NPT Thread Chart
To gage Internal N.P.T. taper pipe threads, screw the N.P.T. thread plug firmly into the product – do not force. The thread is within the permissible tolerance when the gaging notch of the plug is not more than one turn plus or minus from being flush with the end of the product. N.P.T. plugs are regularly furnished with the basic step only. Minimum and maximum steps can be added on request.
To gage External N.P.T. taper pipe threads, screw the N.P.T. thread ring on to the product as far as it will go without using force. If the face of the small end of the ring is flush with the face of the product the thread is basic. The permissible tolerance is one turn plus or minus from basic.