Put an Allen wrench into the chuck and turn it to tighten up the chuck as soon as you have actually eliminated the screw. When the wrench is strongly in position, set the drill's gearbox to the lowest setting to lower resistance from the gears. Then, tap the Allen wrench with a club to loosen the chuck from the drill. Once it's loose, you can unscrew the chuck by hand. Screw your new chuck in its location, tighten it with your Allen wrench, then replace the screw in the center.
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