Here is a post taken from rcmt...
Hi, the ball diff on the T-K is a good unit, it is probably the biggest one out there. Ball diff's are used for racing so they need regular maintenance. What i have found on the Nitro T-K truck's is that the diff MUST be set tight to prevent excessive slippage. Working on the truck from the back, first loosen the diff bolt on the left side, then take out the diff bolt set screw on the right side.(make sure you take it out).Then go back and tighten the diff bolt as much as you can by hand, then go back and put the set screw in and tighten up against the bolt, now go back and tighten the diff bolt into the set screw a little bit more, (you want to jam the bolt into the set screw) that's it. PS; when your putting it back together make sure you dip the end of the dog bone's in a little grease before assembly. Also when set up, do not holed one tire and turn the other backward's on any ball diff because it is sometime's possible to loosen the diff.
For those of you that want to try something diffrent with your NTK s8s ball diff, you can replace the 1/8 balls in the diff with friction slipper pegs from Team associated #25042 or even Traxxas #4685. Once you install them with a bit of grease, you will have to ajust and retighten the diff one or two times after running before they are seated properly. This mod will give you a more posi/spool like diff that can still give/slip a little and helps put more power to the ground. ( Update for pegged diff users, Team Associated has some new slipper pegs out that are better then the traxxas rulon pegs I would suggest using instead. The A/E pegs (part# 25042) seem to be of a better type material that are just a little bit longer and are cut more precisely to length, they also seem to be a bit harder matieral. They come in packs of 6 so you need two packs to do the diff.) I use this mod on my NTK trucks and they still handle great!!! with a little less maintainence and I think it is much better for bashing.
ATTENTION: LOOSE DIFF'S, for those of you that are having problems with your ball diffs coming loose. The diff (SPRING and WASHER) is causing the problem. SOME of the diff springs are cut oddly, so when you go to tighten down the diff, the spring bends to one side also bending the bolt with it and causing it to make contact with the inside of the joint shaft (HOUSING). This contact makes the diff loosen it's self when you go to drive it. Check when you do adjustments on your diff, make sure the diff bolt (head) is in the CENTER of the joint shaft housing hole, if it is not, it is probably making contact with the inside of the housing, (making the diff loosen). (THE FIX IS) cut off as much as 1/2 coil off one side of the spring so when you compress the spring it will stay strait and (NOT BEND TO ONE SIDE). Also the stock spring washer is to large and to thin and sometimes it bends (cups). Change the washer to one that is a little smaller in DIA.. (no bigger than the spring) and is thicker so it won't colapse when tightend or double up on them. The other thing to look out for is to make sure the cuped washer under the spring can turn/move/spin inside the housing, if it can not this will also cause the diff to loosen, some times this cup washer can seeze/crack/break from to much heat or lack of grease or both, just make sure it and the bearings under it have lots of grease and can turn freely. ATTENTION: One last thing when you are doing adjustments, don't forget about the diff bolt SET SCREW on the other side. ALWAYS take it OUT when you are making ajustments. THIS IS THE ONLY WAY TO KNOW FOR SURE THAT YOU ARE TURNING THE SET SCREW and not just thinking you are.