sides extending in the axial direction of the sleeve it has shoulders 13 14 the shoulder 13 serves to connect to the first lever arm 7 the lever arm is mounted pivotably about a shaft 16 by its first end 15 by means of this shaft mounted on two opposite points of the wall of the second member 3 the shaft extends perpendicular to the longitudinal axis of the space keeper at its second end 17 opposite the connection to the shaft 16 the first lever arm is connected pivotably about a shaft 18 to the shoulder 13 via a pin or shaft 18 the pin or shaft 18 extends parallel to the shaft 16 as illustrated in fig2 the second lever arm 8 viewed about the setting screw 9 is constructed or arranged symmetrically relative to the first lever arm 7 at its first end 19 correspondingly located opposite end 15 the lever arm is mounted via a shaft 20 to pivot about the latter the shaft 20 is mounted on the side in the side walls located opposite one another of the first member 2 and extends parallel to the shaft 16 at its end located opposite the end 19 the second lever arm is connected pivotably about a shaft 21 to the shoulder 14 by means of a pin or shaft 21 as may be seen in fig1 and 2 the recess 11 is constructed as an oblong hole extending in a direction parallel to the longitudinal axis of.