I'm resurrecting this old thread just to add a little info in case it may help someone else that is rebuilding one of these sanders or is having trouble with the 6x48 belt tracking properly.
Twenty something years ago I got one of these machines with the 6x48 belt and 12" disc for free. It had been thrown out as scrap and it was in pretty sad shape. It just sat in the corner of my garage for quite a few years until about 15 years ago I drug it out and completely rebuilt it, installed a new motor, fabricated a few missing parts and made it pretty. From day one, I fought with erratic belt tracking. Tried different belts with no luck. I knew that the driven belt pulley should be crowned and always just assumed that it was. Well, after 15 year of fighting with it I finally checked that upper belt pulley. Zero crown. I took it off, set it up in the lathe and turned a .020" taper on it. From the first flip of the switch it has tracked perfectly.
Sure wish I would get over this habit of putting things off for years !
Ted