3-1/2" NH fire hose thread (NST): 107.77 mm major diameter, 6 thread, straight (parallel), 60 degree. What it fits and the adapters to join it to other hose and tap threads.
Updated 05 September 2026.
3-1/2" NH fire hose thread (NST) (3-1/2-6NH)
Where 3-1/2-6NH is used, its real dimensions, and how to connect it to other standards.
Used on: US fire hose, standpipes and large transfer lines at 3-1/2 inch.
The 3-1/2 inch NH (National Hose / NST) thread runs 6 TPI on a 4.243 inch (107.77 mm) outside diameter and seals on a rubber gasket in the female swivel, as defined for fire connections in NFPA 1963. It is not a pipe thread and will not mate with an NPT of the same nominal size without an adapter.
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 3-1/2" NH fire hose thread (NST) (3-1/2-6NH)?
3-1/2-6NH
What is the major diameter of the 3-1/2" NH fire hose thread (NST) (3-1/2-6NH)?
107.77 mm
What is the threads per inch / pitch of the 3-1/2" NH fire hose thread (NST) (3-1/2-6NH)?
6
What is the thread form of the 3-1/2" NH fire hose thread (NST) (3-1/2-6NH)?
straight (parallel), 60 degree