American Standard Taper Pipe ThreadsBasic DimensionsThe basic form of external NPT thread is shown in the figure below. The angle between the flanks of thethreads is 60° measured in an axial plane. The 30 "half angle is measured from a line perpendicular to axisof the thread. The crests of the threads may be truncated either parallel to the pitch line or parallel to theaxis. The height of thread and minimum and maximum truncation are given below for nominal pipe sizes of1/16 thru 2". The crests of the threads may be either flat or rounded. Rounding of the roots, however, shouldbe within the root truncation limits and not extend along the flanks of the thread beyond the maximum trun-cation. Taper on diameter is one inch in sixteen inches, or .75 inches per foot. The angle of taper is 1 °47'from the axis.

Manufacturing ToleranceThe maximum allowable variation in diameters E0 and E, is one turn large or small from the basic dimensionbelow. The external thread is within permissible tolerance when the gaging face of the working ring gage isnot more than one turn, large or small, from being flush with the small end of the thread.

The basic effective length of the external taper thread, L2, includes two usable threads with slightly im-perfect crests located against the the vanish threads. The length, L,, is the normal engagement between ex-ternal and internal taper threads when screwed together hand tight. For general low pressure (less than 300pounds) applications, ring gage practice provides a satisfactory check of accumulated variations of taper,lead and angle deviations. NPT threads require the use of a sealer for tight joints.

NominalPipeSue

1/16

1/8

1/4

3/8

1/2

3/4

1

1-1/4

1-1/2

2

OutsideDiameter

ofPipe

.313

.405

.540

.675

.840

1.050

1.315

1.660

1.900

2.375

ThreadsPerInch

27

27

18

18

14

14

11-1/2

11-1/2

11-1/2

11-1/2

Pitchof

Thread

.03703

.03703

.05556

.05556

.07143

.07143

.08696

.08696

.08696

.08696

PitchDiameter

AtBeginning

ofExterna lThread

Eo

.2712

.3635

.4774

.6120

.7584

.9677

1.2136

1.5571

1.7961

2.2690

Heightof

Sharp VThread

.0321

.0321

.0481

.0481

.0619

.0619

.0753

.0753

.0753

.0753

Heightof Thread

Max

.0296

.0296

.0444

.0444

.0571

.0571

.0696

.0696

.0696

.0696

Min

.0250

.0250

.0383

.0383

.0507

.0507

.0626

.0626

.0626

.0626

TruncationCrest or Root

Max

.0036

.0036

.0049

.0049

.0056

.0056

.0063

.0063

.0063

.0063

Min.

.0012

.0012

.0018

.0018

.0024

.0024

.0029

.0029

.0029

.0029

Hand TightEngagement

Length•Li

.160

.180

.200

.240

.320

.339

.400

.420

.420

.436

DiameterEi

.2812

.3745

.4899

.6270

.7784

.9889

1.2386

1.5834

1.8223

2.2963

Effect iveThreadLength

L2

.261

.264

.402

.408

.534

.546

.683

.707

.724

.757

VanishThreadLength

V

.129

.129

.193

.193

.248

.248

.302

.302

.302

.302

OverallLength

ofExternalThread

Li

.390

.392

.595

.601

.782

.794

.985

1.009

1.025

1.058

*Also length of Thin Ring Gage.

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