I have a 6" Craftsman 4 jaw chuck that came on my South Bend 9" lathe, and it takes a metric hex key (Allen wrench) that isn't in most sets of Allen wrenches, maybe a 7mm if I remember right.? A 1/4" one fits loosely and works if you don't have to turn it very hard, otherwise it slips a bit and I think is hard on the wrench and the screw head.
Jim, in western Wisconsin
