How to Change the Oil in a 2019 Yamaha YZ125

Shop parts for a 2019 Yamaha YZ125 Dirt Bike.

This proprietary MotoSport.com guide will walk you through the process of changing the oil (specifically, the transmission/gear oil) on a 2019 Yamaha YZ125. The YZ125 is a two-stroke dirt bike, so "oil change" refers to the transmission or gear oil, not engine oil as in a four-stroke. Performing this service keeps your bike shifting smoothly and prolongs internal component life.

Tools & Supplies Needed

  • 12mm socket or wrench (for drain bolt)
  • 4mm Allen wrench (for oil check bolt–optional but recommended)
  • Ratchet
  • Drain pan
  • Measuring cup (marked in milliliters and US quarts)
  • Long funnel (for pouring oil under the exhaust pipe)
  • Rags or shop towels
  • New crush washer for drain bolt (recommended)
  • Screwdriver (for body panel removal, if needed)
  • High-quality 10W-40 motorcycle transmission oil (See manual; 660ml or 0.69 US quarts required)

Preparation

  • Start the engine and let it warm up for a few minutes to allow the oil to flow more freely.
  • Turn the engine off and let the bike cool for about 5 minutes.
  • Place the bike on a level surface using a bike stand to keep it upright and stable.

Steps to Change the Oil

  1. Locate the Drain Bolt:
    • Find the 12mm drain bolt under the engine crankcase.
  2. Remove the Oil Fill Cap:
    • The fill cap is located on the right side, near the top of the clutch cover. Removing it before draining helps prevent vacuum and ensures full drainage.
  3. Position the Oil Drain Pan:
    • Place the pan directly under the drain bolt to catch all old oil.
  4. Remove the Drain Bolt:
    • Carefully use your 12mm socket to remove the drain bolt and washer. Oil will begin to drain out rapidly–watch for hot oil if you've just run the engine.
    • Inspect and replace the crush washer if it is squashed or damaged.
  5. Let the Oil Drain Completely:
    • Wait several minutes for all oil to drain–lean the bike side to side gently to remove any trapped oil.
  6. Reinstall the Drain Bolt:
    • Install the cleaned or new washer on the drain bolt, then thread the bolt back in by hand to avoid cross-threading.
    • Tighten with your 12mm socket to 14 ft-lbs (19 Nm).
  7. Measure Fresh Oil:
    • Pour 660ml (0.69 US quarts) of recommended transmission oil (SAE 10W-40 motorcycle-specific oil) into a measuring cup.
  8. Add New Oil:
    • Use a long funnel (the fill cap sits under the exhaust pipe and can be hard to access).
    • Pour the measured oil slowly into the transmission.
  9. Optional: Check Oil Level via Level Bolt:
    • The YZ125 has a 4mm Allen head check/overflow bolt located on the right side cover.
      • After adding about half the oil, remove the level bolt and continue filling until oil just begins to weep from the opening. Replace and tighten the bolt.
      • This ensures you have not overfilled or underfilled the transmission.
  10. Reinstall the Fill Cap:
    • Ensure the cap is clean and free of debris, then tighten it down securely.
  11. Clean Up:
    • Wipe down any oil drips. Store used oil safely for recycling.

2019 Yamaha YZ125 Model-Specific Call-Outs

  • Oil capacity: 660ml (0.69 US quarts) is model-specific and must be measured accurately for proper clutch operation.
  • Oil fill location: The fill plug is partially obscured by the exhaust expansion chamber. A long funnel is required to avoid spillage.
  • Drain bolt torque spec: Tighten the drain bolt to 14 ft-lbs (19 Nm)–do not overtighten to avoid damaging threads on the crankcase.
  • Check/overflow bolt: Use the 4mm Allen check bolt to verify proper level.
  • Recommended oil: Use a high-quality 10W-40, wet-clutch compatible motorcycle oil (not automotive oil).

Additional Tips

  • Change the oil every 5-10 hours of riding, or after any race, muddy, or wet conditions.
  • Store your service manual nearby for torque specs and maintenance intervals.
  • Inspect for metal shavings or clutch debris in the old oil. Excessive contamination may indicate internal wear.

By following these steps and using the correct specification oil, you can keep your 2019 Yamaha YZ125 operating at peak performance. For all your OEM and aftermarket parts, tools, and oil needs, trust MotoSport.com — your authority in motocross maintenance.

Related Shopping Categories

Shop Motor Oil for a 2019 Yamaha YZ125 Dirt Bike.

Shop Oil Filters for a 2019 Yamaha YZ125 Dirt Bike.

Shop Oil Drain Pans for a 2019 Yamaha YZ125 Dirt Bike.

Shop Tools for a 2019 Yamaha YZ125 Dirt Bike.

Shop Oil Fillers and Drain Plugs for a 2019 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.