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Thread: Build, Break-in, and Sand ball diff rings for perfect diff!

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Old 06-23-2011, 10:36 AM   #1
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Default Build, Break-in, and Sand ball diff rings for perfect diff!

Not sure if this tip has been covered, but if not here you go.

MIP and associated make great ball-diffs that help a lot of guys get more performance out of their rig. When properly assembled, broken in, and adjusted, you will see the full advantage and benefit of a great ball-diff. The reason why I wanted to share this info with everyone is because of how much of a difference it really makes! The diff-rings provided in the A.E./MIP kits are not perfectly flat unless they have been sanded or machined flat. This is the remedy!

This will work on any ball diff that uses steel diff rings. I have found that building diffs using this technique makes it easier to adjust properly, and last longer between rebuilds. Try it next time you rebuild your diff, you wont be dissapointed!

ITEMS NEEDED TO SAND DIFF RINGS

250 grit wet/dry sandpapaer
500 grit wet dry sand paper
Electronic motor spray cleaner
Super flat surface like a counter-top or glass surface
small bowl
Tweezers
Papertowel


1. Lay down the 250 grit on a totally flat surface (glass or countertop) and spray a big circle of electronic motor cleaner in the center. The reason for using electronic motor cleaner is because it wont warp the sandpaper like water would. If you dont have access to electronic motor cleaner, its okay. Its just something I prefer to do because it seems to make it slide easier and helps to keep it absolutely flat.
2. Place diff ring into its possition on female outdrive, rotate in circles (wax on wax off style) or in a figure 8 on the sand paper. The spray evaporates pretty quickly, reapply when necessary. Keep even pressure, dont press too hard. Rotate the outdrive 90 degrees occasionally.
Click the image to open in full size.
3. Keep checking the sanded side and you will see where the sand paper is actually contacting the ring, and where it is not. Sand it until the entire ring shows contact with the sand paper. You only have to do one side, but I do both. If you only do one side, make sure you remember which side you did. Repeat these steps for both diff rings and both thrust washers. Hold thrust washer with your index finger and dont press too hard.
Click the image to open in full size.
This is from the BFast website. It illustrates the difference between the rings before and after they are sanded flat.
Click the image to open in full size.
4. Now switch to the 500 grit sand paper and repeat the same process. Just sand them long enough to "polish" the contact surface. You will see it become shinnier and smooth. Do all the rings like this.
5. Put everything (diff balls, spring, rings, thrust bolt, thrust balls) in a bowl and spray with the motor cleaner.
Click the image to open in full size.
6. When you assemble your diff, pull out each piece as you need it with tweezers and spray one last time over a papertowel. The motor cleaner will evaporate almost instantly. Start with the diff rings first. What I do is steal one of my girlfriends make-up brushes and use it to put diff lube on the diff rings. I basically paint on a thin layer on the back and front of the diff rings.
7. A nice trick for placing the diff balls into the diff gear is to pick them up using the tube of diff lube (Part number ASC6591). They will stick right to the lube so you can place them in their spots and lube them at the same time. They dont need much lube on them. I think most people just go with their first instinct as far as amount used. If picking the diff balls up with the tube is not working for you, then just squeeze a small amount of diff lube into the corner of a clean bowl (about half the size of a pea) and put all the 3/32 diff balls in it and push them into the lube. They will clump together like a glob and then I put the glob of diff balls onto the diff gear. They should all have a thin coat of lube on them. Then I push the balls into their slots on the diff gear.
8. With the thrust balls and assembly, I drop one thrust washer onto the diff bolt, then hold it with the hex side downward, put a small amount of black diff grease (Part number ASC6588 or Green Slime part number ASC1105) all the way around the thrust washer, then use tweezers to pick up the thrust balls and place them into the grease. The grease will hold them in place. Then drop the other thrust ring on top of the thrust balls.
9. Now you are ready to insert the bolt and thrust assembly into the FEMALE outdrive.
Click the image to open in full size.
VERY IMPORTANT! Make sure to compress the spring with needle nose pliers to soften the spring to its intended state of tension before you place it into the male outdrive.
10. I use 2 drops of acer sin-lube on the ball bearings.
11. Finish the assembly by putting the 2 outdrives and diff gear together, spring into the male outdrive, then the T-nut. Tighten the diff bolt just until it eliminates the play between the diff gear and the outdrives. Now its ready to break in.
12. Use proper break in method. This is the most common method used.
Click the image to open in full size.

