How to Change the Coolant in a 1978 Yamaha YZ125
Shop parts for a 1978 Yamaha YZ125 Dirt Bike.
The 1978 Yamaha YZ125 is a legendary piece of motocross history that features a liquid-cooled engine–fairly advanced for its era. Keeping the coolant fresh is essential to maintain optimal engine temperature, prevent overheating, and protect internal components from corrosion or mineral buildup. Here is a step-by-step, MotoSport-exclusive guide to performing a coolant change on this classic machine.
What You Need
- High quality ethylene glycol-based coolant (with anti-corrosion properties for aluminum engines)
- Distilled or soft water for mixing (avoid hard water, which causes mineral deposits)
- Drain pan or container
- Socket wrench set
- Copper washer (replace if worn)
- Protective gloves and eyewear
- Shop rags or towels
Steps to Change Coolant
- Let the engine cool. Never remove the radiator cap when the engine is hot–pressurized hot coolant can cause serious burns.
- Secure the bike. Set your YZ125 upright on a flat and level surface.
- Prepare for draining. Place a drain pan underneath the engine to catch the old coolant.
- Remove the drain bolt. Locate the coolant drain bolt at the base of the water pump on the engine case. Loosen and remove the drain bolt and copper washer. Allow the old coolant to drain completely.
Tip: If the coolant does not flow freely, loosen the radiator cap slightly to break the vacuum and allow air in. - Remove the radiator cap. Once the hissing stops and pressure is released, remove the radiator cap fully.
- Flush the system. Pour clean, soft water (preferably distilled) into the radiator, then reinstall the drain bolt and temporarily secure it. Start the engine and let it run briefly (one or two minutes) to circulate the water, then shut it off, remove the drain bolt again, and let the rinse water drain. Repeat if necessary until the water drains clear.
- Inspect the copper washer. Replace with a new washer if it appears damaged or crushed.
- Install the drain bolt. Reinstall the drain bolt and torque to 10 Nm (7.2 ft–lb).
- Mix and add coolant. The 1978 YZ125 requires a 50/50 mix of coolant and distilled water. Total capacity is about 0.95 L (1 US qt). Fill the radiator to the correct level–avoid overfilling.
- Expel air bubbles. With the radiator cap off, gently lean and tap the bike a few times to help trapped air escape. Top up as needed.
- Reinstall the radiator cap. When the coolant level stabilizes, install and secure the cap.
- Check for leaks. Start the engine and let it warm up to operating temperature. Observe for leaks and ensure the coolant level remains stable. Shut down the engine and let it cool; re-check and top off if necessary.
Maintenance Tips Specific to the 1978 YZ125
- This model's aluminum engine components are sensitive to minerals and corrosion–use only fresh, high-quality engine coolant suited for aluminum systems.
- Check the radiator and hoses for signs of aging, dry rot, or leaks, as the original parts may be decades old.
- If your YZ125 is still using original radiator cap, consider replacing it to ensure proper pressure retention in the cooling system. A weak cap can cause overheating.
- After long-term storage, always flush the cooling system before use, as old coolant destabilizes over time and can cause internal corrosion.
Keeping your vintage Yamaha's cooling system in top shape ensures its proven racing performance stays sharp even decades after its debut. MotoSport supports you with the right parts and service knowledge for your ride.
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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.