Ingeniero

AMERICAN PIPE THREADS

1847

American National Standard pipe threads described in the following paragraphs provide
taper and straight pipe threads for use in various combinations and with certain modifications to meet these specific needs.
American National Standard Taper Pipe Threads.— The basic dimensions of the
ANSI Standard taper pipe thread are given in Table 4.
Form of Thread: Theangle between the sides of the thread is 60 degrees when measured
in an axial plane, and the line bisecting this angle is perpendicular to the axis. The depth of
the truncated thread is based on factors entering into the manufacture of cutting tools and
the making of tight joints and is given by the formulas in Table 4 or the data in Table 1
obtained from these formulas. Although the standardshows flat surfaces at the crest and
root of the thread, some rounding may occur in commercial practice, and it is intended that
the pipe threads of product shall be acceptable when crest and root of the tools or chasers
lie within the limits shown in Table 1.
Table 1. Limits on Crest and Root of American National Standard External and
Internal Taper Pipe Threads, NPT ANSI/ASME B1.20.1-1983(R1992)
INTERNAL THREAD

Root

Max.h

Minimum
Maximum
Truncation Truncation Minimum
Truncation

H

Crest

Maximum
Truncation

Crest
Crest

Minimum
Maximum
Minimum
Truncation Truncation Truncation
Maximum
Truncation

Root

EXTERNAL THREAD
Threads
per
Inch
27
18
14
111⁄2
8

Height of Sharp
V Thread, H

Height of Pipe Thread,
h

Truncation,
f

Width ofFlat, F,
Equivalent toTruncation

Max.

Min.

Min.

Max.

Min.

Max.

0.03208
0.04811
0.06186
0.07531

0.02963
0.04444
0.05714
0.06957

0.02496
0.03833
0.05071
0.06261

0.0012
0.0018
0.0024
0.0029

0.0036
0.0049
0.0056
0.0063

0.0014
0.0021
0.0027
0.0033

0.0041
0.0057
0.0064
0.0073

0.10825

0.10000

0.09275

0.0041

0.0078

0.00480.0090

All dimensions are in inches and are given to four or five decimal places only to avoid errors in
computations, not to indicate required precision.

Pitch Diameter Formulas: In the following formulas, which apply to the ANSI Standard
taper pipe thread, E0 = pitch diameter at end of pipe; E1 = pitch diameter at the large end of
the internal thread and at the gaging notch; D = outsidediameter of pipe; L1 = length of
hand-tight or normal engagement between external and internal threads; L2 = basic length
of effective external taper thread; and p = pitch = 1 ÷ number of threads per inch.
E 0 = D – ( 0.05 D + 1.1 ) p
E 1 = E 0 + 0.0625 L 1
Thread Length: The formula for L2 determines the length of the effective thread and
includes approximately two usable threads that areslightly imperfect at the crest. The nor-

1848

AMERICAN PIPE THREADS

mal length of engagement, L1, between external and internal taper threads, when assembled by hand, is controlled by the use of the gages.
L 2 = ( 0.80 D + 6.8 ) p
Taper: The taper of the thread is 1 in 16, or 0.75 inch per foot, measured on the diameter
and along the axis. The corresponding half-angle of taper or anglewith the center line is 1
degree, 47 minutes.
Tolerances on Thread Elements.— The maximum allowable variation in the commercial product (manufacturing tolerance) is one turn large or small from the basic dimensions.
The permissible variations in thread elements on steel products and all pipe made of steel,
wrought iron, or brass, exclusive of butt-weld pipe, are given in Table 2. This tableis a
guide for establishing the limits of the thread elements of taps, dies, and thread chasers.
These limits may be required on product threads.
On pipe fittings and valves (not steel) for steam pressures 300 pounds and below, it is
intended that plug and ring gage practice as set up in the Standard ANSI B1.20.1 will provide for a satisfactory check of accumulated variations of taper, lead,...