Features:  1) Extend operational life by maximizing chain and bar groove alignment while reducing overall costs, 2) Two-piece design includes a drum with a replacement rim, allowing the rim to be replaced multiple times before the entire unit is replaced, 3) Replacement schedule:  every three rims, 4) Some Oregon Power Mate systems have bearings installed, but some do not. Part numbers ending with a number have bearings installed. Part numbers ending with the letter "X" do not. Bearings must be purchased separately for all rim systems having part numbers that end with an "X".