Abstract: A machine tool spindle is associated with an adjustable preload mechanism for varying the preload on a bearing set for the spindle the mechanism comprising an annular piston that is selectively pressurized for loading spring(s) carried by a spring retainer that is engageable with the bearing set; and wherein the annular piston is held in an adjusted position by a position locating device including an adjustment member removably supported within a housing for the bearings and selectively engageable with the annular piston; the position locating device having a plurality of adjusted positions selectively engageable with the annular piston for fixing it in one of a plurality of positions for varying the compression of the spring(s) so as to maintain one of a plurality of preloads on the bearing set.