Fire Hose Thread Types: NST, NPT, and NPSH Explained

Thread type is the single most important specification when ordering a fire hose adapter. Two fittings can look identical and appear to thread together, then leak at 100 PSI or fail to seal at all. NST, NPT, NPSH, and GHT are four distinct thread standards used across fire service hardware, and they are not interchangeable without adapters.

This guide explains how each thread type works, where it appears on apparatus and hydrants, and how to identify what you have before placing an order.

Key Takeaways

• NST (National Standard Thread) governs most U.S. fire hose couplings and hydrant outlets per NFPA 1963.

• NPT (National Pipe Thread) is a tapered pipe thread found on gauges, pump fittings, and equipment ports. It is not interchangeable with NST.

• NPSH (National Pipe Straight Hose) is a straight thread found at pump suction ports and some older hydrant connections.

• GHT (Garden Hose Thread) is the civilian standard used on hose bibbs and smaller hydrant outlets.

• Visually similar threads can cross-connect and appear to seal, then fail under pressure. Always verify thread type with a gauge or thread ID chart.

Why Thread Types Matter for Adapter Selection

Every fire hose adapter has two ends, each with a specific thread type and diameter. When you order a 2.5-inch to 1.5-inch reducer, you need to specify: NST on both ends, or NST on one and NPT on the other, or some other combination. The default assumption (both ends NST) is correct for most hose-to-hose connections. It is wrong for pump-to-hose or hose-to-pipe connections.

The most common ordering mistake: a firefighter needs to connect a 2.5-inch hose to a 2.5-inch pipe fitting and orders a 2.5-inch adapter without specifying NPT on the pipe side. The result is either a cross-thread or an apparent seal that leaks above 50 PSI. Specifying the thread on both ends takes 10 seconds and prevents that problem.

Once you've confirmed the thread type on both ends, browse our fire hose adapters and fittings filtered by size to find the exact configuration you need.

NST: National Standard Thread

NST is the standard thread for fire hose couplings in the United States. NFPA 1963 specifies NST dimensions for every hose size from 3/4 inch through 6 inch. NST is also called NH (National Hose) in older references and on some manufacturer spec sheets; the two names refer to the same thread standard.

NST uses a parallel (straight) profile. The threads do not seal against each other; sealing depends entirely on the rubber gasket in the female coupling. Remove or damage the gasket and the connection leaks even if the threads appear tight.

NST Thread Specifications

NPT: National Pipe Thread

NPT is a tapered pipe thread. The taper is 1/16 inch per inch, which means the thread diameter increases as you go deeper into the fitting. This taper creates a metal-to-metal seal as the threads engage; no separate gasket is required (though thread sealant or PTFE tape is standard practice). NPT is the dominant thread standard for pipe fittings, gauges, and equipment connections across all industries.

In fire service, NPT appears on:

• Pump discharge gauge ports and pressure ports.

• Engine throttle governors and electronic monitor connections.

• Foam concentrate system connections.

• Adapters that transition from hose to fixed pipe systems.

NPT and NST are not interchangeable. They share similar thread pitches at some diameters, so they can appear to thread together. However, NPT's tapered profile will not seal correctly against NST's parallel profile, and cross-threading is common when someone attempts the connection under field pressure.

NPT Thread Specifications

NPSH: National Pipe Straight Hose

NPSH uses the same thread pitch as NPT but with a straight (parallel) profile instead of a taper. It appears at pump suction ports and on older hydrant steamer connections. The distinction matters because an NPT fitting will not seal correctly on an NPSH port: the taper creates an uneven contact with the parallel bore, and the connection leaks.

The most common NPSH connection in U.S. fire service is the 4.5-inch pump suction port. Most structural apparatus have a 4.5-inch NPSH female port at the front intake. Our hard suction hose assemblies use 4.5-inch NPSH male couplings that thread directly into this port. A Storz conversion adapter at this port requires 4.5-inch NPSH x 5-inch Storz.

