How to Change the Coolant in a 1977 Suzuki RM125

Shop parts for a 1977 Suzuki RM125 Dirt Bike.

The Suzuki RM125 from 1977 is a classic air- and liquid-cooled two-stroke dirt bike that requires regular coolant maintenance to prevent overheating and safeguard internal engine components. Changing the coolant is a straightforward process, but certain model-specific details should be considered to keep your RM125 trail-ready and running efficiently.

Tools and Supplies Needed

  • New premixed ethylene glycol-based coolant (50:50 ratio with distilled water)
  • Socket set or wrench
  • Drain pan
  • Clean rags
  • Protective gloves

Preparation Steps

  • Park the RM125 on a level surface and allow the engine to cool completely. Never remove the radiator cap while the engine is hot–scalding fluid or steam can cause serious injury.
  • Ensure the area is well-ventilated and have your drain pan positioned beneath the bike.

Step-by-Step Coolant Change Procedure

  • Locate the drain bolt on the water pump cover at the base of the engine. Remove this bolt to allow old coolant to drain into the pan.
  • Remove the radiator cap slowly and carefully after confirming the engine is cool. This will help old coolant drain thoroughly.
  • Reinstall the drain bolt once all coolant has evacuated.
  • Fill the radiator neck slowly with fresh coolant until it reaches the bottom of the inlet hole. Pause periodically to allow air pockets to escape; gently rock the bike side to side to help coax trapped air out.
  • Start the engine briefly—with the radiator cap still off—and gently blip the throttle to encourage any remaining air to escape.
  • Stop the engine and top up the radiator as needed. Securely tighten the radiator cap: improper sealing can lead to coolant overflow and system pressure loss.

Model-Specific Notes for the 1977 Suzuki RM125

  • This bike does not have an overflow tank, so the engine coolant level may decrease during riding. Always inspect the radiator before each ride and replenish as needed.*
  • Mix coolant with distilled water at a 50:50 ratio for optimal thermal protection and corrosion prevention.
  • Check for any signs of leaks from hoses, radiator, or water pump cover. Damaged seals are a common issue on vintage RM125s and can impact cooling performance.

Final Tips

  • Dispose of old coolant properly at your local recycling center – it is toxic to humans and the environment.
  • Log the date and engine hours/mileage in your service record to maintain a regular maintenance schedule.
  • Inspect your radiator cap for wear or poor seal—replace if necessary for proper pressure regulation.

Maintaining proper coolant levels will help prevent overheating, reduce engine wear, and preserve your classic RM125's reputation as a reliable off-road machine. For a full selection of replacement gaskets, hoses, and coolants custom-fit for your 1977 Suzuki RM125, shop parts for a 1977 Suzuki RM125 Dirt Bike.

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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.