	18	43/64	0.675
8	1/2	14	27/32	0.840
12	3/4	14	1-1/16	1.050
16	1	11.5	1-5/16	1.315
20	1-1/4	11.5	1-21/32	1.660
24	1-1/2	11.5	1-29/32	1.900
32	2	11.5	2-3/8	2.375
40	2-1/2	8	2-7/8	2.875
48	3	8	3-1/2	3.500
64	4	8	4-1/2	4.500