Medium Shaft

Remove the rubber sleeve from the neck of the die grinder
Remove the 4 screws that hold the neck on
set aside the plastic coupler & large washer
While holding the neck in a vice, unscrew the collar nut
Set aside the smaller washer & felt washer that were behind the collar nut
Using the handle of the crescent wrench, unscrew the butterfly nut
Set aside the butterfly nut
Place new shaft in the neck.
Place metal washer and felt washer into neck, then reinstall collar nut.
Reinstall butterfly nut. Note the vice is holding the shaft
Reinsert plastic coupler
Place large washer in neck
How it looks before reattaching the neck
Reattach the neck with 4 screws, be careful to line up exhaust vents to the side
Slide the rubber cover back onto the neck
Voila!