Abstract: A device for adjusting the opening travel of a mold holder unit of a machine for molding plastic materials, comprising a movable plate which is arranged to support a portion of the mold and is actuated by at least one cylinder comprising a piston movable in a casing, said plate being movable, during the opening travel, from a first position, in which said mold is closed, to a second position in which said mold is open, is described. On the casing of the cylinder there being provided a member movable axially with respect to said casing and displaceable axially with respect to the casing by screw means in order to act as a stop for said piston during said opening travel.