Starrett, 52451, Mechanical Micrometer Heads; Minimum Measurement (Decimal Inch): 0; Accuracy: 0.000100 in; Maximum Measurement (Inch): 1/2; Thimble Dia (Decimal Inch): 0.5313; Thimble Dia (mm): 13.49; Digital Counter: No; Spindle Dia (mm): 5.08; Spindle Length (mm): 19.05