I have actually taken a tip from another thread and cut a dog bone in half and it fits perfectly in a dremel tool collett or a cordless drill. So after I build my diff, I break it in outside of the truck. I stick the dogbone into the outdrive, hold onto the diff gear, and run the dremel on its lowest setting or cordless drill on its highest setting for 30 seconds. Then I will tighten the bolt 1/4 turn and do the same thing over and over.
Click the image to open in full size.
I do this about 6 or 7 times until I can start to notice slight resistance when functioning the diff. Then I will make smaller adjustments making sure it still functions smooth after each adjustment and still using the dremel between each turn of the bolt. You will do this until you get to the point where it rotates smooth but with a little more resistance. If I start to feel a lot of resistance all of the sudden, or if it stops functioning smooth, I back off just slightly so its smooth again. If this is your first time building a diff, you can tighten it all the way until you feel the bolt get tight and stop turning. When you feel that, you have basically locked the diff, back it off 1/8 or 3/16 of a turn. Make sure to make mental notes of how the diff feels right at this point. I dont recommend the "lock-it-down and then back it off 1/8th" method. It will flat spot your diff balls and pit your rings. However, for first time builders it might help you to figure out where it should be. After that you are ready to go. Install the diff into your sc with the head of the diff bolt on the passenger side. Once I build my diff, it lasts a very long time because I NEVER LET IT SLIP! I dont think people realize just how much heat the diff builds up during a break in cycle. Heat makes metal expand. If you break in your diff in the truck, at the end of your break in cycle it may seem like its set just right, but as it cools down you will lose a little bit of tension and possably slip a little when you run it after it cools. One little slip and you can flat spot your diff balls or pit your diff rings. It will instantly feel gritty. So I break in my diff outside the truck and put it in the fridge for a few minutes if I feel its getting too hot during break in, then when it cools, I continue the break in. I take a lot of pride in having the smoothest no-slip diff. It doesnt demand that much scruteny, but its just another thing to consider. A lot of guys upgrade to cvd's and have a useless dogbone laying around. Cut that sucker in half and try this break in method.

I used this process on my MIP ball diff and it is absolutely the smoothest diff I've ever seen. I know BFast does this and sells them, but I didnt want to order one and I felt I could do a fine job myself. Turned out perfect, and I will do this to every diff from here on out.
This is a video of my diff after break in. The motor is brushless, but the diff is so smooth that it still functions without turning the motor. At the end of the video it shows what your slipper should do and sound when properly adjusted.
http://www.youtube.com/watch?v=hTlwTymwobg

After you install your diff and are ready to run your truck, then its time to adjust your slipper clutch. I always adjust my diff first, then adjust the slipper second. Put your RC on a table facing away from you and make sure to grab the correct wrench to adjust the slipper nut. Turn the RC on and hold the top of the right rear tire with your hand. Hold the top of the left rear tire with the bottom of your radio. Press down pretty hard with the radio and make sure nobody is in front of the rc. Goose the throttle for just a second. You are trying to lift the front tires up about 2-4 inches. If they dont pop up, tighten the slipper until they pop up when you goose the throttle and hold the rear tires. Tighten the slipper 1/4 turn at a time until its adjusted properly. Also, listen for a noise that sounds like a turkey call. If you hear it and the tires are not popping up, that means your ball diff is too loose. Pop off the passenger side rear camber link and tighten the diff by pulling out the drive axle. If you hear it and the tires are popping up pretty easy, then you have over tightened your slipper and need to loosen it. After the adjustment, do a 1/4 throttle diff break in cycle to smooth out the chatter marks you just made on your diff rings. Just hold one tire and give it 1/4 forward throttle for 20-30 seconds, then hold the other tire and do the same thing. Then go back to adjusting the slipper.

