How to Change the Coolant in a 1981 Suzuki RM125

Shop parts for a 1981 Suzuki RM125 Dirt Bike.

Changing the coolant in your 1981 Suzuki RM125 is essential for maintaining stable engine temperatures and achieving peak performance. The 1981 model marked Suzuki's first use of a liquid-cooled engine on the RM125, delivering more consistent power and improved reliability. Below is the MotoSport step-by-step guide tailored specifically for your bike's unique design and requirements.

What Makes the 1981 RM125 Cooling System Unique?

  • The 1981 RM125 features a single large aluminum radiator mounted on the front frame down-tube, rather than under-seat locations used by other brands.
  • The water pump is integrated into the right side crankcase and uses a six-blade impeller, efficiently circulating about one quart of coolant dozens of times per minute during operation.
  • There is no overflow tank; check coolant level before each ride, as small losses are normal, especially after hard riding.

Tools and Supplies You Will Need

  • Phillips or flathead screwdriver (for radiator shroud removal, if necessary)
  • Drain pan and shop towels
  • Fresh engine coolant premixed with distilled water (50:50 ratio)
  • Gloves and safety glasses

Step-by-Step Coolant Change Instructions

  1. Let the engine cool completely. The cooling system is pressurizedopening the cap on a hot engine may result in dangerous scalding or steam.
  2. Place the bike upright on level ground, using a stand for stability.
  3. Remove the radiator cap. It's located on top of the radiator, in front of the tank. Press down and turn counterclockwise.
  4. Locate and remove the coolant drain bolt. On the 1981 RM125, this is typically found on the water pump housing on the right side of the engine. Place a drain pan beneath. Loosen the bolt and allow all coolant to drain.
  5. Inspect the drained coolant. Look for signs of rust, oil, or debris, which can indicate more serious issues.
  6. Reinstall and tighten the drain bolt securely. Do not over-tighten, as the housing is aluminum and can strip. Clean up any spillage around the area.
  7. Slowly fill the radiator with your premixed coolant (50% distilled water and 50% specified engine coolant), adding until the coolant level is at the bottom of the radiator neck.
  8. Bleed the system for air pockets:
    • With the radiator cap off, tilt the bike gently from side to side and tap the coolant hoses to release trapped air.
    • If possible, briefly start the engine in neutral (no more than 30 seconds) to help purge any remaining air, then top up the coolant as needed while the engine is off and cools.
  9. Reinstall the radiator cap. Press down firmly and turn until it locks. A loose cap will cause coolant to boil at a lower temperature, risking overflow and potential engine damage.
  10. Check for leaks and wipe up any spilled coolant. Run the engine for a few minutes. After another cool-down, recheck the leveltop off if needed.

Tips and Key Considerations for 1981 RM125 Owners

  • Frequent coolant checks are crucial since there is no overflow tank—expect to lose some level over time, especially with hard riding.
  • Use only non-silicate, motorcycle-safe coolant. Avoid automotive coolants with silicates as they can damage the water pump and seals.
  • If you notice repeated coolant loss or milky oil, promptly check for possible engine gasket or water pump seal failures—common issues as these bikes age.
  • The water-cooled system dramatically improved engine reliability on the 1981 model, but its components (hoses, plastic, aluminum) are now vintage. Inspect all cooling system parts regularly for wear and consider replacing old or brittle hoses using MotoSport parts for this vintage RM125.

Enjoy Hassle-Free Rides

Proper coolant maintenance not only preserves the performance that made the 1981 RM125 iconic but also protects against costly engine damage. Follow this guide to keep your classic running cool, efficient, and race-ready for seasons to come.

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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.