this invention provides substitute capping heads to apply a threaded closure on a bottle neck through the utilization of existing machines which were designed to apply an aluminum closure on a bottle neck by in situ formation of the threads in an aluminum shell . the modified capping heads embodying this invention utilize the same rotational and vertical movements of the capping heads relative to the containers , but effect a frictional engagement with a pre - formed , internally threaded plastic closure to effect the screwing of such closure onto a bottle neck with the effective application torque being limited by a slip clutch incorporated in the drive train of the modified capping head . such clutch is isolated from ambient humidity conditions , and in some modifications operates in an oil bath .