Cooling system - Honda CX and GL

All of the Honda CX and GL engines feature liquid cooling. For more information about the cooling system check out the articles below!

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The CX500 and all other variants are water cooled; as such, they have a radiator and a fan. All variants of the 500 (CX, GL, Sports) have a mechanical fan driven by the camshaft, while the 650 models featured a thermostat-controlled electric fan. We have seen one CX400 Custom with an electrical fan. The mechanical fan is attached to the end of the camshaft, and spins at the same RPM as the camshaft - which is half the speed of the crankshaft/engine RPM.

Honda CX500, GL500, CX650, GL650 Cooling system overview

The flow of coolant is driven by the water pump impeller, which is on the rear side of the engine and is attached to the end of the camshaft. This means that it spins at the same RPM as the camshaft, which is half that of the crankshaft. When the engine is cold, the thermostat is closed which allows the coolant to circulate primarily through the engine's water jacket with only a small amount flowing through the bypass hose to the radiator. Once the coolant heats up, the thermostat opens and the coolant is allowed to flow through to the radiator.

The radiator cap has two functions beyond simply preventing coolant from escaping out the hole - it contains a relief valve and a vacuum valve. The relief valve opens when the pressure inside the cooling system exceeds normal levels, allowing coolant/vapour/air to escape into the overflow tank. The vacuum valve works at the opposite end of the scale, where a drop in pressure (e.g. when the radiator is cooling once turned off) opens the valve and draws coolant back into the main system from the reserve tank.

In practice the vacuum valve does not seem to come into use very often; many riders report the overflow tank coolant remaining untouched, evidenced in some cases by the radiator being refilled with a different colour of coolant and the reserve tank colour remaining unchanged through months of riding. This may depend on the climate and manner in which the bike is being ridden, of course.



Honda specs the volume of coolant held in the main system as 1.8L, and the reserve tank as 200ml to bring the total system volume to 2L. Honda recommended a 50:50 mixture of ethylene glycol and distilled water.

The radiator and hoses will empty about 1.4L when drained; to drain the remaining ~400ml you will have to remove the drain bolts in the front of the engine, at the base of each cylinder.

Cooling fan

As mentioned above, the early CX500s had a mechanical cooling fan which is press-fit onto the end of the camshaft. For more information about the mechanical cooling fan, click here.

Cooling fan infopic - click here.

To see the procedure for removing the cooling fan, click here.

For information on converting a CX500 or GL500 mechanical fan to an electric fan, see this page.

See this page for info on the stock electric CX cooling fan.

Cylinder/block drain bolts, collars

There are two bolts on the front of the engine which you can undo to drain the coolant around the cylinders.

Mechanical seal

This sits around the crankshaft and keeps the coolant from getting past. See the infopic below.

Radiator Hoses

Radiator Cap

Overflow tank

This sits behind the engine - here's a photo of one from the front after the engine has been removed:

Photo of the coolant reserve or overflow tank on a Honda CX500, GL500, CX650, GL650 motorcycle

Temperature Sender

The temperature sender sits in the same housing as the thermostat and it's internal resistance changes as the temperature does; the following is a table of the expected Ohm reading as per Honda's spec and matching temperature:

Temp in C Temp in F Ohms
60 140 104.0
85 185 43.9
110 230 20.3





The thermostat is rated to open at 80 to 84 degrees Celsius (176 to 183 Fahrenheit) and is fully open by 93-97 degrees Celsius (199 to 205F). The valve lift is supposed to be at least 8mm at 95 degrees C (0.315" @ 203F).

The boiling point of the system with a 50:50 mixture of coolant and distilled water is 107.7C (226F) unpressurised, or 125.6C (258F) with the cap on and the system normally pressurised.

If the thermostat is faulty, your bike can have difficulty maintaining it's operating temperature.

Water pump

The water pump sits on the rear engine cover, attached to the back end of the camshaft. Around the camshaft sits an oil seal on the engine side, and a mechanical seal on the water pump side. Inbetween the two sits a weep hole, so that any leakages will flow out and alert you to the leak as opposed to having one escaping fluid contaminating the other.

One common fail point on CX500s is the mechanical seal; replacing the seal can be done with the engine still in the bike, though it is often done with the engine out of the bike at the same time as the cam chain and stator are replaced as part of the Triple Bypass procedure.

The water pump thrust washer is a 10mm washer, part number 90453-415-000 - it's used on a bunch of different vehicles and is often stuck to the old polo mint behind the impeller and can be thrown out. You definitely need one, and in case you have chucked it use that part number to buy another.

Water pipes

There are some pipes between each head and the thermostat and another between the water pump to the base of the radiator. The Turbos have a different way of hooking the water pipe up to the water pump cover;

Here's how the CX500 and CX650 Turbo connect the water pipe to the water pump cover

Thanks Joe Roggenbeck for that photo. More information to come.

Weep hole

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