They have a 3/8" (9.5mm) diameter shank. The cutting end (P/N 3061) is designed to work in a 1/4" (6.4mm) minimum hole with a maximum depth of .600" (15.2mm). P/N 3063 is designed to work in a 5/16" (7.9mm) minimum hole with a maximum depth of 1.00" (25mm). P/N 3064 is also 5/16" minimum but has a longer 1.5" reach. If desired, they can easily be ground down to work in a smaller hole. The shank has a flat ground on one side that is for the set screw of the boring head to tighten against.