Cooling: what you forget until it overheats
The cooling system is one of the most neglected parts of a motorcycle. As long as the temperature stays normal, nobody thinks about it. But when the gauge needle climbs into the red, it's often too late — blown head gasket, warped head, seized engine.
A regular check takes 5 minutes and costs almost nothing.
Which coolant to use?
Motorcycle coolant is a mixture of ethylene glycol and demineralised water, with anti-corrosion additives. Never use tap water alone — limescale and minerals cause internal corrosion and radiator deposits.
Specifications:
- Antifreeze protection down to -25 °C minimum (check the bottle)
- Compatible with aluminium alloys (essential — motorcycle engines are aluminium)
- Preferably the same brand/colour as the existing coolant to avoid additive incompatibilities
Some manufacturers require a specific coolant (Honda Type 2, Yamaha Yamalube). Check the workshop manual.
When to replace?
- Every 2 years or 40,000 km, whichever comes first
- Coolant degrades over time: anti-corrosion additives deplete, even if the bike isn't ridden
Level check
Check the level with the engine cold, bike upright (not on the side stand). The expansion tank has MIN and MAX marks. The level should be between the two.
If the level drops regularly:
- Check the hoses (cracks, loose clamps)
- Check the radiator cap (weak spring = pressure leak)
- Check the head gasket (coolant in the oil = milky emulsion on the oil filler cap)
Bleeding: removing air pockets
After a coolant replacement, air pockets remain in the system. These bubbles prevent proper fluid circulation and create hot spots.
Bleeding procedure:
- Fill the system through the radiator cap (not just the expansion tank)
- Open the bleed screw on the cylinder (often on the thermostat housing or top of the cylinder) — let air escape until coolant flows out
- Close the bleed screw and top up the level
- Start the engine, let it idle until the fan kicks in — the thermostat opens between 82 and 88 °C depending on model
- Check for bubbles rising in the radiator cap opening (leave it open during warm-up, then close)
- Let it cool, top up the level in the expansion tank
The bleed screw is often a small brass bolt, 7 or 8 mm. If you can't find it, check the workshop manual — its location varies significantly by model.
The thermostat: the forgotten part
The thermostat regulates engine temperature by controlling coolant flow to the radiator. If it stays stuck closed, the engine overheats. If it stays stuck open, the engine never reaches operating temperature (poor combustion, accelerated wear).
Opening temperature: 82-88 °C on most models. If in doubt, you can test it in a pot of hot water with a thermometer.
Don't mix colours randomly
Green, blue, pink — coolant colours aren't decorative. They indicate additive chemistry. Mixing two different chemistries can neutralise the anti-corrosion agents and create gelatinous deposits that clog the system. When in doubt, drain completely and fill with a single product.
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