How to Change the Oil in a 2020 Yamaha YZ125

Shop parts for a 2020 Yamaha YZ125 Dirt Bike.

Changing the transmission (gear) oil in your 2020 Yamaha YZ125 is a crucial part of regular maintenance that ensures longevity and optimal performance for your dirt bike. This guide, proprietary to MotoSport.com, will walk you through every step, highlight the exact tools you need, and provide YZ125-specific callouts you should know. Whether you're a weekend racer or trail enthusiast, following these instructions will keep your YZ125 running strong.

Tools and Materials Needed

  • 12 mm socket or ratchet (for drain bolt)
  • 4 mm Allen wrench (for oil check bolt)
  • Measuring cup (for precise oil measurement)
  • Drain pan
  • Long funnel (to reach under the exhaust pipe)
  • Rag or shop towels (for spills)
  • New gear/transmission oil (10w40 wet clutch compatible or equivalent)
  • New drain bolt washer (recommended, but not always included in kits)
  • Torque wrench (for precise tightening)

Steps to Change the Oil

  • Warm Up the Engine
    • Start your bike and let it idle for a few minutes. Warming up ensures the oil flows out smoothly and carries debris with it.
  • Stabilize the Motorcycle
    • Place your YZ125 on a flat, level surface using a bike stand if possible.
  • Remove the Oil Fill Cap
    • The fill cap is located above the transmission case, partially beneath the exhaust. Removing it helps oil drain more efficiently.
      YZ125 note: Because the fill cap sits under the pipe, a long funnel is recommended for refilling oil

      .
  • Drain the Old Oil
    • Place the drain pan under the crankcase drain bolt. Using a 12 mm socket, loosen and remove the drain bolt.
    • Allow all the oil to drain completely. This may take several minutes. Inspect the oil for unusual debris (signaling clutch or gear wear) and wipe the magnetic tip of the bolt clean if present.
    • YZ125-specific: The drain bolt is 12 mm and should be torqued upon reinstallation to 14 ft-lbs

      .
  • (Optional but Recommended) Replace the Crush Washer
    • If available, replace the drain bolt's crush washer to ensure a proper seal and prevent leaks

      .
    • Hand thread the bolt, then tighten snugly with a torque wrench (14 ft-lbs).
  • Refill with New Oil
    • Measure exactly 0.69 US quarts (660 ml) of oil – this is critical for the YZ125's transmission. Overfilling or underfilling can affect clutch feel and transmission health

      .
    • Use a long funnel to avoid spills, as the fill port is partially obstructed by the exhaust system.
  • Check Oil Level (YZ125-Specific Step)
    • After adding oil, you can confirm the fill by removing the oil level check bolt (use a 4 mm Allen wrench). This bolt is located on the side cover of the transmission case.
    • With the bike upright and level, remove the check bolt. Oil should just begin to trickle out; if not, carefully add more until it does. If oil pours out, let the excess drain until it stops trickling, then reinstall the bolt

      .
  • Reinstall the Fill Cap
    • Tighten the oil fill cap securely by hand.
  • Clean Up and Inspect
    • Wipe down any oil spills and check around both bolts for leaks once the bike has rested a few minutes.
    • Dispose of the used oil in accordance with local regulations.

Important YZ125 Call-Outs and Tips

  • Oil Type: Use only oil formulated for wet clutch systems. 10w40 motorcycle gear oil or synthetic 2-stroke gear oil recommended by Yamaha. Do not use standard automotive oil

    .
  • Exact Oil Capacity: 0.69 US quarts (660 ml). Overfilling may cause pressure build-up and clutch issues

    .
  • Crush Washer: Always check and replace the washer if deformed. It prevents leaks and ensures proper torque; a new washer also acts as a torque cushion

    .
  • Tightening Torque: The drain bolt requires 14 ft-lbs. Overtightening can damage threads; undertightening risks leaks

    .
  • Check for Leaks: Inspect side cover gaskets for drips. Replace if needed; Yamaha gaskets are commonly back-ordered, so order ahead

    .
  • Oil Change Interval: For racing or heavy riding, change oil every 5–10 hours of use. For recreational use, follow the manufacturer's guidelines but err on the side of caution.

Conclusion

By following these steps, you'll ensure the gearbox and clutch in your YZ125 remain protected and perform at their best. This process is simple but crucial for the life of your bike. Always use the correct oil, proper tools, and pay attention to model-specific details as outlined above. Happy riding, and trust MotoSport.com for all your dirt bike parts and maintenance needs.

Related Shopping Categories

Shop Motor Oil for a 2020 Yamaha YZ125 Dirt Bike.

Shop Oil Filters for a 2020 Yamaha YZ125 Dirt Bike.

Shop Oil Drain Pans for a 2020 Yamaha YZ125 Dirt Bike.

Shop Tools for a 2020 Yamaha YZ125 Dirt Bike.

Shop Oil Fillers and Drain Plugs for a 2020 Yamaha YZ125 Dirt Bike.

Disclaimer: This guide is provided for general informational purposes only and is not intended to replace your motorcycle's official owner's manual. Always refer to your manufacturer's documentation for model-specific instructions, torque specifications, safety procedures, and maintenance requirements. If you are unsure or inexperienced, consider seeking assistance from a qualified mechanic or technician.