With ISO metric screw threads to BS 3643, 1 mm to 20 mm diameter. Dimensions, tolerances, mechanical requirements for countersunk, raised countersunk, pan and cheese head screws with slotted heads, and countersunk, raised countersunk and pan head screws with recessed heads. Dimensions, tolerances, general requirements for square and hexagon machine screw nuts (pressed type). Checking recess penetration, dimensioning of 90 countersunk heads, standard sizes. See BS 1981, BS 3692.