4" BSP parallel (G4 / BSPP): 113.03 mm major diameter, 11 thread, parallel, 55 degree. What it fits and the adapters to join it to other hose and tap threads.
Updated 05 September 2026.
4" BSP parallel (G4 / BSPP) (G4)
Where G4 is used, its real dimensions, and how to connect it to other standards.
Used on: very large transfer, tank and industrial water pipework.
G4 is a BSP parallel thread of 113.03 mm major diameter at 11 TPI, cut to the 55 degree Whitworth form of ISO 228. Being parallel it seals on a washer or bonded seal at the face, not on the thread flanks.
Quick-connect click fittings from Hozelock, Gardena and Karcher are proprietary and do not interchange. They all sit on top of a standard thread underneath, so the reliable way to move between brands is to adapt at the threaded tap or hose-end fitting, then use each brand's own connector above it.
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Questions and answers
What is the designation of the 4" BSP parallel (G4 / BSPP) (G4)?
G4
What is the major diameter of the 4" BSP parallel (G4 / BSPP) (G4)?
113.03 mm
What is the threads per inch / pitch of the 4" BSP parallel (G4 / BSPP) (G4)?
11
What is the thread form of the 4" BSP parallel (G4 / BSPP) (G4)?
parallel, 55 degree