I once saw instructions on a package of a new chuck. Lock the drill in a vice, open the chuck. It should have an Allen screw inside, behind the teeth of the chuck. Find the right wrench and tap with...
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A Phillips screwdriver, or a nail can be inserted into one of the holes for the chuck key, and then use a "pipe" wrench on the collar to break it loose. Once loose, you can then turn it enough to...
Open the chuck all the way and look down inside it, you'll probably see a screw you'll need to remove.
If it's an older model, and you don't see a screw, it's probably got a tapered shaft.
You can...