BS 1981:1991

This standard BS 1981:1991 Specification for unified machine screws and machine screw nuts is classified in these ICS categories:

• 21.060.10 Bolts, screws, studs
• 21.060.20 Nuts

This British Standard specifies requirements for the dimensions, tolerances and mechanical properties of machine screws and machine screw nuts having ISO inch (Unified) screw threads.

Dimensions and tolerances are given for machine screws and nuts in the following diameter ranges:

1. hexagon head machine screws from No. 2 to No. 12;
2. slotted countersunk and pan head machine screws from No. 0 to in;
3. slotted raised cheese head machine screws (non-preferred) from No. 0 to in;
4. slotted raised countersunk machine screws from No. 0 to in;
5. slotted hexagon head machine screws (non-preferred) No. 4 to 12;
6. recessed (types Z and V) pan and countersunk head machine screws from No. 2 to in;
7. recessed (types Z and V) raised cheese head machine screws (non-preferred) from No. 2 to in;
8. recessed (types Z and V) raised countersunk head machine screws from No. 2 to in;
9. square and hexagon machine screw nuts, pressed type, from No. 0 to in;
10. hexagon machine screw nuts, precision type, from No. 0 to No. 12.

Mechanical properties are specified for steel, brass and aluminium alloy machine screws and machine screw nuts.

Appendices are included that relate to the dimensioning of 80° countersunk heads, slot widths and standard sizes (not to be confused with stock sizes, see note 1 to table 27).

NOTE 1. Attention is drawn to the fact that it is claimed that the type V recess is the subject of British patent no. 1 521 141 copies of which can be obtained from the Patent Office, 25 Southampton Buildings, London WC2A 1AY. BSI takes no position as to the validity of the patent or whether it is still in force. The patent is irrevocably endorsed ‘licences of right’ under Section 46 of the Patents Act 1977, which states:

‘(3) Where such an entry is made in respect of a patent –

(a) any person shall, at any time after the entry is made, be entitled as of right to a licence under the patent on such terms as may be settled by agreement or, in default of agreement, by the Comptroller on the application of the proprietor of the patent or the person requiring the licence’.

Licence details may be obtained from the registered proprietor of the patent.

NOTE 2. Details of precision hexagon screws and nuts in diameter and above are given in BS 1768.

NOTE 3. The titles of the British Standards referred to in this standard are listed on the inside back cover.

Applicable to machine screws and machine screw nuts having ISO inch screw threads and hexagon head and slotted or recessed heads with type Z and type V recess. Mechanical properties are specified for steel, brass and aluminium alloy screws and nuts.