# Please Help. I need to know how to build a smooth ball diff.



## fenx42 (Nov 30, 2009)

Here is the thing. No matter what I have tried I can not build a ball diff that is smooth. I don't know what I am doing wrong. My Associated T4 and my Kyosho RB5 SP2 both have diffs that feel like I used stones for diff balls. If I change diff rings and balls it doesn't seem to make any difference either. I used to be able to build smooth diffs I am doing everything I remember doing in the past. Please someone help me. I know my cars would drive better with a smooth diff; I am at a loss. Please Help.

Here is what I do.

1. Clean everything with motor spray

2. Oil the bearings (sealed bearings, not the diff or thrust balls)

3. Build the thrust bearing 
a. put the thrust washer on the screw
b. place a bead of black grease (Associated or Kyosho) 
c. stick the thrust balls around the screw in the grease
d. put another layer of grease to make sure the balls lightly are covered
e. put the second thrust washer on. 
f. Then set the assembly aside.​
4. Pop the diff balls in the diff gear
a. put the bearing in the center of the diff gear and the one out drive
b. put a dot of diff grease on each diff ball (Kyosho or Associated Stealth Lube) on each side of the gear​
5. Put the Diff Rings on the out drives
a. I have tried this all three ways, it doesn't seam to make a difference
i. I glue the rings to the out drives using a thin coat of CA on the out drive
ii. I use a couple dots of grease to hold the rings in place 
iii. I do not attach the rings in any way to the out drive just place them on the out drive.
b. set it aside​
6. Put the diff nut assembly into the proper out drive.
a. Compress the diff spring with pliers a few times
b. put the sprint in the out drive
c. put the nut in the out drive​
7. Put the diff gear on the out drive with the nut

8. Tighten the diff screw until it barely makes contact with the diff balls

9. work the diff for 15 to 20 seconds (hold the gear and twist one side back and forth)

10. tighten the diff screw 1/4 turn

11. repeat 9 and 10 until the screw is mostly tight (still loose enough where it would still slip if I put it on the track)

12. put the trans together and put it in the car. (the diff is normally really smooth at this point)

13. hold one wheel of the car and run the motor slowly (maybe 1/8 or less throttle) for about 15 seconds, and repeat with the other wheel. (I have tried skipping this step)

14. tighten the diff about 1/4 turn

15. place car down and check for slippage (hold car give it a little throttle and listen for slippage, just a short burst of 1/4 to 1/2 throttle)

16. repeat 13, 14, 15 until I don't get any slippage

17. be sad because i just built a really gritty diff. :beatdeadhorse:

I have read about some people taking 400, 800, 1000 grit sand paper to the diff rings first, but I have not tried this.


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## 4ThePinkRacing (Apr 30, 2010)

dont clue the rings. get new rings . get new balls .. sounds like that is only thing there not rite.. rings can get groved n feel like a box of stones ..

also make sure the diff screw balls has 2 rings one on head side of screw then bals then washer then slip it in the outdrive.. i myself use prelube for building real motors lite drop fo thatw orks great ..


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## BRDRACING (Dec 7, 2001)

Every time I build a diff, I always install a new thrust bearing.


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## HammerRacing (Jul 6, 2007)

Could be a bearing(s) as well. Run into this alot with my pan car, could of just rebuilt the diff with new balls and rings and it is feels bad. I took all the bearings out oiled them and free spun them one buy one and found one bad one. Changed that one bearing and reassembled the diff and it was smooth.

Hope this helps..


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## OvalmanPA (Mar 21, 2000)

I've found that most of the time I get a gritty diff in an off road car it is due to needing a new thrust bearing assembly.


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## XXXT4me (Apr 6, 2007)

*B-FAST R/C PERFORMANCE*​*Differential Break-In Procedure*​​*1) **Clean all parts and work area*

*2) **Follow owner’s manual for assembly instructions*

*3) **Lightly Coat Diff Balls with B-Fast Pro Diff lube, thoroughly lube Thrust Bearing with*
*B-Fast Pro Thrust grease. (or equivalent) *

*4) **Snug diff screw just until gear will not turn while outdrives are held.*

*5) **Hold one outdrive from spinning, a hex wrench that fits into the outdrive slot, clamped to your bench or in a vise works good.*

*6) **Use a power drill or Dremel (the cone shaped cotton polishing tip works great) to spin the diff for about 20 seconds (HIGH rpm on drill, LOW on Dremel). Reverse rotation for another 20 seconds.*

*7) **Tighten diff screw about 1/16 of a turn and repeat step 6*

*8) **Repeat step 7 at least two more times.*

​*Follow owner’s manual to set slipper*​*Slipper should slip BEFORE the diff*​*Do Not let the Diff Slip at anytime!!*​


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## fenx42 (Nov 30, 2009)

*Thanks*

Thanks for the replies. I will have to try what you guys said. I ordered up new stuff (Rings, Ceramic Diff balls, Thrust bearings). Hopefully it turns out better this time.


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