How To Rebuild Your 3-Gear Transmission

How To Rebuild Your 3-Gear Transmission


On race day, you only have one goal: to go as fast as possible and to lay down the quickest lap times. As you try to accomplish this, one of the major factors that contributes to your success is your transmission. If your motor is the heart of your car, your transmission is the soul that will translate the motor’s power into perfectly carved turns and intense forward grip. With all this awesome power comes responsibility, and your duty as a dedicated racer is to maintain your transmission and service it so it maintains its highest potential. Follow along and I will show you the steps you’ll need to take, as well as some tips to help you free up that tranny. Let’s get to work.


Step 1: Remove the Transmission
Whether it’s your first time rebuilding your transmission since you put your car together, or you bought your car as an RTR, refer to your instruction manual to determine which screws need to be removed and to help keep track of which screw came out of which part. As you remove each fastener, keep them organized by placing them in a parts tray or on a magnetic pit mat.

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Don’t force it! If the transmission doesn’t come out with a wiggle, chances are you missed a screw.


Step 2: Open Up the Gearbox
Once the tranny is removed and you’ve popped open the case, remove each piece and inspect all of the working parts for damage and wear. After everything is taken out of the transmission case, clean the case and each part with a brush and motor spray to remove any dirt or plastic slivers that have worn off of the gears. Take note on how everything is put together so you can reassemble it correctly later.

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Once you’ve cracked open the case, spin the gears to check for bad bearings and worn or broken teeth.


Step 3: Inspect the Gears
Even if you don’t fi nd anything stripped or broken, you should still look for worn or broken gear teeth, gritty bearings, and wobbly gear shafts. Do the gear teeth still have a nice shape? When transmission gears wear, the teeth will become more pointy and sloppy, and they should be replaced. It’s always a good idea to replace any gears that have chunks missing or have been damaged in any way due to debris entering the transmission or broken gear teeth fl oating around in the case. Clean the gear case with a non-corrosive spray, such as motor spray available at your hobby shop.

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This idler gear is in good shape, with no chipped or missing teeth and a symetrical tooth profile. If a gears’ teeth appear to slope more on one side like a sharkfin, the gear is worn. 


Step 4: Inspect the Differential
It’s now time to pull out your differential, which is the key component in the transmission that allows your car to rotate through corners. With a super clean work area, pull out your instruction manual to view what to expect when your diff is apart (if your RTR didn’t come with instructions, look online). The first items to look at are the diff rings. Examine the surface of the ring to see if a groove has formed; if this is your fi rst time rebuilding the diff and it uses symmetrical diff rings, you can often flip them over and use the shiny, undamaged underside until it’s time for another rebuild. Roll the diff balls on a smooth glass surface to check for fl at spots, as any damaged balls should be replaced. When you inspect the diff gear, inspect to see that the holes into which the diff balls fit are not too worn or damaged in anyway; if they are, replace the gear. Once everything meets your approval for reassembly, use your instruction manual to rebuild everything to factory specs.

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It’s normal for a polished line to appear where the balls contact the diff ring. 


Step 5: Bearing Maintenance
When you remove the bearings from your transmission, ensure that they spin freely. The bearings go through a tremendous amount of load when they put the power down from the motor to the wheels. Therefore, they need to be super efficient. If any bearings have a notchy feeling or have completely seized due to being overheated, save yourself any future trouble and replace them. Spray down the ones that are still in working condition with motor spray to remove all the dirt and grit. If you have the time and have access to an ultrasonic bath like the ones used to clean jewelry, watches, and small parts, that will be your best bet. Let everything dry after cleaning and apply a small drop of light bearing oil.

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Don’t spray your fingers like this. Place the bearings on a rag to clean them, or better yet, use an RPM Bearing Blaster.


Step 6: Slipper Clutch Maintenance
It’s almost time to start putting everything back together and one of the last items that needs your attention is the slipper pads. The surface of your slipper pads should have a slightly coarse surface used to grip on the slipper plates. If the pads have a shiny glaze to them, use a light grit sand paper to rough up the surface of your pads. This trick will usually prolong the life of your pads throughout a few rebuilds. Also, if you haven’t used the other side of your slipper pads, you can flip them like you did with the diff rings. When nothing else can be done to salvage your slipper pads, buy new ones.

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A light scrubbing is all it takes to freshen up a set of glazed slipper pads.


Step 7: Make Sure the Spur and Pinion are in Good Shape
The first contact your motor has to your transmission is your spur and pinion gears. If dirt, rocks, or other small debris have made it past your gear cover, your gears will most likely be chewed up. Replace either the spur or pinion gear if the teeth are damaged, as continuing to run a gear with chunked teeth will simply tear apart the other. You can sometimes salvage a steel/aluminum pinion gear by taking a small file to it and shaving off the jagged damage to restore a normal tooth shape.

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Step 8: Put It All Back Together
It’s now time for your instruction manual to do its thing. Start laying out the transmission pieces in the order they will be reassembled according to the directions. One thing I like to do is to use thread-lock on the screws that hold the transmission case halve even though they screw into plastic. Tighten these screws all the way down and check how freely the whole transmission spins. If there is a bind, loosen each of the transmission case screws an 1/8 of a turn at a time until everything spins. Repeat these steps when you install the aluminum motor plate.

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Screws that pass through the transmission and thread into the motor plate should get a drop of thread locking fluid.


Step 9: Reinstall the Transmission
Before you start to install your newly rebuilt transmission into your car, clean off any dirt or debris in and around where the transmission will be so that it will sit flat and square on the chassis. You want everything to sit symmetrically to avoid any binding so that the left and right tires get the same amount of power.

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Give the slipper shaft a spin before you reinstall the gearbox to make sure it’s working properly. A freshly rebuilt transmission should rotate with almost no resistance.


Mission Accomplished
Now that your car is back up to snuff, you should perform maintenance on your transmission every few weeks to keep it in top shape. The less friction your tranny has, the faster it will spin, the faster your car will go, and the cooler your motor will run.






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Updated: February 3, 2017 — 10:14 AM
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