Machinery's Handbook, 31st Edition
British Pipe Threads 2099 For external threads two classes of tolerance are provided and for internal, one class. The two classes of tolerance for external threads are Class A and Class B. For economy of manufacture the class B fit should be chosen whenever possible. The class A is reserved for those applications where the closer tolerance is essential. Class A tolerance is an entirely negative value, equivalent to the internal thread tolerance. Class B tolerance is an entirely negative value twice that of class A tolerance. Tables showing limits and dimensions are given in the Standard. The thread series specified in this Standard shall be designated by the letter “G”. A typical reference on a drawing might be “G 1 ∕ 2 ”, for internal thread; “G 1 ∕ 2 A”, for external thread, class A: and “G 1 ∕ 2 B”, for external thread, class B. Where no class reference is stated for external threads, that of class B will be assumed. The designation of truncated threads shall have the addition of the letter “T” to the designation, i.e., G 1 ∕ 2 T and G 1 ∕ 2 BT. British Standard Pipe Threads (Non-pressure-tight Joints) Metric and Inch Basic Sizes BS 2779:1973
Threads per Inch a
Depth of Thread
Major Diameter
Pitch Diameter
Minor Diameter
Threads per Inch a
Depth of Thread
Major Diameter
Pitch Diameter
Minor Diameter
0.581 7.723 7.142 6.561 0.0229 0.3041 0.2812 0.2583 0.581 9.728 9.147 8.566 0.0229 0.3830 0.3601 0.3372 0.856 13.157 12.301 11.445 0.0337 0.5180 0.4843 0.4506 0.856 16.662 15.806 14.950 0.0337 0.6560 0.6223 0.5886 1.162 20.955 19.793 18.631 0.0457 0.8250 0.7793 0.7336 1.162 22.911 21.749 20.587 0.0457 0.9020 0.8563 0.8106 1.162 26.441 25.279 24.117 0.0457 1.0410 0.9953 0.9496 1.162 30.201 29.039 27.877 0.0457 1.1890 1.1433 1.0976 1.479 33.249 31.770 30.291 0.0582 1.3090 1.2508 1.1926 1.479 37.897 36.418 34.939 0.0582 1.4920 1.4338 1.3756 1.479 41.910 40.431 38.952 0.0582 1.6500 1.5918 1.5336 1.479 47.803 46.324 44.845 0.0582 1.8820 1.8238 1.7656
1.479 53.746 52.267 50.788 0.0582 2.1160 2.0578 1.9996 1.479 59.614 58.135 56.656 0.0582 2.3470 2.2888 2.2306 1.479 65.710 64.231 62.752 0.0582 2.5870 2.5288 2.4706 1.479 75.184 73.705 72.226 0.0582 2.9600 2.9018 2.8436 1.479 81.534 80.055 78.576 0.0582 3.2100 3.1518 3.0936 1.479 87.884 86.405 84.926 0.0582 3.4600 3.4018 3.3436 1.479 100.330 98.851 97.372 0.0582 3.9500 3.8918 3.8336 1.479 113.030 111.551 110.072 0.0582 4.4500 4.3918 4.3336 1.479 125.730 124.251 122.772 0.0582 4.9500 4.8918 4.8336 1.479 138.430 136.951 135.472 0.0582 5.4500 5.3918 5.3336 1.479 151.130 149.651 148.172 0.0582 5.9500 5.8918 5.8336 1.479 163.830 162.351 160.872
28 {
1 3 ∕ 4
1 ∕ 16
11 { 11 {
28 {
2
1 ∕ 8
19 {
11 {
2 1 ∕ 4
1 ∕ 4
19 {
11 {
2 1 ∕ 2
3 ∕ 8
14 {
11 {
2 3 ∕ 4
1 ∕ 2
14 {
3
11 {
5 ∕ 8
14 {
11 {
3 1 ∕ 2
3 ∕ 4
14 {
4
11 {
7 ∕ 8
1 11 {
11 {
4 1 ∕ 2
11 {
5
11 {
1 1 ∕ 8
11 {
11 {
1 1 ∕ 4
5 1 ∕ 2
11 {
6
11 {
1 1 ∕ 2
0.0582 6.4500 6.3918 6.3336 a The thread pitches in millimeters are as follows: 0.907 for 28 threads per inch. 1.337 for 19 threads per inch, 1.814 for 14 threads per inch, and 2.309 for 11 threads per inch. Each basic metric dimension is given in roman figures (nominal sizes excepted) and each basic inch dimension is shown in italics directly beneath it. British Standard Pipe Threads for Pressure-Tight Joints.— The threads in BS 21:1973, “Specification for Pipe Threads where Pressure-tight Joints are Made on the Threads”, are based on the Whitworth thread form and are specified as: 1) Jointing threads: These relate to pipe threads for joints made pressure-tight by the mating of the threads; they include taper external threads for assembly with either taper or parallel internal threads (parallel external pipe threads are not suitable as jointing threads). 4) Longscrew threads: These relate to parallel external pipe threads used for longscrews (connectors) specified in BS 1387 where a pressure-tight joint is achieved by the compres sion of a soft material onto the surface of the external thread by tightening a back nut against a socket.
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