BSPT threads have on the Whitworth thread form, which is designed to provide a pressure-tight fit through mating of the threads. This is accomplished by cutting the threads on a taper. Because they are tapered, the 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.