Gear differentials have been the norm in 1/8-scale cars for quite a while, but it has only been since the release of short course trucks that they’ve found their way into many RTR and race-level 1/10-scale cars and trucks. A gear diff requires little maintenance and is very durable, which is why they have become the norm for entry-level cars, but their consistency and ability to be tuned with different oils makes them ideal for any track condition. Properly building a gear diff is very easy, and routinely changing the fluid or grease is a great way to keep that diff consistent and functioning properly. It also is a great tuning feature on your car as well. Let’s get greasy and go over building, maintaining and tuning a gear differential.

What does a differential do?

A differential is a fairly complex mechanism to do a simple task, allowing opposing wheels to spin at different rates. Think of a car turning around a corner: the inside wheels have to travel a shorter distance than the outside wheels. If you didn’t have a differential, the inside tire would spin excessively and either cause your car to push through the corner or break loose and spin out. Since neither result is ideal, differentials are used in order to help the vehicle corner smoothly.

Here’s what goes into a gear diff – not too complicated, right?

MAINTENANCE

When rebuilding a gear diff, you want to make sure everything is clean and as fresh as possible. No need to get new gears or outdrives every time you rebuild your diff, but some new O-rings and paper gaskets are always a good idea (where applicable), especially if your diff is leaking.

Step One: CLEAN YOUR DIFF.

A bucket of warm soapy water works great on dirt and grease.

Clean the differential before you take it apart, and then disassemble the diff piece by piece and clean each item individually.

Step Two: CHECK YOUR BEARINGS.

If thick diff fluids have leaked into the bearings, they will need to be thoroughly cleaned and lubed with some bearing oil.

Roll the bearings in your fingers to ensure that they spin freely – if not, blast them clean with motor spray and re-lube them with a drop of bearing oil. If that doesn’t get rid of the grit, toss the bearing and install a new one.

Step Three: INSPECTING FOR WEAR.

Diagnosing outdrive wear is easy: if the dogbone pin wobbles around and gets caught on jagged edges or grooves, the outdrive is worn. Make sure the dogbone pins aren’t flat spotted either.

The outdrives typically wear first. A properly wearing outdrive will have a smooth, U-shaped arch where the bones make contact. If there is a jagged notch or wear is excessive, it is time to replace the outdrives. Take a look at the spider and bevel gears. If teeth are excessively sharp or broken off, it’s time to replace the gears. When diff cases leak, it is sometimes caused by excessive wear or stripped out screw holes. If you have already tried new seals, your screws tighten down without issue, and your diff still leaks, it may be time for a new diff housing.

ASSEMBLY

Step One: GREASE YOUR O-RINGS.

Applying a little bit of grease to the o-rings will not only help prevent leaks, but make assembly go more smoothly as well!

Starting with your clean or brand-new diff, it’s time to assemble it. Start by replacing the O-rings and other seals. Wherever you have an O-ring, it is a good idea to use a good amount of grease to help with the seal and protect them during assembly from binding or cuts.

Step Two: SHIM YOUR GEARS PROPERLY.

Shimming the spider gears properly is essential for smooth diffing action and preventing premature wear.

Check your manual and make sure you have the right shims in your diff. This is extremely important! If you are missing a shim, do not continue with assembly until it is replaced. Improper shimming can lead to premature wear and ultimately, failure of your gear diff.

Step Three: ADD FLUID OR GREASE.

Squeezing a little bit of oil into the diff before you place the spider gears can help make sure there are no air pockets.

At this point, you should have both of your diff halves ready for inner gears and some oil or grease. Install the bevel gears, then add a little oil. There are a lot of small pockets for oil to seep into and it seems to go a bit faster if you put in some oil before the spider gears. Assemble the spider gears on the shafts with proper shimming and drop them in the diff case, making sure that the notches in the shafts line up. Fill up the diff case to the top of the pins holding the spider gears in place. If you are using black grease, filling it to this level is fine too.

Step Four: ALMOST DONE!

Diff screws can be quite long, and it’s okay to use a power driver to thread them in most of the way – make sure the clutch is adjusted to a loose setting and don’t force it! Finish tightening them by hand, working in a cross pattern to place even pressure on the gasket.

Place the diff halves back together, making sure the gears mesh up and the case seals completely. If you are using any sort of power drill to assemble the diff, be very careful! The screws go in so fast that they create a lot of heat, the plastic softens, and you will strip out the threads. If you insist on using a power drill, be sure to have the clutch set very low and only put the screw in about halfway. Finish the other half by hand. Start with snugging one screw down, go to the opposite side, then work in a cross pattern tightening the screws (don’t follow in a circle pattern). No need to crank them down hard as you can.

TUNING

Many RTR cars come with grease in the gear diffs. Lighter oils (or grease) will allow your car to rotate faster around corners but you may sacrifice straight-line traction. Heavier oils will have the opposite effect.

Because of the different sizes of gears used in differentials it’s not really fair to compare the oils used in various vehicles, but the rules of adjusting your car’s performance apply across the board. Thinner oils will help the car rotate in corners more quickly, while thicker oils will increase forward grip and help the vehicle accelerate. Different cars react differently to oil changes, so the best thing to do is just to try some different oils or grease. Diff oil is fairly cheap and a great way to tune your truck for more or less traction.

THAT’S THE DIFF!

Though a gear differential may not require as much critical maintenance and adjustment as a ball unit, keeping your gear diff operating smoothly and knowing how different weight oils will affect how your car handles is just as important to the overall performance of your vehicle. Take your time, try new things, and have fun!