My Sears chain drive garage door opener ‘s main gear is worn. I replaced just the main gear with the sprocket and shaft assembly for or so from a supplier at Amazon. I did not have to remove the unit nor the motor from the installed position making this an easier job. Remove the two screws from the end panel with the controls and leave the circuit board hanging. That way you can have room to unscrew the 3 screws at the top. You have to use the screw you removed to tap the holes in the replacement part. The holes are not threaded. Working on the ladder can be dangerous. If you installed the opener yourself, you can probably replace the gear. Work safely. This video should be viewed from a PC, Otherwise, the annotations clarifying the process will not be seen.