The two are only separated when replacing the drum, otherwise the assembly is supposed to be removed by removing the wheel bearings. You do not have to separate them to have the drums machined as part of a brake job. To separate the two parts, install a lug nut a few turns on one stud, then hammer the stud out. You'll see serrations under the head that bites in and holds the two parts together. Pound out all of the studs and the drum will fall off the hub.
To install the new drum, line the holes up, then use a lug nut to pull the studs into place. To prevent damage to the friction surface of the nut which is required to keep them from working loose, use a couple of washers as spacers under the nut and put the nut on backwards, curved side up away from the washers. The washers must have a hole large enough for the serrations on the studs to pass through freely. Be sure the holes in the drum are centered. If not, the bolt will distort one side of the holes when you pull it in place. That will prevent the wheel from sitting flat. If the drum centers itself later while driving, the nuts and wheel will be loose.
You can also pound the studs in place but that's a lot of work. That will work easier if you're installing a used drum with the grooves already in the holes from the previous studs.
Thursday, November 25th, 2010 AT 7:13 PM