and the liner 36 are helically formed along a selected length of the external tube 38 such that the liner 36 conforms to the helically shaped inner surface 44 so that the at least two lobes 46 formed on the shaped inner surface 44 correspond to the lobes 40 formed in the liner 36 the external tube 38 including the inner surface 44 may be helically shaped by any means known in the art including machining extrusion and the like in some embodiments the shaped inner surface 44 of the external tube 38 is adapted to provide additional support for the liner material the shaped inner surface 44 u201c stiffens u201d the liner 36 by providing support for the liner 36 e g by forming a metal backing thereby increasing power available from the pdm for example shaping the inner surface 44 to form a contoured backing for the liner 36 may stiffen the liner material proximate the lobes 40 by reducing an amount by which the liner 36 may be compressed when contacted by the rotor 44 so that a better seal may be formed between the rotor 44 and the stator 32 moreover reduced flexibility increases an amount of torque required to stall the pdm the thickness 42 of the liner 36 may be increased at selected locations that are exposed to for example increased wear and shear e g proximate the lobes 40 46 so that the longevity of the stator 32 and therefore the longevity of the pdm 30 may be increased in some embodiments the thickness.