How to Shim Traxxas 4×4 Differentials

🛠 What Is Differential Shimming?

Shimming refers to placing thin washers (shims) between the differential gears and the case or bearings to tighten the gear mesh and reduce play or slop. It's a small tweak that makes a big difference in drivetrain performance and durability.

In the Traxxas Slash 4x4, differential shims are typically used:

• Behind the ring gear (to adjust how it meshes with the pinion gear).
• Behind the satellite (spider) gears inside the diff case.
• At the outdrive cups, to reduce side-to-side play.

⚠️ Why Do You Need to Shim Differentials?

Without proper shimming, you can face:

❌ Common Issues from Poor Gear Mesh:

• Stripped ring or pinion gears
• Premature gear wear
• "Clicking" noises under throttle
• Excessive drivetrain slop
• Heat buildup from poor alignment

These issues are common in high-power setups or if you're bashing hard, especially with metal diffs or aluminum housings where tolerances differ from stock.

🧰 How to Shim a Slash 4x4 Differential

Step-by-Step:

1. Remove the differential from the front or rear bulkhead.
2. Disassemble the differential housing (if you’re working inside the diff).
3. Clean and inspect the gears and housing.
4. Test the mesh between the ring gear and pinion (before adding grease).
   • If you feel side-to-side play or backlash, you need to shim.
5. Add shims:
   • Behind the ring gear bearing (usually on the input shaft side) to move the ring gear closer to the pinion.
   • Start with 0.1mm or 0.2mm shims, stack as needed.
6. Reassemble and test gear mesh by slowly turning the pinion.
   • Should feel smooth and tight, but not grinding.
7. Add grease or oil, depending on your setup.
8. Repeat for other diffs, if needed.

🧪 Pro Tip: When to Shim

• New metal differentials: Always shim. Tolerances can vary.
• Plastic housings: Often need at least a light shim.
• Aluminum diff housings: Tighter fit, but check anyway.
• After a gear failure: Shim to prevent recurrence.

🔩 What Shims Do You Use?

You’ll typically want:

• 8x10x0.1mm or 0.2mm shims (for ring gear bearings)
• 10x12x0.2mm (for spider gears inside diff)
• Good brands: Mugen, Kyosho, or Traxxas OEM

🎯 Summary

Shimming your Slash 4x4 differentials:

• Improves durability
• Prevents gear failures
• Optimizes drivetrain performance

It's a cheap and effective upgrade, especially if you're running powerful brushless motors, metal upgrades, or just want your Slash, or other models, to last longer and run smoother.