How to Change the Oil in a 1980 Yamaha YZ125

Shop parts for a 1980 Yamaha YZ125 Dirt Bike.

Changing the oil in your 1980 Yamaha YZ125 is a fundamental maintenance task that can extend your dirt bike’s life and prevent major mechanical issues. This MotoSport.com how-to blog will guide you step-by-step through the process to ensure a smooth, effective, and clean oil change specific to your classic two-stroke.

Tools and Supplies Needed

  • Oil drain pan
  • Funnel
  • Ratcheting wrench with 12mm socket
  • Measuring cup or graduated container
  • 4mm Allen wrench (for oil level check bolt)
  • New transmission/gear oil (10W-40 wet clutch specific is recommended)
  • Clean rag
  • Replacement washer for drain bolt (highly recommended if washer appears worn or damaged)
  • Torque wrench (optional but ideal for correct bolt spec)

Preparation

  • Ensure the bike is on level ground before starting. A stand is helpful for stability.
  • Warm up the engine for a few minutes. This thins the oil, allowing it to drain more thoroughly.

Step-by-Step Oil Change Procedure

  1. Locate the oil drain bolt:
    • On the 1980 YZ125, this is typically found at the bottom of the crankcase. Use your ratcheting wrench and 12mm socket to access it.
  2. Position your drain pan directly underneath the drain bolt to catch the used oil.
  3. Remove the oil drain bolt carefully. Allow all oil to drain out—this can take 5–15 minutes. Be cautious; the oil may be hot if the bike was just run.
  4. Inspect and replace the crush washer:
    • This small washer provides a tight seal. If damaged or flattened, replace it with a new one to avoid future leaks.
  5. Reinstall and tighten the drain bolt:
    • If you have a torque wrench, tighten the bolt to 14 ft-lbs as specified for Yamaha YZ125 models. The washer should compress slightly; this indicates a proper seal.
  6. Remove the oil fill cap:
    • This is normally on the top or side of the engine case.
  7. Measure and add new oil:
    • For the 1980 YZ125, measure out approximately 660 milliliters of high-quality 10W-40 motorcycle oil appropriate for wet clutches. Use a funnel to pour slowly into the oil fill hole.
  8. Check oil level using the oil level bolt:
    • With a 4mm Allen wrench, remove the small oil check bolt on the right side of the clutch cover. Oil should just start to seep out when the correct amount is present. If it doesn’t, add small amounts of oil at a time until it does, then reinstall and snug up the check bolt.
  9. Replace the oil fill cap securely.
  10. Start the engine and let it idle for a few minutes. Re-check oil level (via check bolt or sight window if equipped) to ensure it’s at the proper level. Add oil if needed.
  11. Dispose of the old oil properly at a recycling center or hazardous waste facility.

Specific Call-Outs for the 1980 Yamaha YZ125

  • This is a two-stroke motorcycle; engine lubrication is handled by pre-mix fuel (usually at a 30:1 or 15:1 gas/oil ratio, depending on oil type and riding conditions). The oil change described here is strictly for the transmission/gearbox, not the main engine.
  • The crush washer on the drain bolt is important to prevent leaks. Replace as needed.
  • Inspect the old oil for metal shavings or a milky color (which could indicate internal problems such as bearing wear or a water intrusion).
  • Over-tightening can strip the aluminum case threads—always tighten to spec.
  • Use only wet-clutch compatible oil; automotive engine oils may cause clutch slippage.
  • If your bike has been sitting for a long period, it’s wise to repeat the oil change after a couple of short rides to help flush out any residual contaminants.
  • If the oil level check bolt is rounded, replace it immediately to avoid future access problems.

Final Notes

  • Keep records of each oil change with mileage/hours for best long-term maintenance.
  • If in doubt about procedure or if you encounter damaged hardware or oil contamination, consult a qualified mechanic.

Changing your YZ125’s gearbox oil regularly—every 10 hours of riding or after severe use—will help keep your transmission and clutch in top condition for years to come.

Related Shopping Categories

Shop Motor Oil for a 1980 Yamaha YZ125 Dirt Bike.

Shop Oil Filters for a 1980 Yamaha YZ125 Dirt Bike.

Shop Oil Drain Pans for a 1980 Yamaha YZ125 Dirt Bike.

Shop Tools for a 1980 Yamaha YZ125 Dirt Bike.

Shop Oil Fillers and Drain Plugs for a 1980 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.