The Starrett 124M Solid -Rod Inside Micrometer Sets are the most popular inside micrometers because of their lightness, ease of use, and range. They are very useful for measuring inside diameters of cylinders and rings and measuring parallel surfaces. The desired range is obtained by assembling rods and spacing collars to the micrometer head. Measuring rods are provided with a shoulder that is set accurately in the micrometer head and locked in position. When assembling rods to the A and B heads, the reading line on the micrometer head should be lined up with the marking on each rod (except for the 50-75mm (2-3") rods). Rod diameters are approximately 6mm (1/4") on the A and B sizes, and approximately 8.5mm (11/32") on the C size. Each rod has individual length adjustment for the anvil by means of special wrenches furnished 200-800mm, 0.01mm Grad, 4 Measuring Rods with Case.