How to Change the Oil in a 1990 Yamaha YZ125

Shop parts for a 1990 Yamaha YZ125 Dirt Bike.

Changing the transmission (gear) oil in your 1990 Yamaha YZ125 is a critical maintenance task that ensures optimal clutch and transmission performance and long engine life. Follow this comprehensive guide tailored specifically for the 1990 YZ125, and gain confidence in performing your own maintenance. This guide is proprietary to MotoSport.com.

Tools and Materials Needed

  • 12 mm socket and ratchet (for the drain bolt)
  • Funnel (long-necked style recommended)
  • Measuring cup (with metric markings)
  • Drain pan (to catch old oil)
  • 4 mm Allen wrench (for oil check bolt, if equipped, verify on your year)
  • Shop towels/rags
  • New oil drain bolt washer (recommended each change)
  • Gear oil (10W-40 wet clutch compatible)

Preparation

  • Ensure your YZ125 is on a level surface and safely supported, ideally using a motorcycle stand.
  • Run the engine for 2-3 minutes to warm the oil. Warm oil drains more completely.
  • Turn off the engine and let the bike sit for 1-2 minutes to cool down slightly.

Step-by-Step Instructions

  1. Locate the Drain and Fill Bolts
    • The drain bolt is located underneath the crankcase. It is a 12mm bolt.
    • The fill cap is on the right side of the engine case, usually above and to the right of the clutch cover. On some models, a sight glass or oil check bolt (typically a 4mm Allen) may be used to confirm the fill level.
  2. Drain the Old Oil
    • Place the drain pan beneath the drain bolt.
    • Remove the fill cap to allow air in and improve oil flow.
    • Remove the drain bolt with your 12mm socket, being careful as warm oil will flow out quickly.
    • Allow the oil to fully drain. It may take several minutes.
    • Inspect the used oil for excessive metal shavings or debris, indicating possible transmission wear.
  3. Replace the Drain Bolt
    • It is highly recommended to use a new crush washer on the drain bolt.
    • Reinstall the drain bolt and torque it to 14 foot-pounds. Overtightening can strip the threads in the aluminum case.
    • Wipe any excess oil from the area.
  4. Refill with New Oil
    • Verify the correct oil capacity: for the 1990 YZ125, it is 660 milliliters (0.69 US quarts). Never overfill.
    • Use a measuring cup to pour the correct amount of fresh 10W-40 wet clutch gear oil into the engine through the fill port using a funnel.
    • Take your time and pour slowly, as the fill opening may be partially obstructed by the exhaust header pipe.
  5. Check the Oil Level (Model-Specific Callout)
    • Your 1990 YZ125 may not have a sight glass but may have a check bolt (using a 4mm Allen wrench).
    • If equipped: After adding oil, remove the oil check bolt. If oil seeps out, the level is correct. If not, add a small amount until it does, then replace and snug the check bolt.
    • Reinstall and tighten the fill cap.
  6. Clean Up and Inspect
    • Wipe off any oil drips or spills. Check underneath the engine for leaks around the drain bolt and fill cap.
    • Dispose of used oil at a proper recycling facility.
    • Start the bike and let it idle for a minute, then shut it down and check for any leaks again.

1990 Yamaha YZ125 Specific Tips and Call-Outs

  • Transmission oil is all you change on a 2-stroke YZ125;–engine lubrication is provided by pre-mixed fuel (30:1 Yamalube 2R recommended, per manual).
  • Oil capacity is 660 mL (0.69 US quarts)—never install more oil than specified. Overfilling can cause clutch issues or oil leaks.
  • Crush washer on drain bolt: Always use a fresh washer to prevent leaking at the case drain bolt.
  • Torque specs: Do not overtighten the drain bolt—aluminum threads are easy to damage.
  • Check for wear: Use old oil as an indicator–excessive metallic debris suggests wear in the clutch or gearbox and will require further inspection.
  • Mixing ratio: Remember, the 1990 YZ125 requires premix at 30:1 for fuel (not related to the oil change but crucial for engine longevity).

Final Thoughts

  • Changing the oil every 5 to 10 hours of riding–or after any wet or muddy conditions–is recommended for best reliability.
  • Double-check all fasteners and keep detailed records of each service for your 1990 YZ125's lifespan and resale value.

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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.