Machinery's Handbook, 31st Edition
1808 BRITISH Machine Screws General Dimensions: The general dimensions and tolerances for screws and nuts are given in the accompanying tables. Although slotted screw dimensions are given, recessed head screws are also standard and available. Dimensions of recessed head screws are given in BS 4183:1967 (obsolescent). British Unified Machine Screws and Nuts.— British Standard BS 1981:1953 covers certain types of machine screws and machine screw nuts for which agreement has been reached with the United States and Canada as to general dimensions for interchangeability. These types are: countersunk, raised-countersunk, pan, and raised-cheese head screws with slotted or recessed heads; small hexagon head screws; and precision and pressed nuts. All have Unified threads. Head shapes are shown on page 1809 and dimensions are given on page 1810. Identification: As revised by Amendment No. 1 in February 1955, this standard now requires that the above-mentioned screws and nuts that conform to this standard should have a distinguishing feature applied to identify them as Unified. All recessed head screws are to be identified as Unified by a groove in the form of four arcs of a circle in the upper surface of the head. All hexagon head screws are to be identified as Unified by: 1) a circular recess in the upper surface of the head; 2) a continuous line of circles indented on one or more of the flats of the hexagon and parallel to the screw axis; and 3) at least two contiguous circles indented on the upper surface of the head. All machine screw nuts of the pressed type shall be identified as Unified by means of the application of a groove indented in one face of the nut approximately midway between the major diameter of the thread and flats of the square or hexagon. Slotted head screws shall be identified as Unified either by a circular recess or by a circular platform or raised portion on the upper surface of the head. Machine screw nuts of the precision type shall be identified as Unified by either a groove indented on one face of the front approximately midway between the major diameter of the thread and the flats of the hexagon or a continuous line of circles indented on one or more of the flats of the hexagon and parallel to the nut axis.
Recessed and Hexagon Head Screws
Precision Type
Pressed Type
Hexagon Machine Screw Nuts
Slotted Head Screws Identification Markings for British Standard Unified Machine Screws
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