Ribbed for her pleasure.. These sweet little heavy duty rubber caps get the job done when trying to close off vacuum ports on your head or carburetors. Stop messing with those cheap flimsy caps from Home Depot and move up to the big leagues. No likes waiting around while you fumble with duct tape and zip ties on the side of the road after blowing the cheapys off! With a thick wall construction and interior ribs the babies are guarenteed to stay put!