There is definitely two bearings and a unique size seal fitted, The seal fails due to scale build up and the inner bearing fails soon after. Mine failed after 9 months and neads another at 19 months ! ! ! The motor brushes have also been renewed as has the door rubber, all poor quality short lived components. !
To change, undo all 8 hex head bolts and centrepulley bolt, remove pulley and belt, then cast iron spider.
Alan Webb, July 2005