For those who want to try gear diff for indoor carpet....it is actually the best option for any TC when running indoor. 300,000 at front, 2000 rear...alot better then spool. However, for those who don't want to shell out $80 for 2 diffs and still want to work with ball diff...here is what I did. (I used this method for outdoor racing and works out great)...
Use your existing ball diff...(Mi3 or Mi4...) you don't even need to replace the balls and diff plate at all. But get a whole of a Tamiya Anti-wear grease 53439. Apply the right amount of them into the diff....the grease itself is extreme sticky and thick and adjust the diff pressure to what you need...
I applied the grease onto the Mi3.5 diff on front for my USGT car...tighten it to 90%. And I compare it with my 416 gear diff with 300,000 grease...it is very similar.
Apply 300,000 grease onto the ball diff will only last you a race or so...cause the ball diff does not have enough room to carry the grease and the diff will wear off (get loose) very fast....but not this anti-wear grease. I am actually building another Mi3.5 for USVTA with similar setup.
The problem is...Tamiya/tower discontinued this grease (don't know why) and you guys have to look around for those who still has it... (ebay has plenty)