Comprehensive Explanation Of Imperial Thread Standards

As a commonly used standard system in the machinery industry, imperial threads are classified by shape into cylindrical threads and tapered threads, and by thread angle into 55° Whitworth threads and 60° Unified National Threads. Imperial thread size markings such as 1/8, 1/4 and 1/2 are measured in inches. According to industry conventions, 1 inch equals 8 British fractional divisions; for example, 1/4 inch is called 2 divisions and 1/2 inch is called 4 divisions.

NPT, PT, G and RC are common industrial pipe thread standards belonging to American and British standard systems respectively, with distinct differences in functions, thread profiles and sealing principles.

1. Detailed Classification of Common Pipe Thread Standards

1.1 NPT Thread

NPT (National Pipe Thread) is a 60° tapered sealing pipe thread under American standards, widely used for hydraulic, pneumatic and fuel sealing pipelines in North America. It complies with the national standard GB/T 12716-1991. Featuring self-sealing performance through tapered thread extrusion without additional gaskets, it delivers excellent sealing reliability.

1.2 PT Thread

PT thread is a 55° tapered sealing pipe thread of the British Whitworth thread family with a standard taper of 1:16. It is prevalently applied in water supply, gas and pressure pipeline systems in Europe and Commonwealth countries, conforming to the national standard GB/T 7306-2000. Sealing is achieved by compression and extrusion during thread engagement.

1.3 G Thread

G thread is a 55° non-sealing cylindrical pipe thread also belonging to the Whitworth thread system. With no taper, it cannot achieve self-sealing and requires auxiliary sealing materials such as gaskets and PTFE tape during assembly. It complies with the national standard GB/T 7307-2001. The size markings of G threads represent the nominal pipe diameter in inches.

1.4 RC and ZG Thread Explanation

RC refers to British 55° tapered internal sealing pipe thread. ZG is an obsolete domestic designation for tapered pipe threads, which has been officially replaced by the unified R and RC standard series and is no longer applicable. Tapered thread structures adopted for water pipe and hydraulic joints achieve locking and sealing through tapered surface fitting.

2. Size Comparison and Common Names of Standard Imperial Pipe Threads

Imperial thread conversion standard: 1 inch = 25.4 mm, 1 inch = 8 British fractional divisions

3/4 inch thread: commonly known as 6 divisions, corresponding to a nominal pipe diameter of approximately 20 mm

1/2 inch thread: commonly known as 4 divisions, corresponding to a nominal pipe diameter of approximately 15 mm

1 inch thread: commonly known as 1 inch, corresponding to a nominal pipe diameter of approximately 25 mm

1-1/2 inch thread: commonly known as 1.5 inches, corresponding to a nominal pipe diameter of approximately 32 mm

DN stands for nominal diameter, a unified specification mark for pipeline matching.

3. Functional Differences Between Cylindrical and Tapered Pipe Threads

Tapered pipe threads (NPT, PT, RC) adopt a standard 1:16 taper. The thread tightens progressively during engagement and realizes reliable sealing by thread extrusion and deformation, making them suitable for high-pressure sealing scenarios such as hydraulic circuits, high-pressure oil pipelines and airtight piping systems.

Cylindrical pipe threads (G threads) are straight threads with fixed fitting clearance and no self-sealing capability. They are mainly used for low-pressure water and air pipeline connection focusing on positioning and fastening, and auxiliary sealing materials are required to prevent leakage. The core difference between the two types lies in taper structure and sealing mechanism.

4. Basic Definition and Classification of Threads

A thread is a continuous regular tooth profile machined along a spiral curve on the surface of a cylindrical or conical base. The raised solid part of the thread is defined as thread crest, and the groove between adjacent crests is the thread root. In mechanical processing, when the workpiece rotates one full circle, the tool moves axially by a fixed pitch to form a complete thread structure.

Threads machined on the outer surface are external threads, while those machined on inner hole surfaces are internal threads. The matching of internal and external threads realizes mechanical fastening and connection.

According to cross-sectional tooth profiles, threads are divided into triangular threads, trapezoidal threads, sawtooth threads, square threads and circular arc threads. Imperial and American standard ordinary threads all belong to triangular threads, which are the most widely used thread type in mechanical assembly.