The diameter range is 6-1/2" to 14-1/4", calibrated into 1/32" (1mm)increments. Sets the proper clearance between brake lines and drums. The tool has a locking  knurled knob to keep measurement accurate This quality tool is used to set the proper clearance between brake lining and the drum prior to assembly.  Inside calipers measure drum diameters and outside calipers transfer the measurement to the brake shoes.  A knurled knob locks the tool at measurements.  Its diameter range is 6-1/2" to 14-1/4", and is calibrated into 1/32"(1mm)increments.