Micro 100 QT-312-C4 Boring & Grooving Bar Holders; Inside Diameter (Inch): 5/16 ; Inside Diameter (Decimal Inch): 0.3125 ; Outside Diameter (Decimal Inch - 4 Decimals): 1.5748 ; Overall Length (Decimal Inch): 3.0411 ; Head Diameter (Decimal Inch): 0.