A2B-2.5NK

BSPT threads have the Whitworth thread form, designed to provide a pressure-tight fit through the mating of the threads. Accomplished by cutting the threads on a taper, this assures that the male thread can only screw into the companion female thread a certain distance before it jams. The standard specifies this distance as a length based on hand-tight engagement; a convenient rule-of-thumb for plastic threads is finger-tight plus 1/2 to 1 turn with a wrench. Do not over-tighten plastic threads – stress on the fitting can result in its failure.