If you’ve ever built a gear diff and thought, “It works… but I’m not sure it’s perfect,” this one’s for you.
In my latest Adrenaline RC Racing YouTube video, I revisited an older differential-build tutorial I wasn’t super happy with and re-shot it the way I build diffs every single time now—the clean, consistent, “like a pro” method that keeps your setup repeatable and your diff life long.
This guide walks you through the full process step-by-step, with the exact workflow I use to build a gear differential(shown here on a Sparko center diff using 20K oil).
Why Diff Builds Matter (More Than People Think)
Your diff isn’t just “a part you assemble.” It’s one of the biggest tuning tools on your car. A properly built diff helps you:
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Stay consistent run to run
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Reduce leaks and premature wear
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Prevent binding, roughness, or weird handling changes
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Know exactly what oil is in the car (and where)
And honestly? Diff maintenance is one of the easiest ways to save money in RC long-term.
What You’ll Need
Here’s the basic setup I use:
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Diff oil (example in this build: 20K for the center diff)
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Driver or drill with the correct bit (mine uses a 2.0mm in this build)
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O-ring grease (for sealing + longevity)
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General grease (the “green goop” from the kit, or black grease works too)
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Pliers (helpful for small parts like shims/washer placement)
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Microfiber rag (you’ll use it constantly)
Pro Tip #1: Lay Everything Out in Order
Before you assemble anything, lay every component out in a clean sequence.
If you’re doing a rebuild, this is huge: line everything up exactly in the order it came out, so every part goes back into the same place and nothing gets mixed up.
Typical layout looks like:
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Outdrive → bearing → gear/cup half
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O-ring → washer/shim → pin → sun gear
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Cross pins + 4 spider gears + washers (if your diff uses them)
This takes 30 seconds and saves you from so many “wait… where does this go?” moments.
Step-by-Step: Building the Diff Like a Pro
1) Grease the Outdrives (Don’t Be Shy)
Start with your general grease (the green kit grease in my case).
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Put a generous coat on the outdrive where it contacts/seals
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Too much is fine — you can wipe excess later
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This helps reduce wear and keeps things smooth
Then install your bearings on each side.
2) Build the Bottom Half of the Diff
Now assemble one side fully:
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Slide the outdrive into the diff cup/base
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Grease the O-ring with O-ring grease and install it
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Add your shim/washer if your diff has one
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Install the pin (line it up with the cutouts so it seats correctly)
Quick tip: If the diff cup is brand new and totally dry, I like adding a tiny amount of diff oil inside the cup before dropping the sun gear in. It just helps everything settle smoothly.
Then drop your sun gear onto the pin.
3) Assemble the Spider Gears
Install:
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Washer behind each spider gear (if included in your diff design)
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Cross pin + 2 spider gears
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Second cross pin + 2 spider gears
Important: the pins have grooves and they need to key into each other properly. Make sure the grooves oppose each other so everything locks in and sits right.
4) Fill the Diff Oil + Let It Bleed
Add your diff oil (20K in this example) and make sure you:
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Fill the outer grooves around the edge
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Rotate the gears slowly to move oil everywhere
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Add more oil if it settles and drops in level
Then set it aside to bleed out air bubbles.
Thicker oils take longer. Don’t rush this.
Pro Tip #2: Prep the Gasket So It Doesn’t Slide
While the oil is bleeding, build the top half.
Before installing your gasket, put a thin film of diff oil around the inside mating surface of the gear housing.
Why? Because gaskets love to shift when you flip things over and try to close the diff. That tiny bit of oil helps the gasket “stick” in place and makes the final assembly way less annoying.
Closing the Diff: The “Perfect Fill” Trick
5) Install the Top Sun Gear and Push Out Excess
Once the diff has bled:
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Put the top sun gear in place
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Press it down
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Some oil will squeeze up — wipe it off with a rag
6) Pull Extra Oil From the Pin Slot (Key Step)
This is one of the most important “pro” steps.
Use a small driver/pick and remove the extra oil sitting in the pin slot area before closing the diff.
Why I do it:
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It keeps your fill level identical every time
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Helps prevent issues like overfilling/hydrolock
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Makes your diffs consistent from build to build
7) Install the Spur Gear Housing + Tighten in a Cross Pattern
Now bring the two halves together:
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Line up the pin with the sun gear slot
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Ensure screw holes align
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If needed, “shimmy” the gear slightly so the gasket isn’t blocking a screw hole
Tightening method:
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Use a drill on very low torque just to snug them down
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Then finish by hand in a cross pattern
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Tight, yes — but don’t go full gorilla and strip plastic threads
Once sealed, wipe off any excess grease around the outdrives.
Rebuild Notes: When to Fully Tear Down vs Quick Clean
If you ran one or two packs and the oil weight is clearly wrong, you can sometimes do a quicker clean.
But if you’ve had a full race day or a lot of laps on it, do a full teardown:
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Clean pins, shims, outdrives
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Clean the teeth of all gears
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Re-grease outdrives
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Clean + re-grease O-rings
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Wipe out as much old gunk as possible from the diff cup
That’s how you prevent leaks, wobble, and premature wear.
Pro Tip #3: Label Your Diff Oil (Future You Will Thank You)
At the end, I always mark the diff with a paint pen or silver Sharpie:
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Write the oil weight (ex: 20K → “20”)
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Add a letter: F (front), C (center), R (rear)
It cleans off later if you change oils, but it saves so much time when you rebuild.
Final Thoughts
That’s the full process I use to build an RC gear diff the “pro” way—clean, repeatable, and consistent.
It’s not about being fancy. It’s about building diffs that:
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don’t leak,
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don’t wear out early,
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and feel the same every single time you rebuild them.