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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. 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 NPSH or NPT adapter - 2.84 inches (8 Threads per inch) 3" Hose Couplings Most 3 inch hoses come with 2 1/2 inch couplings Our company is located in North Carolina and we are firefighter owned and operated since 2011.