How to Change the Coolant in a 1979 Yamaha YZ125: A MotoSport.com Guide

Shop parts for a 1979 Yamaha YZ125 Dirt Bike.

The 1979 Yamaha YZ125 is a legendary motocross bike, renowned for its lightweight chassis, robust air-cooled two-stroke engine, and race-ready design. Keeping its cooling system in top shape is essential for both performance and longevity. While late-model YZ125s use advanced liquid cooling, the classic 1979 model uses an air-cooled design. However, if your YZ125 has an aftermarket or upgraded liquid-cooling system (a common modification for increased reliability in vintage racing), follow these steps for a thorough coolant changethese steps also apply to most liquid-cooled YZ125 models with minor differences.

Preparation

  • Let the engine cool completely to avoid injury or damage.
  • Place the bike upright on a level surface using a stand.
  • Have the following tools and supplies ready:
    • Drain pan
    • Socket/wrench set (typically 8mm or 10mm)
    • Funnel
    • Soft or distilled water (never hard or salt water)
    • High-quality ethylene glycol-based coolant (for aluminum engines, mixed 50/50 with water)
    • Clean rags

Step-by-Step Coolant Change Process

  1. Remove the radiator cap. Only do this when the engine is completely cool. Loosen counterclockwise. If there is any hissing, wait for the pressure to release before full removal.
  2. Place a drain pan below the engine.
  3. Locate and remove the coolant drain bolt. On most YZ125 models, this bolt is found on the water pump cover or lower side of the engine. Remove the copper sealing washer as well.
    Tip: Take note of the torque spec for reinstallationtypically 10 Nm or 7.2 ft–lb.
  4. Drain the coolant completely. Let the coolant flow until fully drained. Tilt the bike gently side-to-side to clear any remaining fluid.
  5. Flush the system. Pour clean tap or distilled water into the radiator, temporarily reattach the cap and drain bolt, start the engine briefly, then drain again. Repeat until only clear water drains out.
  6. Reinstall the drain bolt with copper washer. Tighten to factory specs (about 10 Nm or 7.2 ft–lb). Do not overtighten.
  7. Refill with coolant. Use a 50/50 mixture of high-quality coolant and soft/distilled water. The system capacity is about 0.95 L (1 US qt). Fill until the coolant reaches the neck of the radiator.
  8. Bleed air from the system. With the bike upright, slowly tip it side to side and gently squeeze any visible hoses to help air escape. Top off coolant as needed.
  9. Replace the radiator cap. Ensure it is clean and seals tightly.
  10. Dispose of used coolant responsibly. Never pour it down household drains or onto the ground. Most auto parts stores offer recycling.

Model-Specific Notes: 1979 Yamaha YZ125

  • This year– model is an iconic air-cooled bike. If yours is stock, it will not use engine coolant. Always confirm whether your specific YZ125 has had an aftermarket liquid cooling system installed before attempting coolant changes.
  • Mix coolant with soft or distilled water to prevent mineral buildup and corrosion inside the system.
  • Inspect hoses, gaskets, and the radiator cap for signs of wear or leaks while servicing.
  • Changing the coolant every 40–50 hours of ridingor at least once per seasonpreserves peak performance and engine health.

Why Routine Coolant Changes Matter

  • Prevents overheating and significant engine damage.
  • Protects internal engine and cooling system components from corrosion.
  • Ensures the YZ125 delivers consistent performance on rides and at the track.

Regular maintenance is the best investment you can make in your 1979 YZ125. For all your OEM and aftermarket part needs, trust the team at MotoSportyour powersports experts.

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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.