How to Change the Coolant in a 1987 Yamaha YZ125

Shop parts for a 1987 Yamaha YZ125 Dirt Bike.

The 1987 Yamaha YZ125 is a classic two-stroke motocross bike featuring liquid cooling for optimal engine performance. Proper cooling system maintenance is essential to prevent overheating and ensure a long engine life. Whether prepping for a race or offseason maintenance, changing the coolant should be a regular part of your service routine. Below is a proprietary MotoSport guide tailored specifically to the 1987 YZ125 owner.

What You Need

  • New coolant (high-quality ethylene glycol antifreeze with anti-corrosion properties for aluminum engines)
  • Soft or distilled water (if mixing concentrate; 50% coolant – 50% water is recommended)
  • Drain pan
  • Socket wrench (8mm socket works for most drain bolts)
  • Funnel
  • Clean rags
  • Protective gloves and safety glasses

Important Notes for the 1987 YZ125

  • Never remove the radiator cap when the engine is hot—wait until it is absolutely cool to avoid burns and allow built-up pressure to escape safely .
  • Only use soft or distilled water if mixing coolant. Hard or salt water damages engine internals.
  • Total coolant system capacity is approximately 0.95 liters (1.00 US qt) .

Step-by-Step Coolant Change Guide

  1. Let the engine cool completely.
  2. Stabilize the bike upright on level ground using a stand if available.
  3. Place a drain pan beneath the engine.
  4. Remove the coolant drain bolt (typically located at the bottom of the water pump cover) along with the copper washer. Allow coolant to drain fully .
  5. Remove the radiator cap to vent any remaining coolant. Press down and turn the cap counterclockwise after pressure has vented and the hissing stops .
  6. Dispose of old coolant properly. Coolant is hazardous. Follow local disposal regulations.
  7. Flush the cooling system with clean tap water. Install the drain bolt (with the copper washer) and pour water into the radiator. Replace the cap, run the engine briefly, then let cool and repeat the draining process to flush residue .
  8. Reinstall the drain bolt and copper washer to a tightness of 10 Nm (7.2 ft–lb) .
  9. Fill radiator with recommended coolant mix (50% coolant, 50% soft or distilled water) up to the specified level (about 0.95L) .
  10. Gently shake or rock the bike upright to purge trapped air bubbles, then recheck the coolant level. Fill as needed.
  11. Install the radiator cap securely.
  12. Start the engine and idle for several minutes. Watch for leaks and check coolant level again after cooling—top off if necessary .

Model-Specific Insights

  • The 1987 YZ125 is known for its robust two-stroke engine and lightweight chassis—keeping the cooling system clean and efficient maximizes power and reduces risk of overheat, especially on hot track days.
  • Inspect hoses and the radiator cap for cracks or fatigue during each coolant change—older bikes like the 1987 YZ125 may develop weak seals over decades of use .
  • If troubleshooting overheating, check for clogged radiator fins or scale deposits inside the system which can be cleaned during a flush before refilling with new coolant .

Keep your 1987 Yamaha YZ125 ready for action—a well-maintained cooling system is essential for peak motocross performance.

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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.