seal is provided with radial and axial grooves 22 and 25 in its end portions in contrast to the stator 14 of fig2 however the edges 28 and 29 do not have an enlarged outside diameter for supporting the stator instead the stator is supported in the housing 1 by o rings 33 and 34 the embodiment shown in fig3 is designed so that the fore vacuum connection of pipe stub 21 is disposed between the molecular pump stage and viscous seal the fore vacuum space 35 is formed by a groove 36 cut into the rotor 6 and by a bore 37 in the stator 14 which adjoins the fore vacuum pipe stub 21 in the space 38 beneath the stator 14 and the rotor 6 atmospheric pressure prevails the fore vacuum space 35 is sealed thereagainst by the viscous seal screw thread 32 in cooperation with the stator 14 and o ring 34 atmospheric pressure also prevails in the bearing space 12 over which the bell shaped rotor 6 extends and in the motor space 39 so that lubrication and cooling of these components poses no problems a bore 41 in the housing 1 provides a connection to the atmosphere for this as can be seen from fig3 there is a clearance 10 between the stator 14 and the rotor 6 not only at the screwthread like grooves 13 along the rotor but also at the screwthread 32 at the lower section of the rotor 6 the minimum clearance space between the screwthread section 32 and stator.