BVV Male Hollow Stopper Male Hollow StopperMale Hollow Stoppers are used for capping off or plugging a female join to close off a port on a glassware system. These glass stoppers are hollow on the inside and require vacuum grease to create a seal when having 2 ground surfaces touching in a vacuum application.Joint Sizes Available:14/20 Ground Joint24/40 Ground Joint Male Hollow Stopper.