Abstract: A self-adjusting capping chuck includes a support having a longitudinal axis, a guide element fixed with respect to the support and having guides, jaws which engage respective guides and are movable with respect to the support between release and gripping positions, the jaws have respective gripping surfaces and first cam surfaces, and a control ring movable relative to the support along the longitudinal axis between lowered and raised positions and having second cam surfaces cooperating with first cam surfaces of the jaws, so that the movement of the control ring from the lowered to the raised position moves the jaws from the release to the gripping position. The control ring has an abutment wall arranged to abut on a head surface of a closure element so that, during movement of the chuck along the longitudinal axis, the closing element moves the control ring from the lowered to the raised position.