G-thread

What is a G thread?

The G thread is a pipe thread similar to metric ISO threads, commonly used in pipe connections and hydraulic components. It is based on the British BSP (British Standard Pipe) standard and is specifically defined in the ISO 228 standard. The G thread is cylindrical, meaning both the internal and external threads are straight rather than tapered.

Characteristics and uses of the G thread

The G thread is widely used, especially in mechanical engineering, where tight and reliable pipe joints are needed. Its biggest difference compared to the BSPT thread (R thread) is that it doesn’t seal by the thread itself but requires a separate seal, such as an O-ring or washer, to be pressure-tight. This makes it suitable for applications where connections need to be easily opened and serviced.

G thread sizing and standards

The G thread follows inch-based sizing, but its thread pitch and profile are standardized according to ISO 228. Common sizes include G1/4, G3/8, G1/2, and G1, which are often used in pneumatic and hydraulic connections. Compared to metric threads, G threads are specifically designed for pipe connections where the outside diameter is critical, but wall thickness may vary.

Machining G threads

Since it’s a parallel (cylindrical) thread, there’s no need to account for tapering when machining G threads, simplifying the manufacturing process compared to tapered threads like R threads. However, because a separate seal is needed, it’s important to leave sufficient space for the seal to ensure the connection works properly.

Summary

The G thread is a widely used parallel pipe thread based on BSP standards and ISO 228 specifications. It does not seal by thread alone but requires a separate sealing element. It’s commonly used in pneumatic and hydraulic couplings that need to be easily serviced and opened. Machining G threads is straightforward because they are not tapered, but the need for seals must be taken into account when designing connections.