North American Thread (NPT)

NPTF
National Pipe Tapered thread for Fuel is a dryseal thread.  It is used for both male and female ends.

The NPTF male will mate with the NPTF, NPSF or NPSM female.

The NPTF male has tapered threads and a 30 ํ inverted seat.  The NPTF female has tapered threads and no seat.  The seal takes place by deformation of the threads.  The NPSM female has straight threads and a 30 ํ inverted seat.  The seal takes placed on the 30 ํ seat.

The NPTF connector is similar to, but not interchangeable with, the BSPT connector.  The thread pitch is different in most sizes.  Also, the thread angle is 60 ํ instead of the 55 ํ angle found on BSPT threads.

NPSF
National Pipe Straight thread for fuels is sometimes used for female ends and properly mates with the NPTF male end.  However, the SAE recommends the NPTF thread in preference to the NPSF for female ends.

NPSM
National Pipe Straight thread for mechanical joint is used on the female swivel nut of iron pipe swivel adapters.
The leak-resistant joint is not made by the sealing fit of threads, but by a tapered seat in the coupling end.



                                   NPTF                                          NPTF or NPSF                    NPSM
                                 Solid Male                                      Solid Female                 Swivel Female
                                  (NPT)