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Basic Dimensions of Taper Pipe Threads

National Pipe Threads (NPT)
Nominal Pipe Size
O.D. of Pipe
Threads
per
Inch
Pitch
Major Diameters of Plug Gages
Pitch Diameters of Plug and Ring Gages
Major Diameters of Ring Gages
Increase
in
Dia./
Thread
Thick-
ness of Ring
At Small End
At Gaging Notch
At Large End
At Small End
At Gaging Notch
At Large End, Plug Gage
At Small End
At Gaging Notch
1
2
3
4
5
6
7
8
9
10
11
12
13
14
1/16
0.3125
27
0.03704
0.29289
0.30289
0.30921
0.27118
0.28118
0.28750
0.24947
0.25947
0.00231
0.160
1/8
0.405
27
0.3704
0.38522
0.39531
0.40171
0.36351
0.37360
0.38000
0.34180
0.35189
0.00231
0.01615
1/4
0.540
18
0.05556
0.51339
0.52763
0.53850
0.47739
0.49163
0.50250
0.44139
0.44563
0.00347
0.2278
3/8
0.675
18
0.05556
0.64801
0.66301
0.67350
0.61201
0.62701
0.63750
0.57601
0.59101
0.00347
0.240
1/2
0.840
14
0.07143
0.80600
0.82600
0.83936
0.75843
0.77843
0.79179
0.71086
0.73086
0.00446
0.320
3/4
1.050
14
0.07143
1.01525
1.03644
1.04936
0.96768
0.98887
1.00179
0.92011
0.94129
0.00446
0.339
1
1.315
11.5
0.08696
1.27154
1.29654
1.31422
1.21363
1.23863
1.25630
1.15572
1.18072
0.00543
0.400
1-1/4
1.660
11.5
0.08696
1.61504
1.64129
1.65922
1.55713
1.58338
1.60130
1.49922
1.52547
0.00543
0.420
1-1/2
1.900
11.5
0.08696
1.85400
1.88025
1.89922
1.79609
1.82234
1.84130
1.73817
1.76442
0.00543
0.420
2
2.375
11.5
0.08696
2.32693
2.35418
2.37422
2.26902
2.29627
2.31630
2.21111
2.23836
0.00543
0.436
2-1/2
2.875
8
0.12500
2.80278
2.84541
2.87388
2.71953
2.76216
2.79062
2.63628
2.67891
0.00781
0.682
3
3.500
8
0.12500
3.42388
3.47175
3.49888
3.34062
3.38850
3.41562
3.25737
3.30525
0.00781
0.766
3-1/2
4.000
8
0.12500
3.92075
3.97207
3.99888
3.83750
3.88881
3.91562
3.75425
3.80556
0.00781
0.821
4
4.500
8
0.12500
4.41762
4.47038
4.49888
4.33438
4.38712
4.41562
4.25112
4.30387
0.00781
0.844

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Gaging Practices

Internal Threads

When gaging standard Internal Taper Threads, the Plug Gage is screwed hand tight into the Internal Thread of the product. The Thread is within permissible tolerance when the gaging notch of the working Plug Gage is not more than 1 turn, large or small, from being flush with the end of the thread.

External Threads

When gaging standard External Taper Threads, the Ring Gage is screwed hand tight onto the Product Thread. The Thread is within the permissible tolerance when the gaging face of the Working Ring Gage is not more that 1 turn, large or small, from being flush with the end of the Thread.
ANPT & NPTF Pipe Thread Gages

For Gaging Internal Threads - ANPT or NPTF
In gaging internal threads, either ANPT or NPTF, there are three gaging members involved: (1) L-1 thread plug 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 - ANPT or NPTF
In gaging external threads, either ANPT or NPTF, 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.

Note: ANPT and NPTF Gages are not meant to be interchangeable.