Abstract: A shank (10) for retaining a tool or touch probe in the socket (S) of a machine tool spindle comprises 6 areas of surface which, in use, bear against the wall of the socket (S). The areas of surface are provided by three balls (34) resiliently supported for movement radial to a shank axis (A) on small flange section (26) (FIG. 2 ) at one end of the shank (10), and three further balls (36) embedded in the other end of the shank (10). As the shank is drawn into the socket (S) the small flange sections (26) deform allowing the balls (34) to move radially inward until the surface of balls (36) engage the wall of the socket (S). A pull-up stud (38), supported on a frusto-conical block (40) is connected to the shank by a plunger (42) and compression spring (46). Retraction of the shank into the socket by pull-up stud (38) against the resilience of the small flange sections (26), causes the plunger ( 42) to compress spring (46) until the block (40) bears firmly against the wall of the socket (S).