Abstract: A stop valve assembly comprises two closure members one for stopping substantive gas or liquid flow and the other to prevent leakage along a spindle section 33. Each member possesses a closure element in the form of a rigid disc-like member (9 or 34) of accurately predetermined thickness lying in a recess (1 or 3) formed round the end opening of a duct or around the spindle section and having exactly the same depth as the thickness of the disc. Over the disc outer surface 11, and the end face around the recess is located a flat annular deformable seal member (14, 64); this can be secured at in relation to the disc member, so as to travel with the disc member 9 as a stop valve, or can be secured against the said face around the recess so as to define an antileakage spindle seal. Since no deformed seal is used and since no packing is used in the screw spindle mechanism, high actuating force is unnecessary.