Oil Country Gages
The American Petroleum Institute (API) established a tightly controlled gaging program to insure that the API thread form specifications are met in the manufacturing of oil field equipment, tools and pipe. These types of gages are commonly referred to Oil Country Gages. Pratt & Whitney manufactures several instruments outfitted with the necessary accessories to handle these types of gages. The two most popular instruments are the Labmaster Universal and the External Supermicrometer.
Other common terminology is Tapered Thread Inspection. The National Pipe Thread Taper (NPT) is a U.S. standard for tapered threads used on threaded pipes and fittings. A taper thread will pull tight and therefore make a fluid-tight seal. NPT is defined by ANSI/ASME standard B1.20.1.