Abstract: A self-centering chuck with adjustable and replaceable jaws includes false jaws which are slidable radially in corresponding recesses in an annular body of the chuck through the action of a rod coaxial with the chuck itself. The jaws, each of which has tangential teeth facing the corresponding false jaw, are also slidable in the recesses. In each of the false jaws is mounted a peg slidable parallel to the axis of the chuck. The peg has teeth for engagement with a respective jaw. Each peg has a transverse groove facing radially outwardly of the chuck. This groove is engaged by an eccentric which is rotatable in the false jaw about a radial axis parallel to the direction of sliding of the false jaw. This eccentric has a formation engageable by a key or similar operating tool for rotating it in order to disengage and re-engage the peg relative to the corresponding jaw.