How to change the oil in a 2018 Yamaha YZ125 Dirt Bike

How to Change the Oil in a 2018 Yamaha YZ125 – MotoSport.com

Shop parts for a 2018 Yamaha YZ125 Dirt Bike.

Tools and Materials Needed

• 12mm socket and ratchet
• 4mm Allen wrench (for oil level check bolt, if desired)
• Oil drain pan
• Funnel (preferably long and narrow to fit beneath the exhaust pipe)
• Measuring cup (with clear milliliter/ounce marks)
• Paper towels or rags
• New oil drain bolt washer (recommended for each oil change)
• Recommended oil: SAE 10W-40, JASO MA, API SG or higher

Step-By-Step Instructions

1. Warm Up the Engine
   • Start your bike and let it run for a few minutes. This warms up the oil, allowing it to drain more thoroughly from the transmission.
     Tip: Do not skip this step as warm oil carries out more contaminants and drains faster.
2. Stabilize the Bike
   • Place your YZ125 on a stand or ensure it is completely upright and on level ground.
3. Prepare for Drainage
   • Place a drain pan directly under the 12mm drain bolt located under the crankcase.
4. Remove the Drain Bolt
   • Using your 12mm socket, remove the drain bolt and allow the old oil to drain completely into the pan.
     Model-Specific Callout: The 2018 YZ125 shares its basic engine and transmission design with models from several years before and after, including the location and size of the drain bolt.
5. Optional: Check for Debris
   • Inspect the old oil for any metallic debris, which can indicate excessive wear inside the transmission.
6. Replace the Drain Bolt and Washer
   • Install a new crush washer onto the drain bolt. Thread the bolt back in by hand to avoid cross-threading.
   • Tighten the bolt to 14 ft-lbs (20 Nm) of torque. The crush washer helps seal the bolt and prevents overtightening, which can strip threads.
7. Remove the Oil Filler Cap
   • The filler cap is usually located on the right side, just under the exhaust pipe. Remove it to prepare for filling the transmission.
8. Add New Oil
   • Measure out precisely 0.69 US quarts (660 milliliters) of fresh oil. Using a long funnel, slowly add the oil to avoid spills.
     Pro Tip: Use a high-quality 10W-40 motorcycle oil with JASO MA certification for optimal clutch performance.
9. Optional: Check Oil Level with Level Check Bolt
   • Some riders prefer to pull the small check bolt (using a 4mm Allen wrench) on the right crankcase cover after adding oil. A slow drip indicates the correct oil level. If oil pours out, wait for it to stop dripping and re-install the bolt.
10. Replace Filler Cap
    • Install the filler cap securely by hand.
11. Clean Up
    • Wipe down any oil drips or spills with a rag. Dispose of old oil properly at a local recycling center.
12. Final Inspection
    • Briefly run the engine and check for leaks at the drain bolt and side case. If you find any, shut the engine down and fix as needed.

Specific Call-Outs for the 2018 Yamaha YZ125

• This bike requires transmission/gear oil only—do not confuse this with engine oil changes for four-strokes.
• Always use a new crush washer: Yamaha recommends changing this with every oil change for a consistent seal.
• Oil capacity is exactly 0.69 US quarts (660 mL)–do not overfill.
• The oil fill port is positioned under the exhaust pipe, so a long, narrow funnel is advised to avoid spills and mess.
• Check the transmission oil level regularly; leaks from crankcase or cover gaskets are common on well-used bikes and should be dealt with promptly.
• Recommended oil types: YAMALUBE 10W-40, or a high-quality synthetic motorcycle oil with wet clutch compatibility (JASO MA/MA2, API SG or higher).

When to Change the Oil

• After the initial break-in period on a new bike or rebuilt engine
• Every 5-10 hours of riding, or after every race session/extensive use (whichever comes first)
• If the oil appears milky or contaminated after water crossings or severe mud riding

Final Tips

• Always double-check the torque on the drain bolt. Overtightening can result in costly case repairs.
• Properly dispose of old oil at a certified recycling facility—never pour it down drains or into the ground.
• Keeping your bike's transmission oil fresh ensures reliable clutch operation and protects your investment in high-performance Yamaha parts.

For OEM and aftermarket YZ125 parts, as well as more expert maintenance tips, visit MotoSport.com and keep your ride running strong.

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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.