If you have to enlarge the bore to 6mm, lock the pulley in a vice, if you have one and drill it out , carefully, with an 6mm drill bit. doesn't matter if the hole ends up being a little larger at one end, the lock screws will take care of that. At the speeds involved with focusing any run out will be hardly noticeable unless the belt is over tensioned, which again, at the speeds and weight that needs to be moved, the tension does not have to be much.
Alternately, if you know someone with a lathe, would not take long to drill it out and be a bit more accurate also.
When and if you have to drill out the bore, be sure to remove the grub screws first and it goes without saying that aluminium is a soft metal and the drill bit may bite into the metal and grab. If using a hand power drill use a lower torque setting in case it does bite. Good luck.
Turning now to the mount for this motor, the Baader motor fits 25mm - 27mm dovetails, but it's very hard to find anything I could use as a substitute. Ideally, I'd like to get a pre-existing clamp, but I'm not sure if there are any available in these sizes. I thought of a finder scope clamp, but these are too large. Any ideas? I'll attach a Baader pic, and photo of my focuser drive unit with the dovetail. The tension thumbscrew on the drive unit is removed when mounting the motor.
There are plenty of 11mm mounts/clamps (for gun sights) and some 20mm, but I haven't been able to find anything around 25mm - 27mm. I think I'll have to make a mounting block out of wood, and use threaded inserts to run grub screws under the dovetail.
Once I get the belt and pulley I'll be able to size up the mounting block.
I ended up making the mounting base out of wood, and it's come up pretty well. Hopefully some clear skies tonight so I can try it out. Thanks for the help.
It seems ok. I'm taking my cue from the Baader manual:
Now slightly push the drive pinion until the pulley is slightly under pressure
(Img. 4) and tighten the four screws again. A good rule for the right pressure is that you should be able to twist the pulley at its longest part for 90° without applying force. If in doubt: better keep it too loose than too tight. If the pulley is too tight, it can happen that the SD II can't move the focuser and will stall.
So far so good. There is zero shaking now when focusing, and the amount of control is way above what I've ever had with this scope. The belt and pulley are well matched for the focuser, and the response is very direct and immediate. No play at all.
I can use the stock Mak focuser (mirror shift) to get close, and the motor to fine tune. This is great.