This is a 7/8 in. tub and shower door nylon ball bearing roller with an flat edged wheel. It has a threaded hex head hub that uses an 8-32 x 3/8 in. long Philips binder head machine screw to fasten it to the door bracket. Used on a variety of tub and shower top roller systems. See Prime-Line part M 6002 for 3/4 in. roller.