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Connect 2-1/2" NH fire hose thread (NST) to 1" NPT (National Pipe Taper)

How to join 2-1/2-7.5NH and 1-11.5 NPT, and what stops it leaking.

Needs adapter
End A
2-1/2" NH fire hose thread (NST) (2-1/2-7.5NH)
Seals on rubber gasket in the female swivel
via adapter
End B
1" NPT (National Pipe Taper) (1-11.5 NPT)
Seals on on the thread flanks with PTFE tape
Adapter needed2-1/2 NH to 1 NPT fire hydrant adapter

Not a direct fit. Joining 2-1/2" NH fire hose thread (NST) to 1" NPT (National Pipe Taper) needs a 2-1/2 NH to 1 NPT fire hydrant adapter.

The adapter to buy

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A 2-1/2 inch NH hydrant or FDC connection adapts to a 1 inch NPT pipe with a brass reducing adapter. The NH end seals on its rubber gasket; the NPT end seals on the tapered thread. This is the standard way to draw a small line off a hydrant.

The 2-1/2 NH thread is coarse (7.5 TPI) and unrelated to any pipe thread. Only an NH-to-NPT adapter will join them; nothing threads directly.

Related connections

2-1/2" NH fire hose thread (NST)2-1/2-7.5NH1" NPT (National Pipe Taper)1-11.5 NPT2-1/2" NH fire hose thread (NST) to 1-1/2" NH fire hose thread (NST)adapter

Sources: NFPA 1963 (fire hose connections, NH / NST) · NPSH / NH / NHR hose coupling thread table, Fittings.space · ANSI/ASME B1.20.1 (NPT pipe threads) · NPT thread dimensions chart (ASME B1.20.1), Machining Doctor