How to Change the Oil in a 2020 Yamaha YZ125X

Shop parts for a 2020 Yamaha YZ125X Dirt Bike.

Changing the oil in your 2020 Yamaha YZ125X is a fundamental maintenance task that keeps your dirt bike running smoothly and extends the life of its transmission. While the YZ125X is a two-stroke, which means it relies on pre-mix oil and gasoline for engine lubrication, it does require regular transmission (gearbox) oil changes. This guide covers the tools, supplies, step-by-step process, and specific tips for the 2020 YZ125X.

Tools and Materials Needed

  • Oil drain pan
  • Ratchet with 12mm socket (for drain bolt)
  • 4mm Allen wrench (for the oil check bolt)
  • Measuring cup (with clear metric markings for precise fill)
  • Funnel (long neck recommended for easy access)
  • Shop towels or rags
  • Replacement oil: Use quality 2-stroke transmission oil. Many owners use 10W-40 motorcycle oil designed for wet clutches.
  • Replacement drain bolt washer (recommended; not always included with oil kit)

Preparation Steps

  • Warm up the engine for a few minutes to allow oil to flow out more easily. Do not let the engine overheat—just a few minutes at idle is enough.
  • Position the bike upright and level, ideally on a stand, for an accurate fill and drain.

Step-by-Step Procedure

  • Locate the Drain Bolt and Oil Fill Cap
    • The drain bolt is found under the engine crankcase. On the YZ125X, it is a 12mm hex bolt.
    • The oil fill cap is on the right side of the engine, above the clutch cover. Remove the fill cap before draining to speed up the process and reduce vacuum.
  • Drain the Oil
    • Place the drain pan directly under the drain bolt.
    • Use a 12mm socket to remove the drain bolt carefully.
    • Let the oil completely drain out—this should only take a few minutes if the engine was warmed up.
    • Inspect the old oil for signs of metal shavings or unusual discoloration.
  • Replace the Drain Bolt and Washer
    • Install a new washer on the drain bolt if available. The washer serves as a seal and a torque cushion to prevent overtightening.
    • Hand-tighten the bolt, then use the ratchet to finish–do not overtighten. Recommended torque is 14 ft-lbs for the 2020 YZ125X drain bolt.
  • Refill with Fresh Oil
    • Measure out 660 mL (0.69 US quarts) of gear oil using your measuring cup.
    • Using the long funnel, pour the oil slowly into the oil fill port. Take care not to spill oil on the exhaust or engine.
  • Check the Oil Level
    • Some YZ125X models, including the 2020 model, have a small oil level check bolt located on the right crankcase. Use a 4mm Allen wrench to remove this bolt.
    • The bike must be upright and level.
    • If oil begins to seep out of the check bolt opening, the level is correct—reinstall the bolt and torque gently. If not, add oil in small increments until it does.
  • Reinstall the Fill Cap and Clean Up
    • Install the oil fill cap and wipe up any spills.
    • Dispose of the used oil responsibly at a recycling or collection center.

2020 Yamaha YZ125X Model-Specific Callouts

  • Oil Spec: The 2020 YZ125X requires 660 mL (0.69 US quarts) of gear/transmission oil. Yamaha specifies 10W-40 motorcycle oil suitable for wet clutches, but always check your manual for updates.
  • Drain Bolt Torque: Do not exceed 14 ft-lbs to avoid stripping the crankcase threads.
  • Drain Bolt Washer: It’s highly recommended to replace the drain bolt washer each time to ensure a proper seal and avoid leaks.
  • Check Bolt: If you notice persistent oil leakage after replacing the washer and tightening correctly, inspect the clutch cover gasket for cracks or wear, as leaks are common with age or after many maintenance cycles.
  • 2-Stroke Oil Note: This procedure covers the gearbox oil only. The engine itself is lubricated by the oil mixed with the fuel. Always use high-quality pre-mix at Yamaha’s recommended ratio (typically 30:1 with Yamalube 2-R for this model) as per your owner’s manual.

Final Tips

  • Check oil level after each ride–especially if you ride in wet or muddy conditions, or suspect a possible leak.
  • Keep replacement washers in your toolbox for fast, leak-free changes.
  • Always use a stand to level the bike for accurate readings and proper filling during maintenance.
  • Consult your Yamaha Owner’s Service Manual for torque specs and additional details if you have questions or run into issues.

Proper oil maintenance ensures your 2020 Yamaha YZ125X remains ready for every ride. MotoSport.com has all the oils, parts, and tools you need to keep your bike performing at its best.

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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.