What standard define the exact dimensions and tolerances for NPT threads?

The exact NPT thread dimensions and tolerances are primarily defined by the ANSI/ASME B1.20.1 standard. This standard specifies the thread form, pitch, taper angle, and diameter tolerances required for NPT threads used in the United States. Here’s an overview of key standards governing NPT threads:

ANSI/ASME B1.20.1 (Pipe Threads, General Purpose)

This standard provides specifications for general-purpose NPT for sizes ranging from 1/16 inch to 24 inches. It includes detailed guidelines on pitch diameter, taper (typically 1 in 16), and thread depth, along with tolerances for both male (external) and female (internal) threads.

ANSI/ASME B1.20.3 (Dryseal Pipe Threads)

The B1.20.3 standard outlines the specifications for NPTF (National Pipe Taper Fuel) threads, which are similar to NPT but provide a tighter seal without the need for additional sealants. These threads are often used in fuel, hydraulic, and air systems where leakage is not acceptable.

ISO 7-1 / ISO 228 (International Standards for Pipe Threads)

For international applications, ISO standards may be used. While they define similar specifications to NPT, ISO standards typically specify BSP (British Standard Pipe) threads, which differ in thread angle and taper. ISO 7-1 includes specifications for tapered threads, while ISO 228 includes parallel threads.