Abstract: A piston assembly for use in a viscous material dispenser is disclosed. The assembly has a main body with an axial bore for receipt of a push rod. The body has a face including an annular recess and an endless lip surrounding the recess. A camming washer is disposed at least in part in the recess. The washer includes a perimeteral camming surface engageable with the lip. The body and washer are connected together for limited relative axial movement when the piston assembly is advanced against material in a cartridge to dispense such material. The relative movement is effective to cause the camming surface to force the lip outwardly into tight engagement with a wall of such a cartridge.