How to Change the Coolant in a 1986 Yamaha YZ125
Shop parts for a 1986 Yamaha YZ125 Dirt Bike.
The 1986 Yamaha YZ125 is a legendary two-stroke dirt bike known for its lightweight frame and spirited engine performance. To keep your YZ125 running cool and reliably, periodic coolant changes are critical for protecting the liquid-cooled engine, radiator, and internal passages from overheating and corrosion. Here is MotoSport's proprietary, step-by-step guide for changing the coolant on your vintage ride, including model-specific pointers for best results.
What You Need
- High-quality ethylene glycol coolant with anti-corrosion additives formulated for aluminum engines
- Distilled or soft water (if diluting concentrated coolant)
- Drain pan
- 10 mm wrench or socket
- Copper washer (inspect and replace if damaged)
- Rags and water for clean-up
- Protective gloves
Steps to Change the Coolant
- Allow the Engine to Cool: Never open the radiator cap while the engine is hot due to pressurized coolant. Wait until everything is completely cool before beginning–this is especially important for YZ125 models where the cap sits right at the frame's edge–avoid burns and spills.
- Stabilize the Bike: Set the YZ125 upright on a level surface, using a stand if available, to ensure a full and even drain of the cooling system.
- Open the System:
- Place a drain pan beneath the engine, near the water pump cover.
- Locate and remove the coolant drain bolt on the lower engine/water pump using a 10 mm wrench. Be sure to capture the copper washer for inspection.
- Loosen and carefully remove the radiator cap slowly. Wait for any residual pressure to escape before fully removing the cap.
- Drain the Coolant: Allow the old coolant to completely exit the radiator and engine cases. Gently lean the bike from side to side for a full drain if necessary.
- Flush the System (Recommended): Fill with clean, soft water and re-install the drain bolt/washer. Run the engine briefly, then let cool and re-drain to flush any contaminants. Repeat until water runs clear. YZ125 cooling systems are sensitive to hard water and debris, so thorough flushing is a must for long life.
- Reinstall Drain Bolt & Washer: Inspect the copper washer and replace if it is scored or damaged. Torque the drain bolt to 10 Nm (7.2 ft–lb) for a proper seal.
- Refill with New Coolant:
- For best corrosion protection, mix coolant 50/50 with distilled or soft water (capacity: 0.95 liters)—never use hard or salt water.
- Slowly fill the radiator, allowing air to escape and topping up as bubbles exit. Squeeze hoses gently to help purge trapped air.
- Bleed Air from System:
- With the radiator cap off, start the engine and run for a few minutes. Watch for air bubbles escaping; add coolant if the level drops.
- Once bubbling stops and the level is steady, shut off the engine and top off if needed.
- Replace and secure the radiator cap.
- Final Clean-Up: Wipe any spilled coolant from painted or rubber surfaces with water immediately, as coolant can damage finishes and plastics.
Special Notes for the 1986 Yamaha YZ125
- The 1986 YZ125 features a unique aluminum cylinder and head, which are especially susceptible to corrosion if improper fluids are used. Always use a premium coolant designed for aluminum components.
- Do not substitute tap water or use automotive coolants not intended for motorcycles—these may foam or degrade seals over time.
- Inspect the copper washer and replace with each service to minimize the risk of leaks on this vintage model.
- If your YZ125 has aftermarket radiators, check for slightly different drain bolt locations or torque values as some replacements differ from OEM spec.
Checking for Leaks and Testing
- After the first ride, let the engine cool and re-check your coolant level. Top off as needed.
- Inspect all hose connections and the drain bolt for leaks after full engine warm-up.
With the above steps, your 1986 Yamaha YZ125 will stay cool and reliable on every ride, extending engine life and maximizing track and trail 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.