can somebody explain how to build ball diff smoothly? thx
It starts off with a well designed ball diff. Some diff that are not well made or with structural issues can never been built smoothly. Assuming it is a well designed diff, use Associated clear ball diff grease for the main balls and black grease for the pressure washer (smaller balls), they are by far the best I know of.
Ball diff WILL get damage if it is too loose. Years ago, ball diff are built to allow a little slip but nowadays most buggies have slipper clutch, so the need of slip in a ball diff is unnecessary. IMHO, ball diff should be 'just right', that means smooth to hand touch but no slippage on gas throttle.