How To Use NPT Thread Gages - The Gage Store
https://www.thegagestore.com/docs/HowToUse/Thread_Gages/The_Gage_Store-Measurement_Tools_and_Accessories-How_to_Use_NPT_Thread_Gages.htm
NPTF, PTF-SAE Short (external) require the coordinated use of both the L1 & L2 gage rings. NPTF, PTF-SAE, Short (internal) require the coordinated use of both the L1 & L3 gage plugs. NPSF, NPSI require inspection with the L1 gage.
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