How to Change the Coolant in a 1977 Honda CR125: Complete MotoSport Guide
Shop parts for a 1977 Honda CR125 Dirt Bike.
The 1977 Honda CR125 is a legendary air-cooled two-stroke dirt bike that remains popular among vintage motocross enthusiasts and restorers. While its engine is cooled by air, many CR125 conversions or later models use liquid cooling systems with radiators added for increased performance. If your bike features a liquid cooling (radiator) setup or you have a modified model, this guide walks you through the coolant change procedure specific to this configuration.
What CR125 Owners Should Know
- The original 1977 Honda CR125 is air-cooled; liquid cooling setups are only seen on custom builds, conversions, or later models.
- If your bike remains air-cooled, coolant changes are not applicable. Skip this guide and focus on regular cylinder fin cleaning, carb tuning, and exhaust maintenance.
- If it has a radiator/modification, follow the steps below for a safe coolant change.
Tools & Materials Needed
- Ethylene glycol-based coolant suitable for aluminum radiators
- Distilled water (if mixing coolant concentrate)
- Socket wrench – typically 8mm or 10mm
- New water pump crush washer or copper drain washer (recommended)
- Catch pan
- Protective gloves & rags
Step-by-Step Coolant Change Procedure
- Let the engine cool. Ensure the bike is completely cool to avoid pressurized burns when opening the radiator cap or drain bolt.
- Remove the radiator cap. This allows air into the system and helps coolant drain faster.
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Locate and remove the coolant drain bolt:
- The coolant drain bolt is usually found at the lowest point of the water pump cover on the right engine case.
- Position the catch pan underneath, then carefully loosen and remove the drain bolt. Coolant will begin to drain out rapidly.
- Inspect the drain washer. Replace with a new copper or crush washer to prevent leaks.
- Allow coolant to drain fully. Tilt the bike side-to-side to help remove stubborn coolant pockets.
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Flush system (optional but recommended on vintage bikes):
- Reinstall the drain bolt temporarily and fill the system with distilled water. Start the engine and idle for a minute, then repeat the draining process to flush old debris and residues.
- Some vintage owners use a weak white vinegar solution if heavy corrosion is present. If so, thoroughly rinse with distilled water afterward to neutralize any acidity.
- Reinstall and torque the drain bolt using the new washer. Torque to manufacturer spec (typically 10 Nm).
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Add new coolant:
- Fill the radiator with a pre-mixed ethylene glycol coolant or a 50/50 mix of coolant concentrate and distilled water.
- Fill to the base of the filler neck.
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Eliminate air pockets:
- With cap off, gently tilt or rock the bike, squeezing radiator hoses to help burp out trapped air.
- Start the engine and let it idle for several minutes to circulate the coolant. Watch for bubbling or drops in coolant level, indicating air escape.
- Top off coolant as needed once the engine cools and after any air escapes. Reinstall radiator cap securely.
- Check for leaks at the drain bolt and hose connections. Clean any spilled coolant off the bike, as it can be slippery and attract dirt.
Maintenance Notes for the 1977 CR125
- Check and clean the radiator fins regularly if your bike is liquid-cooled to maximize airflow and cooling efficiency.
- Coolant should be changed at least once per season or after heavy use, especially in vintage bikes to avoid corrosion and overheating.
- If running air-cooled, keep cylinder fins and engine exterior clean—don't use this procedure.
- If the coolant continues to drop or your bike overheats, inspect for possible head gasket or water pump issues.
Final Tips for the Vintage CR125 Owner
- Always use distilled water if mixing coolant to prevent mineral scaling.
- Dispose of old coolant safely; never pour it down drains.
- Double-check all fasteners for proper torque before riding.
Related Shopping Categories
Shop Radiators for a 1977 Honda CR125 Dirt Bike.
Shop Coolant for a 1977 Honda CR125 Dirt Bike.
Shop Temperature Sensors for a 1977 Honda CR125 Dirt Bike.
Shop Tools for a 1977 Honda CR125 Dirt Bike.
Shop Radiator Hoses for a 1977 Honda CR125 Dirt Bike.
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.