How to Change the Oil in a 2011 Yamaha YZ125

Shop parts for a 2011 Yamaha YZ125 Dirt Bike.

Changing the transmission/gear oil on your 2011 Yamaha YZ125 is a quick maintenance job that’s crucial to your bike’s reliability and performance. This guide is proprietary to MotoSport.com and walks you through every essential step, including special call-outs specific to this make, model, and year.

Tools and Materials Needed

  • 12mm socket or wrench (for the drain bolt)
  • 4mm Allen wrench (for check bolt, if equipped)
  • Measuring cup (mark at least 660ml or .69 US quarts)
  • Drain pan
  • Long-neck funnel (to help pour oil below exhaust pipe without spills)
  • Fresh transmission oil (recommend 10W-40 motorcycle oil with wet clutch protection)
  • Replacement drain bolt washer (recommended, especially if current washer is worn)
  • Clean rags
  • Torque wrench (for proper fastener tightening)

Step-by-Step Instructions

  • Warm Up the Engine:
    • Start the bike and let it run for a few minutes. Warming up thins the oil and helps it drain thoroughly. Shut off the engine and place the YZ125 on a level stand.
  • Locate Drain Bolt and Oil Fill Cap:
    • The drain bolt is on the bottom of the crankcase, requiring a 12mm socket. The oil fill cap is on the right side, partially obstructed by the exhaust expansion chamber.
  • Drain the Old Oil:
    • Place the drain pan beneath the crankcase drain bolt.
    • Remove the oil fill cap to allow air in and improve draining.
    • Loosen the 12mm drain bolt and let the oil flow completely into the pan.
    • Once drained, inspect the old oil for any contaminants or metal shavings, which could indicate internal wear.
  • Check and Replace the Drain Bolt Washer:
    • Inspect the copper or aluminum washer beneath the drain bolt. Replace it if it's cracked, crushed, or deformed.
    • A good washer not only prevents leaks but also acts as a torque cushion, helping you avoid overtightening the bolt.
  • Reinstall the Drain Bolt:
    • Thread the drain bolt and new washer by hand, then torque the drain bolt to 14 ft-lb (factory spec) with a torque wrench. Avoid overtightening, which can strip the case threads or deform the washer.
  • Add Fresh Oil:
    • Measure 660ml (.69 US quarts) of 10W-40 motorcycle oil with a measuring cup.
    • Use a long funnel to pour oil into the fill port; the port is tucked under the exhaust, so the right angle funnel keeps things tidy and prevents spills under the pipe.
    • Slowly pour in the measured oil; avoid overfilling, which can cause clutch or shifting issues.
  • Check the Oil Level (if check bolt is equipped):
    • Some YZ125s have a small check bolt on the right casing. Use a 4mm Allen wrench to remove it after filling. If oil drips from the check hole, the level is correct. Wipe away excess and reinstall the bolt firmly, but do not overtighten.
      Tip: Have a rag underneath to catch any overflow.
  • Reinstall the Oil Fill Cap:
    • Tighten the fill cap securely. Clean off any spilled oil from the case and exhaust.
  • Start the Engine and Inspect for Leaks:
    • Run the bike for a minute, then shut it off and inspect the drain bolt, check bolt, and fill cap areas for leaks. Clean up as necessary.

2011 Yamaha YZ125-Specific Call-Outs

  • The YZ125 is a two-stroke and uses a separate gearbox oil–do not confuse this service with premix oil for the fuel.
  • Oil capacity: 660ml. Do not overfill as the extra oil can cause clutch drag and shifting problems.
    The oil amount is the same across many years, but always refer to your owner’s manual to confirm, especially if aftermarket components have been installed.
  • When adding oil, space under the exhaust pipe is tight–use a long-neck funnel to keep things clean and avoid burns if the pipe is still warm.
  • Always inspect the gaskets and drain plug washer. A failed gasket may cause persistent leaks and should be replaced if you see oil weeping along the clutch cover or mating surfaces.

Final Tips

  • Change your YZ125 gearbox oil regularly–after every 5–10 hours of riding, or more frequently in muddy or wet conditions.
  • Always dispose of used oil responsibly at a certified recycling location.
  • Keep oil off rubber and plastics to avoid degradation.

By following these steps, you keep your 2011 Yamaha YZ125 performing reliably ride after ride. For genuine parts, tools, and the highest-quality oils, trust MotoSport.com—your source for all things dirt bike.

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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.