This International Standard specifies the sizes of drills to be used prior to tapping parallel screw threads of normal length of engagement, the drill diameter being approximately equal to the nominal diameter of the thread minus the pitch. Drill sizes are given for the following threads: 1) ISO metric threads (coarse and fine pitch series): Tables 1 and 2. 2) ISO inch threads (UNC and UNF): Tables 3 and 4. 3) Pipe threads: Tables 5 and 6 This International Standard specifies metric diameters of drills, which, except for the four sizes indicated by an asterisk, have been selected from ISO/R 235, and the dimensions shall be regarded as the only recommended dimensions to be used in the future for this purpose.