When its time to rebuild your diff or if it becomes gritty feeling, I suggest avoiding the expensive rebuild "kits" and just buying the parts you need. I have actually never bought a rebuild kit. I got a set of carbide diff balls and ceramic thrust balls from acer for my first rebuild even though I didn't need new balls, and got a new bolt and t-nut from the hobby shop. The T-nut and bolt costs 2 or 3 bucks. But now when i do rebuilds, I just clean everything with electronic motor cleaner. Then I just sand all the rings and clean them, clean out the bearings with electric motor cleaner, add sin lube to the bearings, clear associated lube to the diff rings and diff balls, black diff grease to the thrust balls, and use a new bolt and T-nut. Costs at most 3 bucks. If you never let your diff slip and your rings are perfectly flat, your diff balls last a long long time. Just because your diff feels gritty after a few runs, it doesn't always mean you need new diff balls. It ussually means you let your diff slip and the ring surface was damaged. Associated and MIP balls are carbide, the rings are steel. So the rings are softer than the balls and the rings take more abuse. And when i need new rings, I will just buy a new set for a couple bucks. Just buy the diff grease and lube and a new t-nut. If your balls are pitted, buy a set of those from associated for 12 bucks or acer for 14. The balls are the most expensive part, so if you can make those last by not slipping and keeping your rings smooth, you will save money and have a better diff.
Another thing I do is, between races I will do the "hold one wheel 1/4 throttle" process. It seems to help keep it smooth longer. Sometimes in a race I will get a little squirrely and my rear tires will land unevenly causing the inertia of the unloaded tire to force the slightest slip when it lands. You cant even hear it, but it happens. Thats why I do a quick break in cycle between heats. Just smooths things out before they become severe.

Hope this helps
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Last edited by Nismomike; 07-11-2012 at 03:27 PM.
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Old 06-23-2011, 11:21 AM   #2
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This is soo money...makes it smooth! Just make sure to not tighten the diff too much, just enough to get a slight resistance when you hold one wheel and spin the other.
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Old 06-23-2011, 11:30 AM   #3
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Exactly, and DO NOT tighten it all the way snug and back off an eighth of a turn unless it is your first diff build and need help finding that baseline setting. I see alot of guys give that advise, but you should avoid locking it down if possable. You will pit the rings doing that. You wanna sneak up on it slowly.

Last edited by Nismomike; 12-04-2011 at 09:30 PM.
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Old 06-23-2011, 02:34 PM   #4
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+1 you will flatten your balls.
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Old 06-23-2011, 05:52 PM   #5
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And nobody likes flattened balls!!!! The best way that i have found to adjust a ball diff is with either using a "diff adjustment tool" or using two hex wrenches that fit in the diff cups and slowly tighten it until you can just barely rotate the diff gear while both diff cups are locked with the the tool or wrenches. Ceramic balls make it feel like butter!!!
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Old 08-09-2011, 01:00 PM   #6
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I'm giving this thread a bump because it looks like a few guys are about to make the jump to running ball diffs. Build it right the first time and dont get in a hurry to throw it together.
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Old 08-09-2011, 03:18 PM   #7
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Quote:
Originally Posted by Nismomike View Post
I'm giving this thread a bump because it looks like a few guys are about to make the jump to running ball diffs. Build it right the first time and dont get in a hurry to throw it together.
+1, this link is subscribed and bookmarked for when I get mine.
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Old 08-09-2011, 04:25 PM   #8
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Quote:
Originally Posted by Nismomike View Post
I'm giving this thread a bump because it looks like a few guys are about to make the jump to running ball diffs. Build it right the first time and dont get in a hurry to throw it together.
Thanks for bumping this thread. I have tried a FT ball diff and didn't like it. I could never get it adjusted correctly and putting it in and out of the truck was a pain. So I eventually put it in my B4 and it runs great. I think the lighter weight of the B4 made it easier to tune.

