Rebuild a ball differential

Radio Control Car Action, Nov 2001 by Buono, Derek

5 steps to put the smooth back in

Ball differentials are found in all sorts of vehicles, from nitro off-road to on-road electric. Many of us take our RC equipment for granted and only perform a rebuild when it's absolutely necessary. Have you ever thought about when it would be a good time to rebuild your diff? (I mean other than when you just melted it in your gas truck or had it set so tight that it felt like you were using pebbles for diff balls.) You should rebuild the diff at the first sign of a gritty or "crunchy" feel. All differentials are about the same and can be brought back to like-new performance by following these steps.

Step 1. Inspection

Completely remove the diff from the vehicle. C'mon, you can do it; It's just a few screws. They won't bite, I promise. Disassemble the differential over a large white work towel-- the towel will catch any parts you might drop, and they'll be easy to see against the white background. Lay out all the parts in order so you won't forget how they go. It's also a good idea to have your instruction manual handy so you can get an idea how the diff goes together. Check out the main gear for wear on the teeth or in the ball sockets. If there isn't any wear, put it aside until Step 2. Examine the diff rings for signs of wear where the diff balls ride against them; what you are looking for here is a groove that the balls might have made. If you don't feel a groove with your fingernail, then they are fine. If you do feet a groove, you can flip the rings over to reveal a fresh surface. If the flip side of the ring isn't smooth-faced, or the ring has already been flipped and is worn on both sides, you'll have to replace them.

Step 2. Clean the parts

Grease is normally used on both the diff ball and thrust bearing to lubricate the moving parts. It's a simple fact that this attracts dirt, and no matter how you try to seal the tranny, chances are some sort of contamination will occur. Use some motor spray or denatured alcohol to clean all the parts, and be sure to get all the grease out of the thrust-bearing assembly. if you are reusing any of the diff balls or rings, wipe them down and place them on a clean spot on your towel. Clean the diff gear with a pipe cleaner or a cotton swab to get all the diff lube out of the holes. The more thoroughly you clean the parts, the smoother the diff will feel when It's back together.

Step 3. Diff-ring prep

Matt Francis revealed this secret diff-ring trick: he uses 400-grit sandpaper to remove any high spots on the rings; this, In turn, allows the balls to spin more smoothly. Rub the rings on the paper In a circular pattern, wax-on/wax-off style, until there is an even, dull sheen. Some diff rings are one-sided; make sure you put the rings on the drive hubs with the rounded-- edge side facing toward the outdrive. When you place the rings on the drive hubs, use a small dab of diff lube to hold the ring in place; this helps when you put the two sides together.

Step 4. Put it all back together

Slip the thrust washer on the diff screw and liberally apply grease to the top of the washer. Now place all of the thrust balls in the grease, and apply more grease and the second washer. You want a lot of grease here, so don't worry If It oozes everywhere. Slip the thrust assembly in the diff outdrive and set It aside. Insert the diff balls In the diff gear and apply a generous amount of clear silicone grease to the balls on both sides of the gear. Place a dab of grease on each outdrive and press the diff washers into place. Now simply slide all of these pieces together and tighten the diff screw. Voila; your diff is back together.

Step 5. Adjusting the diff

It is extremely important to tighten a ball differential properly; they were not designed to have any slip in them. In electric vehicles, the sound of a slipping diff is very noticeable; it almost sounds like a quack. Drivers of gas vehicles don't have the luxury of being able to hear a slipping diff, so make sure you adjust it correctly; otherwise, you'll be heading to the parts counter and doing this all over again. A too-- tight diff can also spell disaster; it can affect a vehicle's handling and can wear out the diff rings and balls prematurely.

To set the diff to the proper tightness, first check it when the transmission is apart. Hold both outdrives, and try to spin the cliff gear; you shouldn't be able to. Niftech makes a diff stand that makes this job easy. You want to get the proper balance between smooth, efficient diff action and tightness, so the gear doesn't spin. When replaced in the car, the diff setting may change slightly as the cliff breaks in. Run a quick lap, and check the setting. An easy way to see whether the diff is slipping Is to hold the car still and hit the throttle. If the tires don't spin but the slipper nut or main shaft does, then the cliff is slipping. You want the slipper to slip before the nut spins.

Now, that wasn't so bad, was it? Ball diffs are very simple to adjust. With a little patience and experimentation, a ball diff will give you hours of trouble-free running. As with any moving part, some simple maintenance will go a long way, and that means more time on the track or in the backyard.