This uniquely designed key can securely tighten or easily loosen 1/2 to 3/4 in. drill chucks.The size of key required for your chuck can be found by cross-referencing the numbers 1,2,3 or 4 on the key to the number of teeth on the gear and the 'core size' or diameter of the tip to verify chuck fitment:??????#5: Core size (dia.): 9.3mm; Teeth: 12#6: Core size (dia.): 7.8mm; Teeth: 11#7: Core size (dia.): 6 mm; Teeth: 10#8: Core size (dia.): 6 mm; Teeth: 11