gritty diff
One simple item that is often overlooked but that causes a gritty feeling diff and/or one that won't stay smooth for very long.......a wobbly diff nut. Since the threads are basically forced into the all-nylon diff nuts when they are screwed onto the axle for the very first time, it is not unusual for those threads to be forced in a little crooked. If/when that happens, then the face of the diff nut where it meets the diff cone or thrust bearing imparts uneven pressure....which causes premature wear of the outer bearing and a gritty feeling diff. How to check your diff nut? Just screw the diff nut onto the axle's threaded stud partially and spin the axle while checking to see if there is any visible runout on the inner face of the diff nut. If so, then pitch it and try threading on a new one. Or.....you can also substitute an aluminum nyloc nut in lieu of the all nylon one. Most of the time, but not always, the factory made threads in the aluminum nyloc nuts are true. Having said that, it is still important to check even a nyloc nut for runout because some of those wobble too. Thankfully, once you've found yourself a diff nut that does spin true, it will usually last for the life of the car. Trust me, it is well worth the effort to find a true spinning diff nut if you are keen to have really smooth free 1/12th scale diff that stays good for a very long time between refreshenings.