If I spin the diff in my hand it feels a little gritty, but i've never had a diff perfectly smooth.. Put in the car and I can spin my tires in opposit directions and its as smooth as a baby's butt.. I realise cleaning them after every race is a pain the butt, but IMO this car is worth the extra work... For me its not that much work beings I pretty much go through my car completly after every race.