This is a focus rack for major brand stereo microscopes. It comes with ball bearing and 3" (76mm) diameter of annular ring that holds microscope head. Features Specifications: Brand new focus rack with ball bearing Focus gear with coarse focus and tension adjustment Diameter of annular ring holding microscope head: 3" (76mm) Diameter of opening for stand post (arbor): 1-1/4" (32mm) Distance between central line of annular hole and that of microscope body: 5-1/2" (140mm) Focusing distance in vertical direction: 2" (50mm) Shipping Weight: 4 lbs Manufactured under ISO 9001 Quality Control Standard