I've drilled & tapped several cranks, and while it's possible to do it freehand, there's a lot of chances to screw up. I'd suggest making up a bushing that slips into the hole in the balancer and centers the drill bit. A piece of air-hardening drill rod machined to the proper dimensions, then torch-heated to straw color and allowed to air cool will become hard enough to prevent erosion by the drill bit, and can be used numerous times. Another one, machined to a few thousandths over the major diameter of the thread, can be used to guide the tap for the first few turns to prevent a crooked thread. The standard balancer bolt is a Grade 8, 7/16X20 thread.
Jerry
