2006-2008 Polaris Snowmobiles FS/FST Service Manual., Page 177Get this manual
Final Drive and Brake Systems cover, without the reverse motor, then lift the cover up about one inchNOTE: Install Vise Grip pliers on the oil tank breather hose to prevent oil from flowing into the airbox19Slide the spur gear, chamfer toward rear of chaincase, on to the reverse gear shaft, rotating it until it drops down over the splines20Lift the spur gear up so it is flush with the top of the shaft, then slide the washer on to the gear, then over the reverse gear spline shaftDrop the washer and gear down, then completely install the cover