How to Flush a Motorcycle Radiator

motorcycle radiator

Okay, so it's really hot outside. But you still want to ride. I know I do. But you're concerned because the temperature gauge on your bike is always getting close to the red. Not good. The reason could be because you are sitting idle in traffic way too much. Or it could be that your radiator could use a flush. Realize, before we start that many bike will have different components or different procedures based on the bike. Check your service manual before starting.

Water Wetter For The Track

Water Wetter is a product that will cool your engine but is not nearly as slippery as antifreeze if it gets spilled onto the road/track. A 12-ounce bottle of Water Wetter treats 12-16 quarts for six months.

Quickie-lube places charge upwards of $75-$100 for a coolant flush on a car, so it must be complex and entailed, right? Flushing the coolant from your radiator is a pretty basic task and like every other fluid inside your motorcycle should be changed regularly. Not only does antifreeze prevent freezing but it also lubricates your cooling system components. Also, more engines die due to overheating and/or cooling system failure than anything else. But before you start off to do this, let's look at some things.

First, make sure your fluid needs to be changed. You can do this the technical way or the layman's way. The technical way is to get a hydrometer. These devices draw coolant out of your radiator into a chamber filled with balls. If all the little balls float, you are in good shape. The layman's way to check the fluid is to look at it. If it looks dirty or translucent, change it. Every other year for coolant is a good rule of thumb.


If your bike is running hot all the time, even when riding moderate speeds in cool temperatures, you could have another problem. But one of the most basic rules is to start by fixing the cheapest component. In this case the coolant. You could have a bad thermostat and/or a bad radiator cap. Radiator caps not only wear out but are easily damaged even in minor falls. A bad thermostat doesn't allow the fluid to flow through the system properly. There is no easy way to test these components, so when in doubt, throw them out. Don't scrimp by trying to find an automotive replacement, get the motorcycle rated versions.

Radiator Cap

If you need to replace the cap on your bike, ensure you get one designed for your motorcycle, not your car. They might be more money, but you'll be using the product for what it was designed for the make and model of your motorcycle.


If you need to replace the thermostat in your bike, ensure you get one designed for your motorcycle, not your car. They might be more money, but you'll be using the product for what it was designed for - the make and model of your motorcycle.

So let's say you're pretty sure your radiator cap and thermostat are fine, but your fluid is dirty. Before you run out to find the best deal on antifreeze, check the back of the bottle for something very important. Ensure the antifreeze you choose Contains No Silicates. Silicates are sand, and as you can imagine, sand does not work well a with motorcycle's ultralight, made-of-plastic, water pump impeller. I've seen where silicate-rich antifreeze completely eroded the impeller blades down to nubs. Silicates, however, do keep the inside of a cooling system looking really nice and clean, which is why they are used. The new 5 year/150k miles orange antifreeze does not contain silicates. But check just to be safe.

drain bolt


Antifreeze is relatively benign, but very toxic so dispose of it properly. Don't just pour it down a storm drain. Also, if you have any pets keep them away during this task. Apart from antifreeze being very toxic, the glycol has a sweet taste and smell that is very appealing to animals. It doesn't take much for a dog, even a large dog, to drink before it will kill them. Cats are even more prone.

You need to remove any fairings or plastic that will be in the way. Some bikes require removal of the gas tank. Your service manual will show you where the drain bolt is on your bike. Typically, it is the bolt on the front of the motor near the header pipe. Some bikes have two, three, or even four drain bolts. Also, your bike is probably equipped with an air bleeder bolt located on your water pump.

Take off your radiator cap then find a container that will hold the entire amount of antifreeze/coolant currently in your radiator.

Loosen the radiator cap and place a container under the water pump to catch the old antifreeze/coolant. Then remove the air bleeder bolt located on the water pump or near the radiator cap (check the service manual for exact locations of components). Remove the drain bolt and drain that nasty brown/green goo out of your beloved bike. Remove the air bleeder bolt too. Next, put the bolts back in and fill the radiator with water. Then remove the bolt again and drain the system again. Repeat this process until the water coming out is as clear as it is when it goes in.

With all the bolts removed, you should be able to simply let water run through the system, cleaning out all the sludge and grime inside your engine's water jacket. When you feel good about how clean the water is coming out of the bike, drain the system one last time. We rcommend using distilled water but you could go through five or more gallons depending on the condition of your antifreeze/coolant. Don't scrimp on the distilled water, most grocery stores have five gallon jugs of distilled water for just a couple of bucks. All those minerals and added chemicals and hard water won't do your cooling system any favors. If you have access to compressed air blowing air through the system is great for pushing out old, muddy coolant.

You also want to clean out your overflow bottle. It could have a thick level of sludge and grime in the bottom, and you don't want that to get into your freshly cleaned radiator. The complexity of this task is dependent on the reservoir's location. Some bikes allow for easy removal of the bottle, while others can be excessively hard to get to. If you remove it, you can either wash it out in the sink or if you don't want to remove it from the bike, use the same method you used for the radiator. You'll need to disconnect the hose that runs from the neck of the radiator to the overflow first.

filling coolant

A 50/50 Mix is best

Next you want to premix your coolant with distilled water. Your owners manual will have guidelines for mixing ratios depending on what temperatures you plan on riding in. Remember, water actually cools the most efficiently but provides very little lubrication and no protection from the cold. Pure antifreeze has great lubrication and cold protection but does little for cooling. Water Wetter provides no protection from the cold.

A 50/50 mix is a great rule of thumb (50% distilled water and 50% antifreeze/coolant). It's a good idea to you mark your container after it is mixed. You might not remember which container is which the next time you change your antifreeze/coolant.

While pouring the fresh coolant into your system, remove the air bleeder bolt to alow all the air to escape. Then start the engine and let it run until everything gets hot. The bike has to get warm enough to open the thermostat and allow any trapped air to escape through the radiator or the air bleeder bolt. If you do not get all the air out of the cooling system it prevents efficient operation and you will get hot pockets of air inside your engine causing higher operating temperatures. It can also reslut in cavitation that in extreme cases can damage the engine.

Once you feel confident about getting the air out of your engine, you can shut the bike down and move on. Use a little more of your premixed coolant and fill your overflow bottle to the "FULL" position. You'll notice that the full position is only about a third of the way full on the overflow bottle itself. This is to allow for expansion of coolant (overflow) as well as let the system draw more coolant into the radiator if it needs it.

Be sure to check with your service manual to be aware of any special needs your bike may have. The Honda VTRs have two radiators and this changes some details. Also, some motorcycles, like Triumph, are very susceptible to air pockets in the cooling jacket as well as degradation of factory coolant. I think every Triumph should have the factory coolant flushed out of the bike within the first year of ownership - regardless of mileage.

Now all you have to do is put everything back together. Make sure not to forget the overflow hose from the overflow bottle to the radiator inlet. When everything is back together, I like to wash the bike. This will remove any spilled coolant from the finished surfaces like the wheels or paint. A clean bike is a pretty bike! Also, be aware that you didn't get out all the air in the system so the bike will typically belch air into the overflow tank and draw cooling back into the radiator. Keep a close eye on your overflow and coolant levels for the next few hundred miles until everything stabilizes.