They can only be tightened by opening the covers because the gear is connected to the string shaft where it passes thru the bottom mounting plate by peneing the shafts end with a hammer. If that brass gear hasn't losened on the shafts end, the mounting plated has turned up ears that the worm shaft passes thru that hold the shafts worm gear against the brass gear, and sometimes the ears tilt away from wear or from the knob and shaft being hit. Sometimes the brass gear begins to round over on the ends of the teeth from non lubrication against the worm shaft. The problem with removeing the covers is it will devalue your tuner with signs of the work haveing been performed, and the covers tabs may brake off when bent over to remove it from the mounting plate.