A.N.P.T. & N.P.T.F. Pipe Thread Gages
For Gaging Internal Threads – A.N.P.T. or N.P.T.F.
In gaging internal threads, wither A.N.P.T. or N.P.T.F., there are three aging members involved: (1) L-1 thread plug gage which
checks the thread elements within hand tight engagement; (2) L-3 thread plug which checks the threads that will be engaged
when wrench tight; and (3) 6-Step plain plug which is used to check the minor diameter of the female fitting.
For Gaging External Threads – A.N.P.T. or N.P.T.F.
In gaging external threads, either A.N.P.T. or N.P.T.F., there are three more gages employed: (1) 6-Step plain ring which checks
the major diameter of the male fitting; (2) L-1 (thin) ring which checks the thread elements over the distance of hand tight
engagement; and (3) L-2 (thick) ring which checks the thread elements of those threads engaged when wrench tight.
A.N.P.T. and N.P.T.F. are not meant to be interchangeable
TAPER PIPE THREAD GAGES
N.P.T. (National Taper Pipe)
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.
A.N.P.T. (Aeronautical National Pipe Taper)
A.N.P.T. internal taper pipe threads are basically the same as the N.P.T. pipe threads except that for Army and Navy
Aeronautical use they must be more carefully controlled for diameter, taper and thread form.
To gage Internal A.N.P.T