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ISO 7 - Pipe Threads where Pressure-tight Joints are made on the Threads

Pressure-tight joints made on the pipe threads.

The standard ISO 7 - Pipe threads where pressure-tight joints are made on the threads consists of

  • ISO 7-1:1994 Dimensions, tolerances and designation
  • ISO 7-2:2000 Verification by means of limit gauges

The ISO 7 tapered Whitworth threads are equivalent to DIN 2999, BS 21 (BS EN 10226-1), JIS B0203.

Characteristics of ISO 7 tapered Withworth threads are

  • tapered thread 1o47'
  • truncation of roots and crests are rounded
  • 55o thread angle
  • pitch is measured in threads per inch

Note!

There is a distinction between

  • jointing threads - sealing tapered/cylindrical threads for connections according to ISO 7-1 and/or EN 10226-1, BS 21
  • fastening threads - not sealing parallel cylindrical threads for connections according to EN ISO 228-1

Diameter and pitches are the same for ISO 7-1 and ISO 228 threads.

ISO 7-1 is BSPT (R/Rp) – British Standard Tapered Pipe where

  • R - tapered male with external threads
  • Rp - cylindrical female with internal threads
  • Rc  - tapered female with internal threads

G-threads have a parallel shape in accordance with the DIN-EN-ISO 228-1 standard.

Both female Rp - internal cylindrical threads - and Rc - internal taper threads - are used in combination with male R - external threads - to make sealed joints. Male G-threads (parallel) can only be screwed into female G-threads.
Male R-threads (tapered) can be screwed into female G or R-threads.

For full table with Thread Tolerances - rotate the screen!

ISO 7-1 Tapered Threads
Pipe Size
(inches)
Threads per Inch
P
(TPI)
Lead
p
(mm)
Outside Diameter
d = D
(mm)

Pitch Diameter
d2 = D2
(mm)

Minor Diameter
d1 = D1
(mm)

Rp/Rc
R
aTolerances of Internal ThreadTolerances of
External Thread

(mm)

Pitch Diameter
(mm)

turnsturns(mm)
1/16 28 0.907 7.723 7.142 6.561 1/16 4 ± 0.071 ± 1 1/4 ± 1 ± 0.9
1/8 28 0.907 9.23 9.147 8.566 1/8 4 ± 0.071 ± 1 1/4 ± 1 ± 0.9
1/4 19 1.337 13.157 12.301 11.445 1/4 6 ± 0.104 ± 1 1/4 ± 1 ± 1.3
3/8 19 1.337 16.662 15.806 14.950 3/8 6.4 ± 0.104 ± 1 1/4 ± 1 ± 1.3
1/2 14 1.814 20.955 19.793 18.631 1/2 8.2 ± 0.142 ± 1 1/4 ± 1 ± 1.8
5/8 14 1.814 22.911 21.749 20.587
3/4 14 1.814 26.441 25.279 24.117 3/4 9.5 ± 0.142 ± 1 1/4 ± 1 ± 1.8
7/8 14 1.814 30.201 29.039 27.877
1 11 2.309 33.249 31.770 30.291 1 10.4 ± 0.18 ± 1 1/4 ± 1 ± 2.3
1 1/8 11 2.309 37.897 36.418 34.939
1 1/4 11 2.309 41.910 40.431 38.952 1 1/4 12.7 ± 0.18 ± 1 1/4 ± 1 ± 2.3
1 1/2 11 2.309 47.803 46.324 44.845 1 1/2 12.7 ± 0.18 ± 1 1/4 ± 1 ± 2.3
1 3/4 11 2.309 53.746 52.267 50.788
2 11 2.309 59.614 58.135 56.656 2 15.9 ± 0.18 ± 1 1/4 ± 1 ± 2.3
2 1/4 11 2.309 65.710 64.231 62.752
2 1/2 11 2.309 75.184 73.705 72.226 2 1/2 17.5 ± 0.216 ± 1 1/2 ± 1 1/2 ± 3.5
2 3/4 11 2.309 81.534 80.055 78.576
3 11 2.309 87.884 86.405 84.926 3 20.6 ± 0.216 ± 1 1/2 ± 1 1/2 ± 3.5

The outside diameter of a pipe or fitting must be measured and compared to a table for size identification. A 3/4" ISO 7-1 pipe thread has an outside diameter - OD - of 1.060 inches. Each thread size has a defined number of threads per inch - TPI, or pitch. The 3/4" NPT pipe thread has 14 threads per inch. Both the TPI and OD of the thread are required for positive identification of thread size because more than one size have the same TPI.

ISO 7-1 threads may look similar to NPT threads. However, ISO and NPT threads should not be mixed. ISO threads have 55o taper angle versus 60o for NPT. The NPT root and crest configurations are also different from ISO. For ISO threads pitch is usually measured in millimeters ( may be expressed in Inch). The pitch are different.

Length of ISO 7-1 Thread to make a Tight Joint

Length of ISO 7-1 Thread to make a Tight Joint
Nominal Pipe Size
(inches)
Length of Thread
(inches)
1/8 0.27
1/4 0.39
3/8 0.41
1/2 0.53
3/4 0.55
1 0.66
1 1/4 0.68
1 1/2 0.68
2 0.70
2 1/2 0.93
3 1.02
4 1.09
5 1.19
6 1.21

DIN 2999 is the German version of BS 21 and BS 2779 which were the original British Whitworth screw thread pipe joint standards. DIN 2999 has been withdrawn, is obsolete and has been replaced with ISO 7 and ISO 228.

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