NPSH Thread Specifications

GHT: Garden Hose Thread

Garden Hose Thread (GHT) is a civilian thread standard with a 3/4-inch nominal size. It appears on residential hose bibbs, smaller hydrant outlets (especially blowoff valves), and hazmat adapter configurations. GHT has the same thread pitch as 3/4-inch NPSH, but the diameters are different enough that they are not reliable substitutes for each other.

Fire service applications for GHT include decontamination setups, hazmat foam operations, and fill operations at residential water sources. For large-diameter operations that bypass thread standards entirely, browse our Storz adapters for quarter-turn connections that need no thread matching at all.

Thread Compatibility Quick Reference

How to Identify Thread Type in the Field

When ordering an adapter for an existing fitting, you need to identify both the thread type and the thread size. Four methods work reliably:

• Thread pitch gauge: A set of gauges measures threads per inch. Match the gauge to the threads and confirm TPI. Cross-reference against the tables above to identify the standard.

• Thread OD measurement: Measure the outside diameter of male threads (or inside diameter of female threads) with calipers. Cross-reference against the OD column in the tables above.

• Connection test with a known fitting: Thread a known NST coupling onto the suspect port. If it threads smoothly and seals with a gasket, it is NST. If it threads but leaks or feels uneven, it may be NPSH or NPT.

• Apparatus spec sheet: Most apparatus manufacturers publish complete port thread specifications. Check the OEM data for pump inlet, discharge, and equipment ports.

Once you've identified the thread on both sides of the connection you need to make, browse our full range of fire hose fittings to find the correct adapter by size and thread combination.

Thread Standards and NFPA Compliance

NFPA 1963 (Standard for Fire Hose Connections) governs thread dimensions for fire hose couplings in the U.S. It specifies NST dimensions for hose couplings and requires that all new fire hose purchased by U.S. fire departments use NST threads unless the department adopts an alternative and documents it in SOGs.

ANSI/ASME B1.20.1 governs NPT thread dimensions. AWWA standards govern the thread specifications on water main fittings and hydrant ports. When connecting across these two standards (fire service to utility water supply), an adapter that explicitly bridges NST to NPSH is the correct solution.

FAQ

What does NST mean on a fire hose?

NST stands for National Standard Thread. It is also called NH (National Hose). NST is the NFPA 1963 standard for fire hose couplings and most U.S. hydrant outlets. NST uses a parallel (non-tapered) thread profile and seals against a rubber gasket in the female coupling.

What is the difference between NST and NPT?

NST is a parallel (straight) thread that seals with a gasket. NPT is a tapered thread that creates a metal-to-metal seal as the taper tightens. They are not interchangeable. At 2.5-inch nominal size, NST has a 2.938-inch OD at 7.5 TPI; NPT has a 2.875-inch OD at 8 TPI. Similar but distinct. Use a dedicated NST-to-NPT adapter when connecting fire hose to pipe fittings.

What thread do fire hydrants use?

Most U.S. municipal fire hydrants use 2.5-inch NST on the 2.5-inch outlet ports and 4.5-inch NPSH on the larger steamer (pumper) port. AWWA C502 sets these standards for wet-barrel hydrants; AWWA C503 covers dry-barrel hydrants. Some older municipalities have non-standard threads, so departments that mutual-aid across jurisdictions should carry thread adapters.

Can NST and NPSH fittings connect?

They should not be directly connected. At 1.5-inch size, NST has 9 TPI and NPSH has 11.5 TPI. They will not thread together. At other sizes, similar pitch does not mean identical profile. Use a dedicated NST-to-NPSH adapter for any connection between hose and suction port hardware.

What is NPSH thread used for in fire service?

NPSH is the standard thread at the pump suction port on most U.S. fire apparatus. The 4.5-inch NPSH female port on the front or side of the pump accepts soft suction hose with 4.5-inch NPSH male couplings. It also appears on some older hydrant steamer ports.

How do I know what thread type my pump has?

Check the apparatus manufacturer's spec sheet or the pump manufacturer's documentation. Most Waterous and Hale pumps use 4.5-inch NPSH at the suction inlet. The discharge outlets are typically 2.5-inch NST. If documentation is unavailable, measure the port diameter with calipers and compare against the thread OD tables in this guide.