NPT vs NPTF
The requirements for NPT (National Pipe Taper) are spelled out in ANSI B1.20.1
The requirements for NPTF (National Pipe Taper 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 within 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 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 are designed to screw together. In most cases, there will be no interference between the root and crest of the threads at assembly. The thread is designed to be assembled with some form of sealant to assure a leak-free joint.
NPTF threads will screw together with NPT threads and should have no noticeable assembly problem. There will most likely be an interference fit between the root and crest on either the major or minor diameter of the thread, depending on which part is NPTF. To accomplish a seal on the joint, a sealant will be required.
NPTF threads are designed to screw together. They are designed to have interference at the roots and crests of the threads on both the major and minor diameters. This interference should cause no assembly problem. The thread deformation caused by the interference fit and the wrench tightening is designed to make a dry mechanical seal.
Because of the gauging requirements of the NPTF Class 1 product thread, a sealant should be used to assure a leak-proof connection. Because of the gauging requirements of the NPTF Class 2 product thread, the seal accomplished through mechanical deformation should require no sealant to make a leak-proof joint. This is critical in some applications where the pipe content is corrosive to the chemical sealant.