How to Change the Coolant in a 2026 Yamaha YZ125: MotoSport.com Guide

Shop parts for a 2026 Yamaha YZ125 Dirt Bike.

Changing the coolant in your 2026 Yamaha YZ125 is a vital maintenance step to keep your two-stroke running cool and performing at its peak. The YZ125 is designed for performance and longevity, but regular coolant replacement prevents unnecessary overheating and corrosion. This guide, built for MotoSport customers, walks you through every step—from draining the old coolant to refilling with the right mix for your motocross machine. The steps provided reflect the 2026 model, which maintains Yamaha's proven liquid-cooled system and all-aluminum engine components.

What You'll Need

  • Drain pan or suitable container
  • 8mm wrench or socket
  • Funnel
  • Soft (distilled) water
  • High-quality ethylene glycol-based coolant with anti-corrosion additives safe for aluminum
  • Shop rags
  • Gloves and safety glasses

Step-by-Step Coolant Change Procedure

  1. Preparation
    • Ensure the bike is completely cool before you begin.
    • Place the bike upright on a stand in a well-ventilated area.
    • Remove the right shroud or anything obstructing access to the radiator cap and drain bolt if necessary.
  2. Remove the Radiator Cap
    • Slowly remove the cap to release any pressure and set it aside. This helps the coolant drain freely.
  3. Drain Old Coolant
    • Locate the coolant drain bolt at the base of the water pump on the right side of the engine.
    • Place the drain pan beneath the drain bolt.
    • Loosen and remove the 8mm drain bolt and its copper washer. Allow all coolant to drain out.
    • To ensure thorough draining, you can tilt the bike slightly side-to-side.
  4. Flush the System (Recommended)
    • If the old coolant is dirty, pour clean, soft water into the radiator and let it drain. Repeat until the water runs clear.
  5. Reinstall the Drain Bolt
    • Check and clean the copper washer.
    • Reinstall the drain bolt and tighten to 10 Nm (7.2 ft–lb) for the 2026 YZ125.
  6. Mix and Add New Coolant
    • Mixture: Use a 50/50 mix of recommended coolant and soft/distilled water. Only fill with coolant suitable for aluminum engines to prevent corrosion.
    • Coolant Capacity: 0.95 L (1.00 US qt) for the YZ125 cooling system.
    • Slowly fill the radiator to the top using a funnel, allowing bubbles to escape.
  7. Bleed the Air
    • With the radiator cap off, lean the bike side-to-side and gently squeeze the radiator hoses to help release any trapped air.
    • If needed, top off the coolant as the level drops.
  8. Install Radiator Cap
    • Install the cap securely once the coolant level is stable and no more bubbles rise.
  9. Check for Leaks & Final Inspection
    • Start the bike and let it idle for a few minutes. Check for leaks around the drain bolt and hoses.
    • After the bike cools, recheck the coolant level and top off if necessary.

YZ125 Model-Specific Tips

  • The 2026 Yamaha YZ125 uses precision-machined aluminum components—always use a coolant safe for aluminum systems.
  • Check the condition of the radiator hoses and clamps during each coolant change. Replace if any cracks or leaks are found.
  • Never use only water as coolant in performance riding. Use only recommended premix or mix your own with soft water and high-quality coolant.
  • Repeat coolant changes every 1-2 years or after 40-50 hours of riding for optimal performance and engine life.

Disposal & Safety

  • Never reuse coolant. Used coolant must be disposed of according to local hazardous waste regulations—never pour into drains or soil.
  • Store new coolant and used fluids away from children and pets.

By following this MotoSport.com guide, you help extend the life and reliability of your 2026 YZ125, ensuring the optimal blend of power and protection on every ride.

Related Shopping Categories

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Shop Tools for a 2026 Yamaha YZ125 Dirt Bike.

Shop Radiator Hoses for a 2026 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.