On model pages you will always see a label next to the engine: "air-cooled" or "liquid-cooled". In everyday speech, liquid cooling is also called "water cooling". This difference shapes the engine's character, its maintenance needs, how it behaves in summer traffic and even how the engine looks. In this guide we explain how the two systems work, their pros and cons, who each one suits and why an engine overheats.
How do the two systems work?
With air cooling, thin fins on the outside of the cylinder and cylinder head carry heat to the surface, and the air passing over the engine picks that heat up and takes it away. There is no radiator, pump or coolant. On some models the engine oil helps with this job through a separate oil cooler.
With liquid cooling, a coolant circulates through passages inside the engine. A water pump circulates the liquid, a thermostat keeps it away from the radiator until the engine has warmed up, and the radiator, together with a fan that switches on when needed, releases the heat into the air. The system is closed and pressurised.
Two examples: on Honda's Turkey website the CB125F is listed with an air-cooled single-cylinder engine, while the CB650R is listed with a liquid-cooled inline-four. The parallel-twin engine of the Kawasaki Z650 is also liquid-cooled.
Comparison
| Air-cooled | Liquid-cooled | |
|---|---|---|
| Number of parts | Few; no radiator, pump or hoses | Radiator, pump, thermostat, fan, hoses |
| Maintenance | No coolant maintenance | Checking the coolant level and periodic replacement |
| Temperature control | Depends on outside air and speed, fluctuates | Stable, within a narrow range thanks to the thermostat |
| In summer traffic | The engine can get very hot when stationary for long | The fan kicks in; hot air from the fan may reach your legs |
| Power and performance | Generally in low and medium-power engines | Most high-power, high-revving engines |
| Risk of damage | No radiator to be hurt in a fall | The radiator needs protection from stones and falls |
| Appearance | Finned, classic look | Radiator and hoses are visible |
Pros and cons of air cooling
The biggest advantage of an air-cooled engine is simplicity. There are few parts to go wrong and few maintenance items; you do not have to worry about a radiator leak, a pump failure or a burst hose. That is why it is common on small-capacity work and city bikes and on classic-looking models. Light weight and no risk of radiator damage in a fall are further plus points.
The downside is limited temperature control. The engine's temperature depends on the outside air and your speed; idling for a long time in heavy mid-summer traffic can overheat both the engine and the engine oil. Also, because high power and strict emission targets require temperature to be held in a narrow range, liquid cooling is preferred on most powerful modern engines.
Pros and cons of liquid cooling
Liquid cooling keeps the engine close to its ideal operating temperature at all times. That means more power, more consistent performance and fewer temperature problems on long journeys, on climbs and in summer traffic. The water jacket also dampens engine noise somewhat.
The price is more parts and more maintenance: the coolant level must be monitored and the coolant changed at the interval in the manual, the radiator fins kept clean, and the hoses and clamps checked. The radiator is exposed to stones thrown up by the front wheel and to falls; if you ride off-road, consider a radiator guard. In hot weather, the hot air blown out by the fan can reach the rider's legs.
Which one suits whom?
- City use, short distances, low budget: Air-cooled 125s and small bikes make sense with their simple construction. If you will be idling for long periods in summer, do not leave the engine running needlessly.
- Long distance, motorway, high power: Liquid-cooled engines cope better with sustained high revs and hot weather.
- Those who like a classic look: If the finned cylinder is part of the appeal, look at air-cooled or air/oil-cooled classic models. See the best-of lists.
- Those who do their own maintenance: An air-cooled engine creates less work in a home garage.
The cooling type alone should not decide it; consider it together with your licence, budget and intended use. To help you choose, you can use the motorcycle finder and the first motorcycle guide.
Why does an engine overheat?
The most likely causes of overheating in liquid-cooled engines:
- Loss of coolant (leak, hose, clamp, radiator),
- The fan not working (fuse, fan motor, sensor),
- Radiator fins clogged with insects, mud or leaves,
- The thermostat stuck in the closed position,
- Water pump failure.
In air-cooled engines, long idling in hot weather, a low oil level, fins caked in mud and a lean mixture all increase heating. In either type, thick white smoke from the exhaust together with overheating may indicate a serious problem; see the exhaust smoke guide.
What should you do if the overheating light comes on?
- Pull over somewhere safe and stop the engine. Continuing to ride while the temperature rises can cause permanent engine damage.
- Never open the radiator cap while the engine is hot. The system is pressurised; boiling liquid can spray out.
- Once the engine has cooled, check the level in the reservoir and look for leaks around the hoses and under the engine.
- Check whether the fan runs the next time you start the engine.
- If the level is low, top it up with the coolant specified in the manual. If you added water in an emergency, have the mixture ratio corrected at a workshop later; frost and corrosion protection is reduced.
- If the problem recurs, go to a service centre.
Coolant in winter
Coolant does not only cool the engine; thanks to the antifreeze in it, it also prevents freezing in winter and corrosion inside the system. Before winter, check the level and colour of the coolant; if it looks cloudy or rusty, or if it is due for replacement, renew it. For preparing the engine for cold weather, see the winter guide. Follow the maintenance schedule in your own motorcycle's manual; an example schedule is in the maintenance guide.
Conclusion
Air cooling is simple, light and low-maintenance; it makes sense for city riding and small-capacity bikes. Liquid cooling keeps the engine at a stable temperature and suits high-power bikes and those used a lot on long journeys better, but it needs coolant maintenance. Whichever you choose, take the overheating light seriously, never open a hot radiator cap and renew the coolant at the interval in the manual.
Frequently asked questions
Is a water-cooled engine better?
Not in every case. Liquid cooling provides stable temperature and high power; air cooling is simpler and needs little maintenance. For a small bike used in the city, air cooling may be enough.
Does an air-cooled engine overheat in summer?
Idling for a long time in hot weather can make an air-cooled engine very hot. Switch the engine off during long waits, keep the oil level correct and keep the fins clean.
When should antifreeze be changed on a motorcycle?
The replacement interval varies by model and is given in the maintenance schedule in the manual. Checking its level and appearance before winter is a good habit.
Can you put water in the radiator?
Use the coolant specified in the manual. If you added water in an emergency, frost and corrosion protection is reduced; have the mixture ratio corrected as soon as possible.