"NPTF" (National Pipe Thread Fuel-Tapered) designates the thread form most commonly found on "U.S. inch" pipe plugs. The NPTF thread form is designed for crest-to-root contact. These "crush" immediately prior to the mating thread flanks making contact, to achieve maximum metal to metal interference fit. NPTF threads offer the best potential for leak-free taper thread assemblies. Pre-applied sealant or adhesive should be used to compensate for any variations in the condition of the mating threads and the assembly process.