Dimensions, tolerances and general requirements for sizes above and below ¼ in. Covers screws of the following types: 90 degrees countersunk and raised countersunk, round, pan, cheese and mushroom with slotted and with recessed heads. Appendices on dimensions of hexagon head screws and hexagon precision nuts in sizes below ¼ inch; flushness gauge for countersunk heads; penetration of standard gauge points for recessed heads; formulae for dimensions; tables of standard sizes.

The provisions of this British Standard are applicable to both ferrous and non-ferrous screws and nuts having B.S.W. or B.S.F. threads. General dimensions and requirements are given for countersunk, raised countersunk, round, pan, cheese and mushroom head screws with slotted heads, and countersunk, raised countersunk, round, pan, raised cheese and mushroom head screws with recessed heads.

General dimensions for a restricted range of non-standard sizes of some of the above types of screws, together with dimensions of hexagon head screws and hexagon precision nuts in sizes below ¼ in. are given in an appendix. Other appendices relate to a suitable head flushness gauge for countersunk head screws, a method of checking recess penetration with standard gauge points, the formulae from which the dimensions are derived, and tables of standard sizes.

NOTE 1 Round head screws appeared in the 1932 edition of BS 450 and they are therefore included in this revised standard. It is considered, however, that pan head screws provide for most of the applications in which round head screws are normally used and it is recommended that they be regarded as “preferred?? since their shape enables them to offer better slot engagement. NOTE 2 Machine screw nuts, pressed type (B.A. and Whitworth form threads) are covered by BS 2827.