These 3-jaw, self-centering Chucks feature semi-steel bodies with plain backs. Each includes two sets of jaws, a wrench and mounting screws. Requires a mounting plate. For those of you who are not familiar with chuck mounting, "plain back"" means that a backing plate is required to mount the chuck to your lathe. Since there are so many different types of spindles on lathes, you would require a back plate that fits your lathe spindle and then mount the chuck to the plate. The mounting bolts go through the back of the plate into the back of the chuck, usually on the edge of the chuck since there is a scroll mechanism for self-centering the jaws.</p>This top quality chuck has a semi-steel body and comes with two sets of hardened jaws. It is self- centering and has plain backs. Requires mounting plates. Through hole: 0.865".