The 20439R.S replacement inner rack or teeth can be used to rebuild the part of the screw drive carriage that wears out most frequently. It includes (1) inner rack. Part is not returnable! Make sure the rest of the parts on the carriage are not damaged or missing. No other parts are included. Rack Works with Part Numbers: 34107R, 34107R.S, 20414R, 25589R, 25589S, 36179R, 36179R.S.