How to Change the Coolant in a 2021 Yamaha YZ125

Shop parts for a 2021 Yamaha YZ125 Dirt Bike.

Maintaining the proper coolant level is essential for the longevity and performance of your 2021 Yamaha YZ125. This iconic two-stroke motocross bike features a liquid-cooled single-cylinder engine, which relies on a clean, effective coolant system to manage high engine temperatures during demanding rides. Follow this proprietary MotoSport guide for a safe, thorough coolant change, and pick up top-level coolant and parts for your YZ125 to keep your bike race-ready.

What You Need

  • High quality ethylene glycol-based coolant designed for aluminum engines
  • Distilled or soft water (for mixing if coolant is concentrate)
  • Drain pan
  • Socket wrench set (including 10mm for drain bolt)
  • Rags and gloves

Instructions: Changing YZ125 Coolant

  1. Let the bike cool completely. Ensure the engine is cold to prevent injury from hot fluid or pressurized steam. Never open the radiator cap when the engine is hot.
  2. Stabilize the bike. Place the bike on a level surface using a stand or support. Hold it upright for accurate draining and refilling.
  3. Remove the radiator cap. Slowly turn the radiator cap counterclockwise to release pressure, then remove it. Keep rags handy in case of residual coolant spillage. Avoid letting coolant touch painted surfaces; rinse immediately if it does.
  4. Drain the old coolant. Position a drain pan below the engine. Use a 10mm socket to remove the coolant drain bolt (usually located at the bottom of the water pump cover). Let the coolant drain fully. Inspect and keep the copper washer.
  5. Flush the system (recommended). Pour clean distilled water into the radiator and let it flush out old residue and contaminants. Repeat this step until the drained water runs clear. Do not use hard or salt water, as it may damage engine components.
  6. Reinstall drain bolt and washer. Check the condition of the copper washer, replace if damaged. Tighten the drain bolt to 10 Nm (7.2 ft–lb) torque.
  7. Mix and add new coolant. Prepare a 50/50 coolant/distilled water blend if using concentrate. Pour the mixture into the radiator until the level reaches the bottom of the radiator filler neck. The 2021 YZ125 holds approximately 0.95 L (1.00 US qt) of coolant.
  8. Bleed air from the system. Gently rock the bike side to side and squeeze radiator hoses to help dislodge air bubbles. Top off the coolant if needed.
  9. Reinstall the radiator cap. Secure the cap by pressing down and turning clockwise until tight.
  10. Check for leaks and test run. Start the engine and let it idle for several minutes. Watch for leaks and monitor the coolant level after the engine cools. Inspect hoses, the cap, and the drain bolt for tightness. Add coolant if the level has dropped after air bleeding out.

2021 Yamaha YZ125 Notes & Tips

  • The 2021 YZ125 uses a high-revving two-stroke engine that generates substantial heat during aggressive riding—maintaining coolant quality prevents engine seizure and performance drop.
  • Routine maintenance: Yamaha recommends inspecting coolant levels and changing the coolant at regular intervals or sooner if high-performance riding is frequent.
  • Coolant choice matters: Use only high quality, silicate-free coolant designed for aluminum, since the YZ125 cooling system uses aluminum components which can corrode if the wrong coolant is used.
  • Proper coolant system sealing: Always check the radiator cap seal and spring for fatigue or cracks to ensure system pressure is maintained and coolant boiling is prevented.
  • Environmental tip: Always dispose of old coolant at a hazardous waste facility. Never pour coolant into drains or onto the ground.

Keeping your YZ125's cooling system fresh ensures maximum power and engine reliability on the track or trail. For genuine parts, performance upgrades, and maintenance items, trust MotoSport to support every aspect of your ride.

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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.