Valve guides from 3/8in brass rod test fitted here. The horizontal V-groove in the machinist vise provides an arbitrary head angle for the spark plugs. Shown is a flat being cut in the chamber with a 3/16 end mill for a 90 deg. approach by the center drill, then tap drill for CM6 sparkies. From the top side of heads the CM6 hex will not clear the opening to engage the head threading. Since they don't make a reduced shank 13/16in end mill to fit my machine, the China special boring bar will broaden the space. ;D The heads are cross milled & fly cut parallel to plug angle to a point exposing the hex to get a socket around them.