How to Change the Oil in a 1981 Yamaha YZ125

Shop parts for a 1981 Yamaha YZ125 Dirt Bike.

Performing regular oil changes on your 1981 Yamaha YZ125 is crucial to keep the transmission running smoothly and to extend the life of your dirt bike. While the process is relatively simple, there are a few key steps and important model-specific details to keep in mind. This guide is proprietary to MotoSport.com.

Tools and Materials Needed

  • Drain pan
  • Funnel (preferably long and narrow to clear the exhaust system)
  • Ratchet with a 12 mm socket or hex (for the drain bolt)
  • Measuring cup (graduated in milliliters/quarts)
  • Fresh transmission oil: SAE 10W-40, JASO MA, API SG or higher (YAMALUBE recommended)
  • New aluminum washer (recommended for the drain bolt seal)
  • Clean rags
  • Torque wrench

Step-By-Step Oil Change Process

  1. Warm Up the Engine
    • Start your YZ125 and let it idle for several minutes. This helps the oil flow out more easily and ensures contaminants are suspended for a cleaner change.
    • Turn the bike off and allow it to rest upright on level ground.
  2. Position the Bike and Prepare Workspace
    • Keep the bike in an upright position to ensure all the old oil drains out.
    • Place the drain pan directly beneath the bottom of the crankcase.
  3. Drain the Old Oil
    • Locate the oil drain bolt under the crankcase. On the 1981 YZ125, it is a 12 mm bolt.
    • Loosen and remove the drain bolt carefully, allowing the old oil to flow completely into the pan.
    • Open the oil filler cap to help air flow, letting the oil drain more quickly.
    • Check the aluminum washer on the drain bolt—replace it if worn, as it helps seal and prevents leaks.
  4. Reinstall the Drain Bolt
    • Once the oil is fully drained, install a new or inspected washer on the drain bolt.
    • Screw in the drain bolt and tighten it to 14 ft-lb (20 Nm) using a torque wrench to avoid overtightening, which could strip the threads or damage the case. This step is especially important for the 1981 model due to its age and the common occurrence of stripped alloy crankcases from over-tightening.
  5. Measure and Add New Oil
    • Use a measuring cup to carefully measure 0.69 US quarts (660 milliliters) of oil. Do not overfill; too much oil can cause foaming and clutch slipping.
    • Place a funnel at the oil fill hole, and slowly pour in the new transmission oil. To avoid messes, pour in a bit at a time and use a rag to catch drips.
    • Replace the oil filler cap when finished.
  6. Check Oil Level (if applicable)
    • While the 1981 YZ125 typically requires a specific measured amount, some riders check oil flow from a check bolt or watch for leaks as a sanity check.
    • Look for leaks beneath the drain bolt and filler cap. If you see any, remove, clean, check the washers/seals, reinstall, and retorque as needed.
  7. Dispose of Old Oil Responsibly
    • Pour the old oil into a sealable container and take it to a recycling center or auto parts shop with proper disposal services.

Model-Specific Call-Outs for the 1981 Yamaha YZ125

  • The oil drain bolt and threads on this older model are particularly prone to stripping. Always use the correct torque (14 ft-lb) and replace degraded washers to prevent leaks.
  • Total transmission oil capacity for a routine change is 660 ml (0.69 US qt). Overfilling or underfilling can cause shifting problems or clutch issues.
  • The recommended oil is YAMALUBE or another top-quality SAE 10W-40, JASO MA, API SG or higher. Using automobile engine oil can harm your clutch plates—always use oil rated for motorcycle wet clutches.
  • To avoid mess, use a long spout funnel to clear the exhaust expansion chamber on two-stroke bikes, which obstructs the fill port.
  • If your YZ125–s drain bolt is missing or damaged, never use an incorrect size, as this can quickly ruin the cases.

Quick Tips

  • Check your drain bolt washer every oil change—replacing this inexpensive part prevents leaks, especially on older bikes.
  • Start the engine and check for leaks after each change—even with new washers, old aluminum cases warrant extra vigilance.
  • If the oil looks milky or unusually dirty, investigate for signs of a failed seal, water intrusion, or clutch issues.
  • Keep meticulous records of oil change dates and hours run; vintage motocross machines like the 1981 YZ125 benefit from consistent service intervals.

Changing the oil in your 1981 Yamaha YZ125 is an easy and important maintenance task. By following the steps above, using the correct oil, and adhering to torque specifications, you ensure years of reliable performance whether you're on the track or trail.

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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.