Are NPT pipe threads tapered or straight?

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Prepare for the NFPA 54 National Fuel Gas Code Test. Enhance your knowledge with multiple-choice questions and flashcards, complete with hints and detailed explanations. Get exam-ready now!

Multiple Choice

Are NPT pipe threads tapered or straight?

Explanation:
NPT pipe threads are tapered. The taper is what lets a threaded joint seal as you tighten it—the diameter of the threaded portion changes along the length, so the threads wedge more tightly together and with thread sealant to form a leak-tight connection. This tapered engagement (1/16 inch of change in diameter per inch of length) is the reason NPT is used for gas piping. If the threads were straight or parallel, they wouldn’t seal reliably without a separate gasket or compression fit, which isn’t how NPT fittings are intended to seal. The term “helical” describes the thread’s spiral shape, not whether it seals by taper, and “parallel” describes straight threads, which won’t provide the intended gas-tight seal.

NPT pipe threads are tapered. The taper is what lets a threaded joint seal as you tighten it—the diameter of the threaded portion changes along the length, so the threads wedge more tightly together and with thread sealant to form a leak-tight connection. This tapered engagement (1/16 inch of change in diameter per inch of length) is the reason NPT is used for gas piping. If the threads were straight or parallel, they wouldn’t seal reliably without a separate gasket or compression fit, which isn’t how NPT fittings are intended to seal. The term “helical” describes the thread’s spiral shape, not whether it seals by taper, and “parallel” describes straight threads, which won’t provide the intended gas-tight seal.

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