Are merchant steel fittings with NPTF (ASME B1.20.3) threads compatible with NPT threads?

NPTF and NPT threads can be threaded together, however the thread crests (major diameter) and thread roots (minor diameter) differ slightly as well as the allowed tolerances. While NPTF thread crests have the same shape and major diameter as NPT, their tolerance range is narrower and more carefully controlled. NPTF minor diameters are slightly smaller than NPT, making their thread roots truncated or shallower than NPT threads.

While NPT and NPTF thread forms are almost identical, NPT threads are general-purpose threads that require a thread sealant for a leakproof seal, whereas NPTF to NPTF joints creates a strong, leakproof mechanical seal without using a sealant. When mixing threaded pipe fittings with dry-seal NPTF threads and NPT threaded fittings the use of a thread sealants is required for leakproof connections.

NPT fittings can be reused but require careful inspection to ensure there is no thread damage from overtightening.

WARNING: Never reuse NPTF fittings for flammable and combustible gases or liquids service or systems carrying hazardous and aggressive gases or liquids. While it is not recommended to reuse NPTF fittings, if an NPTF fitting is reused in a non-hazardous application careful inspection for thread distortion and proper thread engagement is critical. The use of a thread sealant for a leakproof seal maybe required.