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DIY! How to Change the Coolant to Prevent Engine Damage and Rust


The characteristic of a liquid-cooled engine against an air-cooled engine that cannot be cooled if the outside air will not reach the engine is that the liquid-cooled engine kept at an appropriate temperature by coolant even when the motorcycle is going to stall.

Most current motorcycles use a liquid cooling system, as controlling the combustion chamber temperature is also effective for exhaust gas control. The coolant maintenance is required to maximize its performance. If the coolant in the reserve tank looks unrefined, replace it as soon as possible.

1. The performance of the coolant will decrease when keep using for a long time.


When removing the radiator cap, wait for the engine and the coolant to cool down sufficiently. Depending on the model, there may be screw around the cap to prevent rotation, so perform the work after checking. When the radiator cap deteriorates, it causes the boiling point to drop and leak.

The cooling principle of a liquid-cooled engine is to absorb the heat generated by the engine and dissipate it when passing through the radiator, and the important role of exchanging the heat is entrusted to the coolant.

A liquid-cooled engine, which receives air from an electric fan through a radiator with coolant circulating through a water pump, can easily control the temperature than an air-cooled engine that relies on the outside air, and it is also effective for high-performance engines with large heat generation and small engine displacement vehicles that regularly use high rev.

The performance at low temperatures is also important for a liquid-cooled engine. The freezing point at which tap water turns into ice is 0°C, but if the water that reaches the engine is freezing below the freezing point, the volume of the water expands and the cooling path may be damaged.

Therefore, the coolant of the liquid-cooled engine uses a liquid with a freezing point that does not freeze below 20℃, and also contains anti-rust components that do not rust the metal parts inside the engine, as well as an absorbent that does not create air bubbles when pumped by a water pump.

The coolant only goes back and forth between the engine, the main body, and the reserve tank according to the change of temperature, but when the temperature rises, the liquid may volatilize and the liquid quantity in the reserve tank may decrease, and when the liquid content of the reservoir tank is poured into the reservoir, the concentration of the coolant decreases and the freezing point rises, which leads to deterioration.

In addition, when the usage time is prolonged, dirt accumulates inside the radiator and the cooling path, and since there are cases where the cooling effect is reduced, a periodic replacement of coolant is required.


・For the liquid-cooled engine, coolant plays a very important role in the engine.
・Unlike tap water, the coolant that does not freeze even under 20℃ below the freezing point plays a role in preventing rust and engine damage even at low temperatures.

2. Remove the radiator cap after the engine has cooled down.

The CBR250R here is being used as an example of changing the coolant.

The CBR250R water pump can be easily checked because the radiator lower hose is stuck on the right side of the crankcase. Since the drain bolt is at the lowest position of the pump cover, remove the drain bolt by a socket wrench so that it will not be stripped.


In case the radiator cap has been removed in advance, the coolant will be discharged at the same time as the drain bolt is removed, so prepare a washbasin or a tray prior. The coolant capacity is 1.1 liters, but you should use 0.79 liters at the time of replacement because it remains in each part of the engine.

The timing of coolant replacement varies on vehicle type, and some models have been designated to change every 4 years after or every 5 years from brand new, while others are recommended to change every vehicle inspection. But either way, the procedure is the same. Most importantly, change the coolant only when the engine is cold.

There is a law in every liquid that the boiling point raises by increasing the pressure, and the boiling point reduces by decreasing the pressure. Therefore, there is a mechanism in the cooling system of the engine to increase the internal pressure by the function of the radiator cap to prevent the coolant from boiling at 100°C.

So, when the radiator cap is opened while the engine is still hot, there is a risk that the coolant will boil and spew out due to the difference between the high-pressure cooling system and the atmospheric pressure. It is important to remove the drain bolt located near the water pump when draining the coolant after ensuring that both the engine and the coolant have reached to its normal temperature.

In case you do not know where your motorcycle’s drain bolt is, follow the lower hose with two hoses connected to the radiator, and there should be a water pump at the end of the hose. If you have an air compressor, air blowing from the radiator cap after the coolant has drained from the drain bolt can further push out the coolant remaining in the path, and in case the drained coolant is contaminated with water scale, it is also effective to use a flushing agent to clean up the dirt and rust in the cooling path.

After the coolant is drained from the engine, the remaining amount in the reserve tank is also discharged. The reserve tank is made of a milky white plastic making the coolant easy to check the amount of liquid, and in case the coolant has not been changed for a long period of time, you may want to check with a thin brush as there is often water stain on the inside.


・The timing of coolant replacement varies on vehicle type.
・Do not open the radiator cap until the engine cools down during coolant replacement.


Keep in mind that the reserved tank is connected to the radiator cap mounting part with a siphon hose, and the amount of water in the tank increases or decreases in response to the change of coolant capacity or temperature.

3. Perform without air bleeding after pouring the coolant.


When restoring the water pump’s drain bolts, replace the sealing washer with a new one to prevent the coolant from leaking. Reusing the washer increases the risk of damaging the screw as the torque increases to further tighten the stripped part.

When restoring the drain bolts, use a new sealing washer and tighten with the right torque. There are 2 types of coolant: a type in which the undiluted solution is diluted with tap water according to the desired freezing point, and the other type is being sold in a pre-diluted solution.

In the former case, considering the lowest possible temperature in the area where you use your motorcycle, it should be diluted with a mixing ratio that is suitable for it. Since a jug with a wider spout increases the amount overflowing from outside, it is recommended to fill the diluted coolant into the radiator with an empty plastic bottle.

In this case, since air still remains in each part of the cooling path, start the engine before closing the cap to remove any residual air inside the radiator core and engine. Since the liquid level of the coolant drops when the air exits, refilling the reduced amount and attaching the radiator cap will complete the coolant replacement.

The coolant replacement does not make the engine sound quieter or change its feeling, but when performing periodic maintenance before cooling efficiency is reduced by water scale and rust, you should be able to enjoy your motorcycle for a long time without extra trouble.


Whether undiluted type or pre-diluted type, the coolant is poured into the radiator using a plastic bottle or funnel. Since the tap water alone freezes at 0°C and has no anti-rust, rust will occur in the cooling pipe made of steel.


Even if it is poured to the port of the radiator at the time of cold, when the engine is started and the coolant will circulate, the liquid level spreads to the air reservoirs of the respective parts and drops. After a few minutes of idling and air bleeding, insert the cap into the port again.

4. Deterioration of coolant is the great enemy of liquid-cooled engines.

If the bright liquid coolant becomes cloudy, it is a sign of deterioration. Since the rise of the freezing point and the rust of the cooling path becomes a big risk, practice regular replacement.

*The replacement should be done after the engine has cooled down.

See HONDA Moto Index Page
See Accessories for HONDA CBR250R (2011-)

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Our mission is to look for unique and valuable news that are from Japan and Asia related to motorcycles and publish to the world. "Webike staff" is name of a team who edit, translate and draft the articles we publish that are brought from all our branches around the world. Not all of our employees ride motorcycle, but find worth living to bring reader say "wow".

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