ISO 2306:1972

Drills for use prior to tapping screw threads

Specifies the sizes of these drills for ISO metric threads (tables 1 and 2). ISO inch threads (tables 3 and 4) and pipe threads (tables 5 and 6). Applies to screw threads of normal length of engagement, the drill diamter being approximately equal to the nominal diameter of the thread minus the pitch. The sizes (except for those sizes indicated by an asterisk) have been selected from ISO 235 and are to be used in the future.