Steels for spanners and tommy bars; hardness figures, performance test. Two grades of spanner. Dimensions: unified precision hexagons, normal series, in BS 1768; Unified black hexagons, heavy series, in BS 1769; Unified machine screw hexagons in BS 1981; precision hexagons B.S.W. and B.S.F. in BS 1083; black hexagons in BS 916; B.A. hexagons in BS 57; Whitworth large hexagons (originally in BS 190, now obsolete); metric hexagons. Cross References:BS 57BS 192BS 860BS 916BS 970BS 1083BS 1750BS 1768BS 1769 BS 1981BS 2708A 102Supplement No. 1 to BS 2558