[Biff]

The shaft is really 8 mm we just used a 5/8" by 1" piece of keyed shaft and drilled a 8 mm hole in the center with a drill press. Starting with a 1/8" bit held near the tip vertically with the shaft in the drill press. With care it's not too hard to center. We set the speed all the way up for this, Then reduced to normal speeds to drill the final 8 mm hole through. Then tapped for the second set screw on the sprocket, drilled in place and tapped so the treads between the sprocket and shaft with the 8 mm hole line up. Put a small dimple in the cim shaft add key (before you tap) to help with balance. One or our mentors has a 8 mm bit. You can get metric drill bits on-line or from a good tool supply, most hardware and home building places don't carry metric. As mentioned above a broach and a arbor press can be a big help, But there are other ways to get what you need.