Whitworth British Screw Threads Size Chart BSW & BSF

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 Fastener and Screw / Bolt Design, Formula and Calculations

The Whitworth British Thread standard is obsolete and not in use as a standard for new products. The information contained herein is for reference only in the application of spare and replace parts on existing products and equipment.

Fastener Thread Designations and Definitions Pitch, Minor, Major Diameters

Coarse Thread Series (BSW)
Nominal Inch's
Threads/ Inch
Pitch, Inches
Thread Depth, Inches
Major Diameter, Inches
Effective Diameter, Inches
Minor Diameter, Inches
Tap Drill Dia. (mm)
1/8
40
0.02500
0.0160
0.1250
0.1090
0.9030
2.55 mm
3/16
24
0.04167
0.0267
0.1875
0.1608
0.1341
3.70 mm
1/4
20
0.05000
0.0320
0.2500
0.2180
0.1860
5.10 mm
5/16
18
0.05556
0.0356
0.3125
0.2769
0.2413
6.50 mm
3/8
16
0.06250
0.0400
0.3750
0.3350
0.2950
7.90 mm
7/16
14
0.07143
0.0457
0.4375
0.3918
0.3461
9.30 mm
1/2
12
0.08333
0.0534
0.5000
0.4466
0.3932
10.50 mm
9/16
12
0.08333
0.0534
0.5625
0.5091
0.4557
12.10. mm
5/8
11
0.09091
0.0582
0.6250
0.5668
0.5086
13.50 mm
11/16
11
0.09091
0.0582
0.6875
0.6293
0.5711
15.00 mm
3/4
10
0.10000
0.0640
0.7500
0.6860
0.6220
16.25 mm
7/8
9
0.11111
0.0711
0.8750
0.8039
0.7328
19.25 mm
1
8
0.12500
0.0800
1.0000
0.9200
0.8400
22.00 mm
1 1/8
7
0.14286
0.0915
1.1250
1.0335
0.9420
24.75 mm
1 1/4
7
0.14286
0.0915
1.2500
1.1585
1.0670
28.00 mm
1 1/2
6
0.16667
0.1067
1.5000
1.3933
1.2866
33.50 mm
1 3/4
5
0.20000
0.1281
1.7500
1.6219
1.4938
39.00 mm
2
4.5
0.22222
0.1423
2.0000
1.8577
1.7154
44.50 mm
2 1/4
4
0.25000
0.1601
2.2500
2.0899
1.9298

Tap drill diameters shown in this column are recommended sizes listed in BS 1157:1975.

These provide from 77 to 87% of full thread.

2 1/2
4
0.25000
0.1601
2.5000
2.3399
2.1798
2 3/4
3.5
0.28571
0.1830
2.7500
2.5670
2.3840
3
3.5
0.28571
0.1830
3.0000
2.8170
2.6340
3 1/4
3.25
0.30769
0.1970
3.2500
3.0530
2.8560
3 1/2
3.25
0.30769
0.1970
3.5000
3.3030
3.1060
3 3/4
3
0.33333
0.2134
3.7500
3.5366
3.3232
4
3
0.33333
0.2134
4.0000
3.7866
3.5732
4 1/2
2.875
0.34783
0.2227
4.5000
4.2773
4.0546
4 3/4
2.75
0.36364
0.2328
5.0000
4.7672
4.5344
5 1/2
2.625
0.38095
0.2439
5.5000
5.2561
5.0122
6
2.5
0.40000
0.2561
6.0000
5.7439
5.4878

Fine Thread Series (BSF)
3/16
32
0.03125
0.0200
0.1875
0.1675
0.1475
4.00 mm
7/32
28
0.03571
0.0229
0.2188
0.1959
0.1730
4.60 mm
1/4
26
0.03846
0.0246
0.2500
0.2254
0.2008
5.30 mm
9/32
26
0.03846
0.0246
0.2812
0.2566
0.2320
6.10 mm
5/16
22
0.04545
0.0291
0.3125
0.2834
0.2543
6.80 mm
3/8
20
0.05000
0.0320
0.3750
0.3430
0.3110
8.30 mm
7/16
18
0.05556
0.0 356
0.4375
0.4019
0.3363
9.70 mm
1/2
16
0.06250
0.0400
0.5000
0.4600
0.4200
11.10 mm
9/16
16
0.06250
0.0400
0.5625
0.5225
0.4825
12.70 mm
5/8
14
0.07143
0.0457
0.6250
0.5793
0.5336
14.00 mm
11/16
14
0.07143
0.0457
0.6875
0.6418
0.5961
15.50 mm
3/4
12
0.08333
0.0534
0.7500
0.6966
0.6432
16.75 mm
7/8
11
0.09091
0.0582
0.8750
0.8168
0.7586
19.75 mm
1
10
0.10000
0.0640
1.0000
0.9360
0.8720
22.75 mm
11/8
9
0.11111
0.0711
1.1250
1.0539
0.9828
25.50 mm
11/4
9
0.11111
0.0711
1.2500
1.1789
1.1078
28.50 mm
1 3/8
8
0.12500
0.0800
1.3750
1.2950
1.2150
31.50 mm
1 1/2
8
0.12500
0.0800
1.5000
1.4200
1.3400
34.50 mm
1 5/8
8
0.12500
0.0800
1.6250
1.5450
1.4650

Tap drill diameters shown in this column are recommended sizes listed in BS 1157:1975.

These provide from 77 to 87% of full thread

13/4
7
0.14286
0.0915
1.7500
1.6585
1.5670
2
7
0.14286
0.0915
2.0000
1.9085
1.8170
2 1/4
6
0.16667
0.1067
2.2500
2.1433
2.0366
2 1/2
6
0.16667
0.1067
2.5000
2.3933
2.2866
2 3/4
6
0.16667
0.1067
2.7500
2.6433
2.5366
3
5
0.20000
0.1281
3.0000
2.8719
2.7438
3 1/4
5
0.20000
0.1281
3.2500
3.1219
2.9938
3 1/2
4.5
0.22222
0.1423
3.5000
3.3577
3.2154
3 3/4
4.5
0.22222
0.1423
3.7500
3.6077
3.4654
4
4.5
0.22222
0.1423
4.0000
3.8577
3.7154
41/4
4
0.25000
0.1601
4.2500
4.0899
3.9298

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