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American Standard Pipe Thread

American Standard Pipe Thread, often referred to as National Pipe Thread (NPT), is a type of screw thread commonly used on pipes and fittings to ensure a tight, leak-proof seal. Developed in the United States, NPT threads are widely utilized in plumbing, gas, and hydraulic systems.

Historical Overview

Development and Adoption

  • Early 19th Century: The industrial revolution and the growth of mechanical engineering created a need for standardized pipe fittings and connections.
  • William Sellers: In 1864, William Sellers proposed a standardized thread for mechanical fasteners, which laid the groundwork for future standardization in piping systems.

Establishment of Standards

  • American Society of Mechanical Engineers (ASME): By the early 20th century, ASME played a crucial role in formalizing standards for pipe threads.
  • NPT Standard: The National Pipe Thread standard was established to ensure compatibility and uniformity across different manufacturers and industries. This standard is now widely recognized and used throughout North America.

Key Characteristics of NPT

Thread Design

  • Tapered Threads: NPT threads are tapered, which means the diameter of the thread decreases along its length. This taper helps create a tight seal as the threads are screwed together, preventing leaks.
  • Angle: The thread angle is 60 degrees, which is standard for many types of screw threads.
  • Flat Crests and Roots: The crests and roots of NPT threads are flat, which enhances the sealing capability.

Sealing Mechanism

  • Interference Fit: As NPT fittings are tightened, the tapered threads create an interference fit that compresses the threads and forms a seal.
  • Thread Sealant: To ensure a completely leak-proof connection, a thread sealant or Teflon tape is often applied to the threads before assembly. This sealant fills any gaps and enhances the seal.

Types of Pipe Threads


  • NPT (National Pipe Taper): Standardized tapered threads used in many plumbing and industrial applications. Requires sealant for a leak-proof connection.
  • NPTF (National Pipe Taper Fuel): Similar to NPT but designed to provide a dry seal without the need for sealant. The threads are machined to closer tolerances to ensure a tight seal.

Other Variants

  • BSPT (British Standard Pipe Taper): Similar to NPT but with different thread angle and dimensions. Not directly compatible with NPT.
  • BSPP (British Standard Pipe Parallel): Uses parallel threads instead of tapered and requires a sealing ring to create a seal.



  • Water Systems: NPT threads are extensively used in residential and commercial water supply systems.
  • Gas Lines: Common in natural gas and propane lines due to their reliable sealing capabilities.


  • Hydraulic Systems: Used in hydraulic machinery and equipment where secure, high-pressure connections are essential.
  • Pneumatic Systems: Employed in air compressor systems and other pneumatic applications.

Oil and Gas

  • Oil Rigs and Refineries: NPT threads are crucial in oil and gas extraction and processing equipment, ensuring safe and leak-proof connections.

Standards and Specifications

ASME B1.20.1

  • Primary Standard: The ASME B1.20.1 standard details the dimensions, tolerances, and design specifications for NPT threads. It provides guidelines for the manufacturing and testing of NPT fittings.


  • ANSI/ASME Standards: In the United States, ANSI/ASME standards govern the specifications for NPT threads.
  • ISO Standards: Internationally, the International Organization for Standardization (ISO) provides similar guidelines for pipe threads, ensuring compatibility and quality.

American Standard Pipe Thread (NPT) has been a critical component in the plumbing, gas, and industrial sectors for over a century. Its design, characterized by tapered threads, ensures a tight and reliable seal that is essential for leak-proof connections. Governed by ASME standards, NPT threads are widely recognized and used across North America and beyond, making them a foundational element in various applications. The evolution and standardization of NPT threads underscore their importance in ensuring safe and efficient piping systems.

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