Multistage vacuum pump and a stator for a multistage pump

A stator for a multistage pump, such as a Roots or claw vacuum pump, comprises two half shell components 10 defining a void for receiving two rotors 30 and forming a plurality of pumping chambers. Two end plates are mounted at either end of the half shell components and at least one of the end plates comprises apertures for receiving shafts of the rotors. An outer casing 20 surrounds and encloses the half shell components in a substantially gas tight manner, and the end plates form part of the outer casing. The outer casing may define an end and side walls of an inlet pumping chamber 44 and/or an exhaust pumping chamber 46. The outer casing may be formed of multiple parts arranged so that junctions 28 between adjacent parts are sealed together in a substantially gas tight manner with a single sealing surface. At least a portion of one part of the outer casing may be integral with one of the half shell components, and at least a portion of a second part of the outer casing may be integral with another of the half shell components, the outer casing parts having a different form to the half shell components.