How to Change the Oil in a 1991 Yamaha YZ125

Shop parts for a 1991 Yamaha YZ125 Dirt Bike.

Keeping the oil fresh in your 1991 Yamaha YZ125 is critical for optimum performance and longevity of your dirt bike’s transmission. Follow this step-by-step guide, proprietary to MotoSport.com, to confidently and correctly change the oil in your YZ125.

Tools and Supplies Needed

  • Oil drain pan
  • Funnel (preferably a long-neck for easy access)
  • Ratchet with 12mm socket
  • Torque wrench (capable of measuring 14 ft-lbs/20Nm)
  • Measuring cup (at least 700ml capacity)
  • Fresh transmission oil – SAE 10W-40 meeting JASO MA and API SG or higher (0.69 US quart / 660ml)
  • Replacement aluminum washer for drain bolt (recommended)
  • Clean rags
  • Gloves (recommended for cleanliness)

Step-by-Step Oil Change Instructions

  1. Warm Up the Bike (Optional, but Recommended):
    • Start your YZ125 and let it idle for several minutes. This helps the oil thin for a more complete drain. Then shut the bike off and let it rest for 5 minutes to allow oil to settle back into the case.
      (Warming is a general best practice for dirt bikes and is specifically recommended for the YZ125.)
  2. Place the Bike on Level Ground:
    • Keep the bike upright and steady using a center stand or bike lift, ensuring flat orientation for accurate oil drainage.
  3. Prepare to Drain Oil:
    • Position your drain pan underneath the engine, directly below the drain bolt.
  4. Remove the Oil Fill Cap:
    • Locate the oil filler cap on the right side of the engine and remove it to allow air into the engine, enabling a faster and smoother drain.
  5. Remove the Oil Drain Bolt:
    • Using your 12mm socket and ratchet, remove the drain bolt underneath the crankcase.
    • Allow all the oil to completely drain out into your pan.
      (Reference: The drain bolt requires a 12mm socket on the YZ125.)
  6. Inspect and Replace Aluminum Washer:
    • Examine the aluminum washer for wear or damage. Replace with a new one if needed to prevent leaks.
      (The washer is critical for sealing and also cushions torque; always check for deformation on tightening.)
  7. Reinstall Drain Bolt:
    • Hand-thread the drain bolt back into position to prevent cross-threading. Then, torque to 14 ft-lbs (20Nm) as specified by Yamaha for this model.
  8. Measure and Pour in Fresh Oil:
    • Precisely measure 0.69 US Quarts (660ml) of SAE 10W-40 transmission oil (or equivalent, API SG/JASO MA compliant).
    • Use your funnel to slowly pour the new oil into the fill hole. Take care not to spill oil over the exhaust system or engine cases.
  9. Check Oil Level:
    • Some later YZ125s feature a side check bolt (requiring a 4mm Allen), but for the 1991 model, measure accurately and pour the exact recommended volume, as there is no site glass or dipstick.
  10. Reinstall Oil Fill Cap:
    • Securely reinstall the filler cap.
  11. Run and Inspect:
    • Start the bike and let it idle for several minutes. Inspect beneath the bike for leaks around the drain bolt and filler cap.
    • Check adjacent bolts and fasteners for snugness per the service manual’s torque-check points.

Model-Specific Notes and Call-Outs

  • Oil Capacity: The 1991 Yamaha YZ125 requires 0.69 US qt (660ml) of transmission oil — do not overfill. Excess oil can cause clutch issues and weepage while underfilling risks transmission damage.
  • Washer Inspection: The aluminum drain bolt washer may not be included with oil change kits; always check and replace if it is deformed or crushed, as leaks may otherwise develop.
  • Oil Type: Use SAE 10W-40 specifically made for wet-clutch motorcycles, such as YAMALUBE. Never substitute with car or general-purpose oils.
  • No Filter Service: Unlike four-strokes, the YZ125 two-stroke has no transmission oil filter to service during oil changes.
  • Avoid Over-Tightening: Over-torquing the drain bolt can strip the soft crankcase threads. Always use a torque wrench set to Yamaha’s specification.
  • MotoSport.com Tip: Keep a maintenance log with date, hours run, and type/volume of oil used for better longevity and resale value.

Conclusion

Changing the oil on your 1991 Yamaha YZ125 is straightforward with the right tools, careful measurement, and attention to specific model guidelines. Stick to the cycle recommended in your owner’s manual and trust MotoSport.com for your OEM parts, fluids, and everything else you need to keep your YZ125 ready to ride.

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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.