A chuck require a key to provide the necessary torque to tighten and loosen the jaws.Our chuck key which is suitable to tighten and loosen a chuck. Chuck Key Selection Method:For 0.3-4mm drill chucks: key diameter about 3mm, gear diameter about 11.5mmFor 0.6-6mm drill chucks: key diameter about 5mm, gear diameter about 15mmFor 1.5-10mm drill chucks: key diameter about 6mm, gear diameter about 12.5mmFor 1.5-13mm drill chucks: key diameter about 6mm, gear diameter about 17.5mmFor 5-20mm drill chucks: key diameter about 9mm, gear diameter about 25mmFor 1-13mm, 3-16mm, 1-16mm drill chucks: key diameter about 8mm, gear diameter about 21mmThe above data is for reference only, only suitable for general models, except for special models