the bearing in the drum has 4 parts. 2 bearings and 2 bearing cases, which are inside drum which run against bearings. so to replace all parts, both old bearings should be removed along with bearing cases by tapping out of drum with punch/chisel. then when the new bearing cases and bearings are installed with correct grease the seal goes in the back of drum to stop grease escaping. it just taps in. make sure all parts are right way round when you fit them.
me
July 2009