I watched the video AStanton mentioned and it is fairly consistent with the procedure for reinstalling the track on your 121-3. Before reinstalling your track I would recommend investigating why the track came off in the first place. First and foremost check the track for damage. It would be a waste of time to reinstall a damaged track. Second, check the upper idler roller. It may be rough to turn, loose, or possibly missing. Thirdly, assess the drive sprocket teeth to make sure there are no missing teeth or they are not badly worn. The last check on the machine is to see how far the track on the other side is sagging. It may just be that the track was loose and needed adjustment. The video says two inches if sag is good. That is incorrect for your Kubota. 10-15 mm (3/8"-1/2") is what you want. To check the sag of your track first you need to find the infinity symbol (the number 8 on it's side on the inner portion of the track) and turn the track until that symbol is over the upper idler roller. With the machine lifted in the air, take a measurement from the lower idler roller to the inside of the track below and if needed add grease to increase the tension. If you do have to adjust the tension, run the track a few rotations and recheck the measurement. One last note, make sure the track is facing the correct direction prior to install. It's a terrible feeling to be all done and then to realize it's on backwards. Good luck!
