Gaging NPTF Internal & External Pipe Threads

Answer: 

As a follow-up to NPT gaging; we will complete the subject of gaging pipe threads with NPTF.
NPTF (National Taper Pipe Fuel) or dry seal threads are for more critical applications than NPT, therefore a greater variety of gages are required to check size, taper and the actual thread form. The crests of the external are "sharper" or more pointed than the roots of the internal threads, which are flat. When hand tightened, these interfere but the flanks remain separated. When wrench tightened, all these areas are crushed together and form a leak proof seal.

There are 2 classes of NPTF threads; Class 1 and Class 2 and the gaging practices are different.
Internal NPTF Class 1: Step A. Exactly the same as NPT, but make a note as to the location of the step within the minimum and maximum range.
Step B. Remove the L1 gage and insert a L3 gage which checks the wrench tight threads in the threaded section beyond that checked by the L1 gage. The threads on this gage are absent in the large end, but the notches are in the same place. This gage must stop within +/- a half turn of the position noted from the use of the L1 gage. The L3 gage checks the assembly and function size of the threads in this region and the correlation to the L1 gage results provides an indication of excessive steep or shallow taper deviation.
Note: Each step must coordinate with the preceding step, e.g. if the L1 is at max, the L3 must also be at Max, +/- one half turn or it does not qualify.
External NPTF Class 1: Step A. Exactly the same as NPT using an external L1 gage, but also note the position of this gage.
Step B. Use a L2 gage to check the wrench tight threads. The small end must also correlate within +/- a half turn to the L1 gage results. The L2 gage has thickness equal to the standard L2 gage but the threads near the small end are removed. This also checks the assembly and functional size beyond the L1 and the coordination of the L1 and L2 gives an indication of taper deviations.
Note: Each step must coordinate with the preceding step, e.g. if the L1 is at max, the L2 must also be at Max, +/- one half turn or it does not qualify.

Internal/External NPTF Class 2: Threads requirement is specified in this critical application to prevent leakage and thus crest and root truncation must be checked in the following steps.
Step A. 6-step Plain Taper Plugs and Rings to Check Thread Crest: Each step is marked to indicate the step used to coordinate with the preceding thread gages. Each gage is really 3 pairs of steps as follows.
B=Basic thread size at minimum truncation.
BT= basic thread size at Maximum truncation.

MN=Minimum thread size at minimum truncation.
MNT=Minimum thread size at maximum truncation

MX= Maximum thread size at minimum truncation.
MXT= Maximum thread size at maximum truncation.
Only one pair of steps is used on any given part and is established by the L1 gage, which separates the gaging into one of three categories of Minimum, Basic or Maximum. If the L1 gage determines the thread is Basic, the 6-step gage must be between the B and BT steps etc.
Step B. 6-Step Root Truncation Gaging: This thread gage is a 50 degree thread flank angle to fit inside the 60 degree thread flank to insure they contact only at the roots of the threads. The steps and process is the same as the 6-step crest gages above B, BT, MN, MNT, MX and MXT.