Aluminum plugs are fabricated from seamless extruded 6061-T6 aluminum and hard coat anodized per MIL-A-8625 Type III, Class 1. Threads have a blunt start Higbee cut with indicators on adjacent lugs. Part identifications are permanent and stamped prior to hardcoat. Plugs and components meet the requirements of NFPA 1963.

Note - 3 inch NH / NST is very rare and is often confused with 2.5 inch NH / NST which is very common. If you have a fire department connection (FDC), hose coupling or an adapter check the female coupling for stamped numbers and letters. Most 3.0 inch hoses use 2.5 inch NH / NST couplings. The most common fire fitting thread in the United States is 2 1/2 inch NH /NST.

If you have two swivel female FDC brass connections that need plugs measure the INSIDE diameter of the swivel fitting. If it measures approximately 3 inches, you need 2 1/2 inch NH / NST plugs. 3 inch NH / NST fittings measure approximately 3 5/8". If you have any doubt it is best to contact your local fire department for size and thread type in your area.

If possible it is best to measure the MALE fitting. The outside diameter of a 2.5 inch NH / NST male hose coupling or fire hydrant is 3.06 inches. The outside diameter of a 3.0 inch NH / NST male hose coupling or fire hydrant is 3.62 inches. The outside diameter of a 2.5 inch NPT or NPSH male hose coupling is 2.84 inches. The outside diameter of a 3.0 inch NPT or NPSH male hose coupling is 3.47 inches.

• Outside diameter of a male GHT adapter: 1 1/16 inches (1.06) - 11.5 Threads per inch
• Garden hose thread is not the same as 3/4 inch pipe thread (NPT)

• Inside diameter of a male adapter: 1 inch
• Outside diameter of a male NH / NST adapter: 1 3/8 inches (1.375) - 8 Threads per inch
• Outside diameter a male NPSH or NPT adapter: 1 5/16 inches (1.295) - 11.5 Threads per inch

• Inside diameter of a male adapter: 1 1/2 inches
• Outside diameter of a male NH / NST adapter: 2 inches (1.99) - 9 Threads per inch
• Outside diameter a male NPSH or NPT adapter: 1 7/8 inches (1.87) - 11.5 Threads per inch

• 1 3/4 inch hoses come with 1 1/2 inch couplings

• Inside diameter of a male adapter: 2 inches
• Outside diameter of a male NH / NST adapter: 2 1/2 inches (2.51) - 8 Threads per inch
• Outside diameter a male NPSH or NPT adapter: 2 3/8 inches (2.35) - 11.5 Threads per inch

Note: 2 inch NH or NST is very rare. If the outside diameter of male adapter is approximately two inches, you need 1 1/2 inch NH or NST.

• Inside diameter of a male adapter: 2 1/2 inches
• Outside diameter of a male NH / NST adapter: 3 1/16 inches (3.06) - 7.5 Threads per inch
• Outside diameter a male NPSH or NPT adapter: 2 7/8 inches (2.84) - 8 Threads per inch

• Most 3 inch hoses come with 2 1/2 inch couplings
• Inside diameter of a male adapter: 3 inches
• Outside diameter of a male NH / NST adapter: 3 5/8 inches (3.62) - 6 Threads per inch
• Outside diameter a male NPSH adapter: 3 1/2 inches (3.47) - 8 Threads per inch
• Outside diameter a male NPT adapter: 3 3/8 inches (3.38) - 8 Threads per inch

• Inside diameter of a male adapter: 4 inches
• Outside diameter of a male NH / NST adapter: 5 inches (5.01) - 4 Threads per inch
• Outside diameter a male NPSH adapter: 4 1/2 inches (4.47) - 8 Threads per inch

• Inside diameter of a male adapter: 4 1/2 inches
• Outside diameter of a male NH / NST adapter: 5 3/4 inches (5.76) - 4 Threads per inch
• Outside diameter a male NPSH or NPT adapter: 5 inches (4.97) - 8 Threads per inch

• Inside diameter of a male adapter: 5 inches
• Outside diameter of a male NH / NST adapter: 6 1/4 inches (6.26) - 4 Threads per inch
• Outside diameter of a male NPSH adapter: 5 9/16 inches (5.56) - 8 Threads per inch
• Outside diameter of a male NPT adapter: 5 7/16 inches (5.44) - 8 Threads per inch

• Inside diameter of a male adapter: 6 inches
• Outside diameter of a male NH / NST adapter: 7 1/32 inches (7.025) - 4 Threads per inch
• Outside diameter of a male NPSH adapter: 6 5/8 inches (6.62) - 8 Threads per inch
• Outside diameter of a male NPT adapter: 6 1/2 inches (6.50) - 8 Threads per inch

• The female side of a garden hose will only connect to a male garden hose connection
• The male side of a garden hose will only connect to a female garden hose connection

NH / NST: The most common thread type in use by fire departments today is NST or National Standard Thread. NST also goes by the initials NH or National Hose. It can even be referred to as NHT or National Hose Thread but NST or NH is more common these days. NH = NST.

• The female side of a NH / NST coupling will only connect to a male NH / NST connection
• The male side of a NH / NST coupling will only connect to a female NH / NST connection

NPSH: National Pipe Straight Hose (NPSH) or Iron Pipe Thread (IPT) has the same threads per inch (TPI) as National Pipe Tapered (NPT) but the thread does not taper. NPSH is a straight thread and seals by a gasket in the female coupling which allows for a swivel, just like NH / NST, which make both ideal for hose couplings.

• The female side of a NPSH coupling will only connect to a male NPT pipe or a male NPSH connection
• The male side of a NPSH coupling will only connect to a female NPSH connection
• It will NOT thread into a female NPT pipe. For example, a male NPSH coupling will NOT thread into a female metal or pvc pipe.

NPT: National Pipe Tapered is common in plumbing and some industrial uses. Sometimes referred to as pipe thread because PVC pipe and schedule 40 pipe are typically threaded in NPT. As its name implies this is a tapered thread so that the outside diameter of the male (ODM) gets smaller towards the end of the fitting and the inside diameter of the female narrows with depth. As the two fittings are threaded together the opposite tapers force thread friction and makes the seal without the use of a gasket.