This is a 3.13/16 Straight Flute Shell Reamer. Shell reamers are fluted almost their entire length, they have a tapered hole(1/8 inch per foot) which fits on a tapered arbor. The slots in the reamer engage lugs on the arbor providing ample strength of drive. This is only one of the items we have available. Our part number on this one is 5-150-250