BS 1981

BS 1981 specifies dimensions, tolerances and mechanical properties for Unified machine screws and machine screw nuts. The drawing shows a slotted pan-head screw; the standard also covers other head styles and nuts.

Line drawing of a slotted pan-head machine screw with head diameter, head height, nominal diameter, length and slot detail marked.

Slotted pan-head screw dimensions

Nominal size	Basic diameter d (in.)	UNC TPI	UNF TPI	Head diameter A max. (in.)	Head diameter A min. (in.)	Head depth B max. (in.)	Head depth B min. (in.)	Slot width H max. (in.)	Slot width H min. (in.)	Slot depth J (in.)
No. 4	0.112	40	—	0.219	0.205	0.068	0.058	0.039	0.031	0.036
No. 6	0.138	32	—	0.270	0.256	0.082	0.072	0.048	0.039	0.044
No. 8	0.164	32	—	0.322	0.306	0.096	0.085	0.054	0.045	0.051
No. 10	0.190	24	32	0.373	0.357	0.110	0.099	0.060	0.050	0.059
1/4	0.250	20	28	0.492	0.473	0.144	0.130	0.075	0.064	0.079
5/16	0.3125	18	24	0.615	0.594	0.178	0.162	0.084	0.072	0.101
3/8	0.375	16	24	0.740	0.716	0.212	0.195	0.094	0.081	0.122
7/16	0.4375	14	20	0.863	0.838	0.247	0.227	0.094	0.081	0.133
1/2	0.500	13	20	0.987	0.958	0.281	0.260	0.106	0.091	0.152
5/8	0.625	11	18	1.125	1.090	0.350	0.325	0.133	0.116	0.189
3/4	0.750	10	16	1.250	1.209	0.419	0.390	0.149	0.131	0.226

Other forms covered

Fastener form	Dimension reference
Countersunk and raised countersunk machine screws	Separate head and slot schedules
Raised cheese-head machine screws	Separate head and slot schedules
Hexagon-head machine screws	Across-flats, corners and head dimensions
Machine screw nuts	Pressed and precision types

Note: The selected inch values are reproduced from a BS 1981:1953 (R2004) table in Machinery's Handbook. Check the required BS 1981 edition and drawing before specifying a screw; the complete standard has separate schedules for other head and nut types.