Carbon motor brushes for Hoover Conquest commercial uprights and Hoover Concept One and Concept Two series of upright vacuums. New carbons in the motor are like starting with a full tank of gas in a car. Replace carbons when they are the about the width of a large screw driver blade as they tend to run hot at the end of their life. Concept One models include U4201 series and U3301 series self propelled models. Requires insertion into black brush holder. OEM Hoover. Replace both at same time. Requires 2.