However I like the break-in process out of the truck that is listed by the OP. I may have to pull it out of the B4 and try it again in my SC10.
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Old 08-09-2011, 09:39 PM   #9
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Quote:
Originally Posted by Lanson View Post
+1, this link is subscribed and bookmarked for when I get mine.
Right on, glad I could help! Hopefully you wont have any problems like Baby Huey did (AZSC10)
My SC10 started life as a brushed rtr and when I got my MIP ball diff I just pulled up the online instructions and put it together carefully, threw it in the truck, barely broke it in, and romped on my truck. Slipped a few times, tightened it, slipped a couple more times, tighter, and once I finally got it set and stopped slipping it was already gritty and not so smooth. Then I took it out and used this process. Night and day difference.

Last edited by Nismomike; 08-10-2011 at 07:56 PM.
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Old 08-10-2011, 08:47 PM   #10
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how often do you have to "rebuild" your diff? and by rebuild i mean resand your rings. thanks
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Old 08-10-2011, 09:02 PM   #11
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how often do you have to "rebuild" your diff? and by rebuild i mean resand your rings. thanks
I practice for 20min monday, wednesday, and friday. Then saturday I practice for about an hour before the heats begin. Then 2 heats 5min each and a 5min main. So every week I put about 2.5 hours on my truck. My current diff has 3 full weeks on it. I will rebuild it after this weekend of races. So I've got 6 hours and 45 minutes on it now, and its still smooth.

The diff balls in this diff have been re-used 3 times. Then I just swap out the diff with my spare when the time comes. Then that week I rebuild it and put it in a ziplock bag until its needed.
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Last edited by Nismomike; 08-10-2011 at 09:10 PM.
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Old 08-15-2011, 12:37 PM   #12
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Ok, so I pulled the Ball diff out of my B4, followed these instructions on a rebuild. How free spinning or smooth should it be turning the diff by hand? I feel resistance but inconsistent, not notchy or crunchy though. I think this was part of why I kept the gear diff installed. Building the diff following the instructions is easy enough, but that fine tuning is driving me nuts. I couldn't get the bolt to be flush with the t-nut. I am close but I am within a 1/4 of the diff being really tight. My final adjustment was to take it to the point that is just started to feel tight and then back off about 3/8th of a turn until is started to feel loose, but I still didn't get it flush with the t-nut.

I am motivated to try the ball diff because for some reason this last weekend I averaged .3 sec slower lap time then the previous weekend with no changes to my truck (running 7k in gear diff). The track was more damp this weekend then the previous, and was slick, I just couldn't get onto the throttle as quickly without the rear end stepping out. So I want to give the ball diff another try since I may have been experiencing the dreaded "Diff out".

Oddly another guy in the class, that I was faster then the previous week, shaved off about a second a lap on the more damp track. He is running a ball diff.
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Old 08-15-2011, 01:03 PM   #13
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Their may be a slight difference in the size of your t-nut. The molds aren't always perfect. When you say it was within a 1/4 of being really tight, I would only back it off 1/8. Really tight in your hand is gonna be a lot easier to turn than it might seem because the ouside diameter of the tire creates a lot more torque. You wanna be right on the edge of when you start feeling it get tight. When you feel it get dramatically tight all of the sudden, it means the spring is fully compressed. Thats too tight. Back off a little, but not more than a half turn.
A gear diff spins pretty free compared to where you want your ball-diff to be.

