Gaging NPT Internal and External Pipe Threads

Gaging NPT is really just a single step process as opposed to the multiple steps involved with gaging NPTF threads. We will address the NPTF gaging steps in a future e-tip.

Internal NPT Gaging Process: Using a L-1 NPT Plug Gage, rotate the gage into the threaded hole until it stops. This referred to as "Hand Tight." If the notch on the gage stops flush with the top of the threaded opening, the part is "Basic Size." The threaded opening is further allowed +/- 1 full rotation to still be considered basic size. Note that minus 1 turn above the notch is minimum size allowable and 1 full turn below the notch is the maximum size allowable.

External NPT Gaging Process: Using a L-1 NPT Ring Gage, (which is the total thickness of "Hand Tight.") rotate the gage onto the threaded section until it stops and if the small end is flush to the end of the threaded section it is considered to be "Basic Size." The gage may stop up to 1 turn short of the thread and be considered as maximum size or stop 1 turn beyond the end and still be considered to meet the allowable minimum size.

Remember that NPT class threads require sealant or tape wrap to prevent leakage.-David Miskinis, Sr. Technical Engineer