it is a round plate ~.030" thick tacked to the vanes, behind them on a BB pump (so you cut it in half & slip the halves behind the vanes on a BB then weld it) & it goes in front of them on a SB. EDIT I cut a circular plate out of the side of an old washing machine for my last one. you want the OD slightly less than the OD of the vanes and a hole in the center to tack the vanes/shaft/plate. first you press the vanes back to get .040" clearance on the back side then the proper thickness of the plate to get .040" on the front side, keeping in mind gasket thickness & this front/rear terminology is reversed so to speak when the pump is installed.