How to Change the Oil in a 2021 Yamaha YZ125

Shop parts for a 2021 Yamaha YZ125 Dirt Bike.

Changing the oil on your 2021 Yamaha YZ125 is a crucial element of maintaining optimal performance and ensuring the longevity of your engine. This guide, proprietary to MotoSport.com, will detail the tools required, every step of the process, and call out specific tips for the YZ125 model year.

Tools and Supplies Needed

  • Drain pan
  • Long funnel (to reach under the exhaust)
  • Ratchet with 12mm socket
  • 4 mm Allen wrench (for the oil level check bolt)
  • Measuring cup
  • New crush washer (recommended for the drain bolt)
  • Shop rag
  • Fresh gear oil (SAE 10W40, wet clutch compatible, 0.69 US qt/660 ml)

Steps to Change the Oil

  • Preparation
    Place your YZ125 on level ground using a stand. It helps to start and let the motorcycle idle for a few minutes to warm up the oil, making it easier to drain.
  • Locate the Drain Plug
    The oil drain bolt is on the lower side of the crankcase. Position the drain pan directly underneath to catch all old oil.
  • Remove the Drain Plug
    Using the 12mm socket, loosen and remove the drain bolt. Inspect the crush washer–replace with a new one if damaged or flattened.
    Let all the oil drain out completely before proceeding.
  • Reinstall the Drain Plug
    After the oil is completely drained, clean the drain bolt and area, fit the (new) crush washer, and hand-thread the bolt back in. Tighten the bolt using the wrench to 14 ft-lbs of torque. Do not overtighten; proper torque is important for preventing leaks and damage.
  • Remove the Oil Fill Cap
    The oil fill cap is partially hidden beneath the exhaust. Use a rag to keep this area clean. Remove the cap.
  • Refill with New Oil
    Using the measuring cup, measure out 0.69 US quarts (660 milliliters) of fresh gear oil. Insert the long funnel under the exhaust and pour the oil slowly to avoid spilling.
  • Check Oil Level
    Locate the oil level check bolt (requires 4mm Allen wrench). Remove this bolt; oil should gently seep out when the correct level is reached. If not, add oil slowly until seepage arises. Reinstall and snug the check bolt.
  • Reinstall the Fill Cap
    Hand-tighten securely.
  • Final Inspection & Clean-Up
    Wipe away any excess oil and dispose of used oil properly. Start the bike and check for leaks, especially at the drain bolt and oil fill cap.

Specific Call-Outs for 2021 Yamaha YZ125 Owners

  • The gear oil change interval on a YZ125 can vary depending on how hard you ride. For routine maintenance, changing after every race or every 5–10 hours of riding is ideal—especially for aggressive or dusty environments.
  • Oil type matters: Always choose a high-quality oil rated for wet clutch systems, such as Valvoline 10W40 with wet clutch protection. Using automotive oils can cause clutch slipping.
  • The oil fill cap is positioned such that a long funnel is necessary for hassle-free filling. Plan ahead to avoid spills.
  • The drain bolt torque spec is 14 ft-lbs. Overtightening can strip threads or crack the case; undertightening can cause leaks.
  • Check and replace the crush washer with every oil change to avoid leaks. It acts as both a seal and a torque indicator; when the washer "collapses," that's your visual cue that torque is sufficient.
  • The official specified capacity is 660 ml (0.69 US quarts)—do not ignore this, as overfilling can foam the oil and under-filling can risk transmission damage.
  • Monitor for drips or leaks under the crankcase and clutch cover after each change. If gaskets are suspect, consult your owner's service manual or contact your Yamaha dealer for replacements.

Conclusion

Routine oil changes keep your 2021 Yamaha YZ125 running crisp, safeguard the gears, and provide predictable clutch performance. Follow the steps and call-outs above for trouble-free results every time. For questions or factory parts, MotoSport.com remains your best source for dirt bike maintenance and OE supplies.

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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.