NPT L1 plug (American National Standard Taper Pipe) is used to check an internal threaded hole. The NPT L1 plug inspects the functional size or hand tight engagement of couplings or an internal thread. Parts being checked are generally acceptable when they come flush with the notch to plus (+) or minus (-) one turn.

NPT L1 taper pipe ring gages are used to check external threaded parts. NPT L1 rings inspect the functional size or hand tight engagement of the pipe, nipple or male threads. When gaging NPT threaded parts, thread the marked side on first. Parts being checked are generally acceptable when they come flush to end of ring to plus (+) or minus (-) one turn.