Last edited by Nismomike; 08-15-2011 at 01:07 PM.
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Old 08-15-2011, 01:48 PM   #14
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Originally Posted by Nismomike View Post
Their may be a slight difference in the size of your t-nut. The molds aren't always perfect. When you say it was within a 1/4 of being really tight, I would only back it off 1/8. Really tight in your hand is gonna be a lot easier to turn than it might seem because the ouside diameter of the tire creates a lot more torque. You wanna be right on the edge of when you start feeling it get tight. When you feel it get dramatically tight all of the sudden, it means the spring is fully compressed. Thats too tight. Back off a little, but not more than a half turn.
A gear diff spins pretty free compared to where you want your ball-diff to be.
I may be in the correct range then, just tightening a little more. My gear diff spins really smooth and consistent compared to the feel of the ball diff, so I was expecting a similar feel. I must be close though, so I will tighten it down a little more, run the dogbone dremel (great idea) and install it. I am going to have my gear diff ready if my times are way off during practice though.

So once you get to the track how do you tune it for the conditions? I like the feel of 7k in my gear diff but it seemed to do better on a drier track. I am hoping to take advantage of the quick adjustments the ball diff is supposed to give me.
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Old 08-15-2011, 02:00 PM   #15
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Originally Posted by Gigasipke View Post
I may be in the correct range then, just tightening a little more. My gear diff spins really smooth and consistent compared to the feel of the ball diff, so I was expecting a similar feel. I must be close though, so I will tighten it down a little more, run the dogbone dremel (great idea) and install it. I am going to have my gear diff ready if my times are way off during practice though.

So once you get to the track how do you tune it for the conditions? I like the feel of 7k in my gear diff but it seemed to do better on a drier track. I am hoping to take advantage of the quick adjustments the ball diff is supposed to give me.
You should actually be doing the dremel technique as you tighten the diff from square one. As soon as you get the diff together, tighten it just enough to remove the slack in the assembly, then dremel, tighten a little, dremel, tighten a little, and so on. Do that until you get to that almost tight feeling when you function it. I thought you had already done that. Back out the bolt a couple turns and break it in as you go.
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Old 08-15-2011, 02:07 PM   #16
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It will feel really smooth and consistant, but you gotta be breaking it in with each turn of the wrench. When you break it in, you are actually compressing a groove in the diff rings where the balls contact. Thats why you have to do the break in a little at a time.
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Old 08-15-2011, 02:55 PM   #17
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Actually I have been doing the dremel technique from the beginning. I followed this thread exact for the rebuild. The only exception was that I have 1500grit sandpaper instead of the 1000grit. By coincidence I even have the exact same can of Electronic Cleaner.

The issue has just been the final adjusting. Basically I am at almost tight right now after using the dremel technique, probably 10 iterations. However it is not smooth. If I back off 1/4 turn it gets smooth but I don't want to go too far back. I was expecting to get the bolt flush but couldn't do it without it feeling too tight.

So once you have set, is there no track adjustments needed if using this technique?
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Old 08-15-2011, 03:20 PM   #18
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You got it right then. If it feels a little inconsistant when you spin it, just back off till the exact point when it feels smooth. And it sounds like thats what you've done. You should be good to go. As for adjusting it, it will depend on your track and if it is loose or high grip. If its loose, you can slightly back off. If the grip is high, you can leave it as is. I dont change my diff setting unless it is really starting to drift and the rear end is coming around on me. You will need less adjustment with it though because it will function so much smoother in the turns. Thats due to the sanding of the rings. Sounds like your ready to go hit the track. Let me know if you pick up on your lap times. I went and measured my thrust bolts with calipers and their was a slight difference in lengths. I will have to edit my post to eliminate any confusion.
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Old 08-15-2011, 06:37 PM   #19
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Excellent, then I will adjust it back a little until it is smooth and leave it. Hopefully I will see some improvement, the track is changing layout after this race so I will only have one more data point. I also realized that I mis-read the grit of sand paper. I started with 500 and then went to 1500. For some reason I thought it was 500 and then 1000 but I only had 1500. The rings still came out mirror like but it probably took me longer then it should have.
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Old 08-15-2011, 06:52 PM   #20
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Youre gonna like it. Ive never used 1500, but it should be fine. I use a little lower grit so the balls have some traction and dont slip as easily. If you hear a sound like a duck call, tighten your diff. Thats the balls chattering against the rings when it slips.
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