I am putting together a wifi 'Pushto' DSC system for my GSO 12" dob and needed to modify the ALt bearing to mount the encoder. Considered a number of different approaches and in the end it proved pretty simple.
The Alt bearing (whether the one with the aluminium knob or the black nylon/plastic knurled snob) is the same internally. There is a cast aluminium spindle which is also doubles as the tube mount. On the outer end of the spindle is the roller bearing with the knob providing the friction locking. When unscrewed (after removing the lock screw) the entire spindle is open to the back and requires no drilling, only the knob and it's aluminium insert requires drilling.
The knob insert is machined with a centre depression so it's pretty easy to drill that out with a normal hand drill if your careful. You may need a small bearing around the encoder bolt through the knob hole but it's possible to go without it. You will need to provide some sort of bearing to the rear in the spindle but this could be anything that holds the bolt in place laterally, from a carved piece of dowel rod to pourng epoxy resin into the spindle and drilling it out, of course there is always a nylon sleeve if you can find the right size. There is a lip on the rear of the spindle which serves as a great spot to epoxy in place to epoxy in a large washer. Your encoder bolt just needs to be long enough to reach above the spindle lip at the back so you can put a nyloc on it. I am using a cap screw type bolt as my encoder shaft will fit into the head nicely and I just need to drill a lock screw hole (and tap it) to secure the encoder shaft.
