How to Change the Coolant in a 1998 Yamaha YZ125

Keeping your 1998 Yamaha YZ125 running cool means regular maintenance of the cooling system – especially the coolant. Over time, coolant breaks down, loses efficiency, and can allow corrosion or overheating. This guide provides a step-by-step process for changing the coolant on your YZ125, along with key notes every owner should know about this iconic 2-stroke dirt bike.

What You Need

  • Fresh, high-quality coolant designed for motorcycles (avoid using plain water)
  • Drain pan or suitable container for used coolant
  • 8mm socket or wrench
  • Funnel
  • Distilled water if you want to flush the system
  • Gloves – optional but recommended

Preparation

  • Make sure the YZ125 engine is completely cool before starting to avoid burns from hot coolant or steam.
  • Place your dirt bike securely on a stand for stability.
  • Position the drain pan underneath the water pump area (right side of the engine).

Drain the Old Coolant

  1. Remove the radiator cap slowly and carefully to release any residual pressure in the cooling system.
  2. Locate the coolant drain bolt on the water pump cover (usually 8mm). Place your drain pan below and loosen the bolt completely to allow coolant to drain out.
    Tip: If needed, remove the skid plate for easier access to the drain bolt.
  3. Allow all coolant to fully drain. Slightly tipping the bike side to side can help remove trapped fluid.

Flush the Cooling System (Recommended)

  • Pour a small amount of distilled water into the open radiator filler neck.
  • Let the water drain out through the same drain bolt, repeating until the fluid runs clear.
  • This step helps remove old coolant residue and contaminants.

Reinstall the Drain Bolt

  • Reinstall and securely tighten the drain bolt. Do not overtighten – just snug is sufficient. Wipe away any residual coolant around the area.

Refill with New Coolant

  1. Using a funnel, slowly pour in fresh coolant through the radiator neck until it reaches just below the filler cap opening.
  2. Gently rock the bike or squeeze radiator hoses to remove trapped air.
  3. Reinstall the radiator cap securely.

Bleed the System & Final Check

  • Start the engine and let it idle for a few minutes. This bleeds remaining air and circulates coolant.
  • Stop the engine, let it cool, and re-check the coolant level. Top off if needed.
  • Inspect for leaks around the drain bolt and hoses.

Key Notes for 1998 Yamaha YZ125 Owners

  • The 1998 YZ125 uses a lightweight aluminum radiator system – always use a motorcycle-safe coolant to prevent corrosion inside the cooling system.
  • Avoid using tap water. Only use distilled or deionized water for flushing, as minerals in tap water can cause scale buildup and overheating issues.
  • Check radiator hoses for cracks or brittleness during every coolant change. Replace if aged or damaged to avoid leaks on the trail or during a race.
  • The YZ125's higher-revving two-stroke motor generates more heat than many believe. Maintaining fresh coolant guards against overheating, which can quickly ruin a piston or cylinder on these bikes.
  • Change coolant every 1–2 years or every 40–50 hours of riding for optimal performance and protection.

Disposal

  • Never pour used coolant into the ground or drains. Take it to an authorized recycling or disposal center to protect the environment.

Proper coolant maintenance ensures your 1998 Yamaha YZ125 stays reliable, responsive, and ready for every ride – whether practice, race day, or just tearing up your favorite trails.

Related Shopping Categories

Shop Radiators for a 1998 Yamaha YZ125 Dirt Bike.

Shop Coolant for a 1998 Yamaha YZ125 Dirt Bike.

Shop Temperature Sensors for a 1998 Yamaha YZ125 Dirt Bike.

Shop Tools for a 1998 Yamaha YZ125 Dirt Bike.

Shop Radiator Hoses for a 1998 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.