Brass 3" NPT to 2 1/2" NH Double Male (Hex)

Description

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.
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. Warning - 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.
2 1/2" Male Adapter
Inside diameter of a male adapter - 2 1/2" inches
Outside diameter of a male NH / NST adapter - 3.06 inches (7.5 Threads per inch)
Outside diameter a male NPT adapter - 2.76 inches (8 Threads per inch)
3" Hose Couplings
Inside diameter of a male adapter - 3" inches
Outside diameter of a male NH / NST adapter - 3.62 inches (6 Threads per inch)
Outside diameter a male NPT adapter - 3.38 inches (8 Threads per inch)
Most 3 inch hoses come with 2 1/2 inch couplings