NPT stands for National Pipe Taper. For NPT, pipe should be thread per ANSI B1.20.1. For the National Pipe Tapered for Fuels, or NPTF, the pipe should be thread as per ANSI B1.20.3.
Differences in NPT and NPTF threads lie in the major and minor diameters, as well as the root and crest of the threads. NPTF thread crests call within the same parameters as NPT requirements, but NPTF crests have a smaller range.
NPTF thread roots are designed to interfere with crest of the mating thread, which creates a mechanical 
seal through thread form deformation at assembly, while NPT thread roots are designed to allow clearance with mating thread crests on assembly.
While NPT and NPTF are both designed to screw together, NPT requires a seal to be leak proof while NPTF do not. The L1 gauge requirements for NPT and NPTF differ for threads more than ½ in. in size, and thus, are not interchangeable. The difference lies in the Plug Gage Major Diameter and Ring Gage Minor Diameter.
But the most significant difference is the inspection required for each. With NPT, only a single plug with a step, an L1 plug for internal threads and a single thin L1 ring for external, are required to check size. For NTPF, additional threads in assembly, L2 and L3, and the major and minor diameters are inspected with either a special plug or ring gages.
I am attaching a 1/2 inch fuel hose to a sump from Bean Performance Diesel. A tech salesman told me the outlet port is 1/2 in NPT threads. I was planning on putting a 1/2″ crimp – male 1/2″ NPTF fitting on the fuel line and then threading it on the 1/2″ NPT fuel port. Will this work? Should i just use NPT regular fitting and some Tephlon? I’m not even sure if the hose can be crimped. Are most types of fuel hose able to be crimped? it is a FASS fuel system. any help would be greatly appriciated
Perfect timing for this article. Just had this discussion with mechanics during a training session when someone had mentioned that he had been told that no sealant was required on a PIPE THREAD. The levels of understanding when properly understood, can be used to refute rumors and incomplete training, which carries on for years until more knowledge is gained and problems actually become eliminated.
I have never come across a tapered thread that formed a leak-proof connection without the addition of some form of sealing assistance, either seal tape or thread compound, or a nut with an deformable seal. And never would I believe that an NPT connection would not leak without some seal material.
Thus the claim in tghe article isamazing.
Thats the point, they’re talking about NPTF.
i believe your reading this backwards… PTF ( NPTF) threads have the crush thread to provide sealing. . NPTF to NPTF would not require ( in theory) any sealing compound NPTF to NPT would, as with any NPT-NPT connection.
I concur. I’ve never had a pipe thread maintain a seal without Teflon tape or liquid equivalent.
The third paragraph states as follows:
“NPTF thread roots are designed to interfere with crest of the mating thread, which creates a mechanical seal through thread form deformation at assembly, while NPT thread roots are designed to allow clearance with mating thread crests on assembly”
When we read this we see that the writer never stated that NPT threads were designed to use no sealant, but in fact designed to have clearance in the thread profiles.
NPTF when manufactured properly, are designed to be like a crush washer, with engineered deformation forming the seal barrier.
My take away is that if both male and female are manufactured properly, NPTF will not require a sealant, NPT will.
You are an idiot. It literlly describes that an NPTF is design to seal and NPT is not designed to seal.
WIlliam,
NPT requires a sealant while NPTF does not (allegedly). That is clearly stated above in the article.
I AM an IDIOT and I clearly understood what was written. NPTF to NPTF roots & crests deform to create a mechanical seal that should NOT require any sealant.
NPT to NPT REQUIRE sealant because the Roots * Crests of the thread have some clearance. Thus Sealant REQUIRED.
NTP to NPTF would REQUIRE sealant because the threads are of a ever so slight different shape. IE: While they CAN be threaded together, There NOT SPECIFICALLY designed to work together to make a leak proof seal as NPTF x NPTF are.
I’m no expert on pipe threads but that seemed pretty easy to understand!
Not saying which is correct but i do know that using teflon as a sealant is also a lubricant which would reduce run down torque effort. In effect you will be deforming the thread metal further to make a tighter seal. More torque applied, less grunting.