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 .