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.
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.