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NPTNATIONAL PIPE TAPERED NPT is a US standard for tapered threads used to join pipes and fittings. ANSI/ASME standard B1.20.1 covers threads of 60 deg; form with flat crests and roots. The taper on NPT threads allows them to form a seal when torqued as the flanks of the threads compress against each other. However a clearance remains between the crests and roots of the threads - resulting in a leakage around this spiral. This means that NPT fittings must be made leak free with the aid of thread seal tape or a thread sealant compound. For each 12 increase in distance from the end - the diameter increases by 3/4.