FMJ guy says in the groove supposedly to hold grease down in the gear. Seems logical but when I push gear down over shaft O-ring seemed to interfere and possibly move but it's the only logical place I can see so it's probably just crappy manufacturing of gear and fit. That's where I put it but ran out of time with jamming my arm up in the small space to mount the motor. I've been having problems with this particular one eating gears. Regulator seems fine but keeps eating these gears. Don't feel any missing teeth on regulator and don't feel any wobble. My last guess is possibly the harness was interfering with the window once I pressed on the door panel. I'm hoping that was the problem so I'm trying again and going to ensure the big wire harnesses are completely out of the way this time. It's not so much the cost of the kit, it's the PITA of reaching up in there and the fear somethings wrong with the regulator.