Pipe Threads NPT versus ANPT

[awol]

Using the standards ASME B1.20.1 for National Pipe Taper (NPT) threads and SAE AS71051 for Aeronautical National Pipe Taper (ANPT) threads, is it practical to manufacture NPT threads and check them as ANPT threads? Can NPT and ANPT L1, L2, and six-step gages be interchanged or must they be used as sets? Example, use an NPT L1, ANPT L2, and ANPT six-step to check ANPT threads?

My vendor is creating "sharp" external pipe threads in order to satisfy all three ANPT gages. The sharp external threads are (I believe) causing an interference issue when mating with other ANPT and NPT threads because the internal thread roots can be shallower.

We didn't have this great of a galling issue before the ANPT threads became a requirement. Also, corresponding with the sharp external threads, external threads that gage ANPT correctly and are assembled one time will not gage ANPT. The once-assembled threads allow the six-step to fall too far. Any comments??

These connections are make and break, meaning the threads are intended to be used several times without damage.

 

[metalonis]

awol,

Sorry I don't have an answer because I have the same question! I have compared both standards and can't find any difference between NPT and ANPT threads.

 

[awol]

After some investigation, here is what I found. NPT and ANPT threads are made the same way. The ANPT threads are required to meet two additional gaging methods, the L2 and the 6-step. There is no difference in the NPT L1 gage and the ANPT L1 gage. No such gages as L2 and six-step for NPT.

The L2 gage check the 4th, 5th and 6th threads , I believe.

While this sounds very exact, I have discovered that threads will gage differently after being assembled a few times, near to the point of failing the inspection. Why? Are there machining burrs that "fool" the gage during inspection that are removed during assembly? Or are the threads and material 'yielding'?

The sharp threads seem to be correct, the deep roots are correct. I have determined my joint issue is the type of thread sealant, not the thread profile. NPT threads should not be the first choice for a make-and-break appliation. Teflon tape should not be used if the joint is to be disassembled. The ductile iron got some bad publicity at my company, but I believe the material is OK. -awol

 

[ramogon]

What is the difference between NPT and ANPT threads? Manufacturing? Gaging? One type of thread better than another?