Whitworth British Screw Threads Size Chart BSW and 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The Whitworth British Thread standard is obsolete and not in use as a standard for new products.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	0.0903	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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