Cooling system

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(Written by FADM Stern GNSF)

Coolant Systems Function and Overview

The coolant system in the bike is used to transfer heat from the engine into the air. In some bikes, this is done simply with fins on the engine (air cooled engine). While these engines can work well, they sure don’t like running continuously without air moving over them.

In the CX/GL bikes, the engines are cooled by a coolant circulating throughout the engine (liquid cooled). Liquid cooled engines are more efficient at heat transfer simply because liquids transfer heat far better than air. For an engine to operate properly, it must maintain its temperature within an operating range. It is vital that the cooling system is working properly to prevent damage or complete engine failure, as most heat-related damage results in catastrophic failure of moving parts.

The engine consists of many moving parts, with most of them involving metal-to-metal contact. We know that our oil reduces the friction of the junctions, as well as removes heat and debris to keep the parts cool and clean. The problem is the oil must be cooled to prevent thermal breakdown of the oil and to keep the oil functioning properly. This is where the cooling system comes in. It transfers heat from the oil to a coolant, then dissipates the heat to the outside air via the radiator.

For the cooling system to function properly, it must be a closed system, meaning it is isolated from the outside atmospheric pressure, and is able to create a positive pressure (higher than the ambient outside pressure). This positive pressure has the effect of raising the boiling point of the coolant (which is important as vapor is less effective at cooling than a liquid) and thus allows the system to function at higher temperatures.

If the cooling system does not function properly, or it has a poor heat transfer rate, the bike will overheat and if taken past a point can cause permanent damage.

The coolant system consists of several important sections:

  • Engine water passages and jacket
  • Water pump
  • Thermostat
  • Radiator and fan
  • Reserve bottle
  • Radiator cap

All of these sections must be functioning properly to keep the bike within its recommended operating temperature. .

Engine Water Passages


The engine is designed with water channels running throughout the engine, mainly located within the cylinder heads. Inside the heads the pistons move inside a “shell” (cylinder sleeve) that is surrounded by a void space (water jacket) that is filled with coolant. These coolant spaces are designed to suck heat from the engine and remove it to the radiator where it can be dissipated to the air.

The main problem that can cause inefficiency in this area is the formation of a scale coating on the walls of the passages. This scale (rust and/or calcium) buildup when present acts as an “insulating barrier” preventing the coolant from removing the heat from the engine.

Flushing/Descaling Your Cooling System

Water pump

The water pump is a simple centrifugal impeller that moves water through the system. When the system is running on a cold bike, it circulates water around the engine and distributes the heat, helping to warm up the engine uniformly. Once the system is at operating temperature and heat builds, water is diverted to the radiator through the thermostat.

Several issues can affect the proper operation of the water pump, mainly scaling (covered above) and impeller damage. For the impeller to work correctly, there must be only a small space between it and the case which has cast raised sections (guides) to help build the pressure differential between the water arriving and the water leaving. Damage to the case guides or impeller fins can cause the pump to become inefficient at moving water, thus slowing the flow rate. If the flow rate is reduced enough, the coolant will heat faster than the radiator can cool it, and the engine will overheat.

Mechanical Seal (Water pump seal)

Engine Out Seal Replacement (Triple Bypass)

The mechanical seal provides the "wet" part of the cooling system (coolant) to remain isolated from the mechanical parts of the engine. It resides behind the water pump impeller and consists of a spring tensioned ceramic disk in contact with another disk (polo mint) nestled in the rear of the impeller . The interface of these two parts provides the seal to keep the coolant out of the engine case and from leaking out of the system. As these two parts rotate against one another, years of non-use or contaminates may compromise this seal.

A provision has been made to allow any escaped coolant to flow to the outside, rather than be forced past the camshaft oil seal and into the engine. This is called the weep hole and is located on the left side of the engine, behind the transfer pipe. This hole should never be blocked or coolant will be forced into the oil, if the seal leaks.

Got a leak you can't find? Maybe it's the weep hole pushing coolant through from the mechanical seal. It's a safety precaution. If the coolant doesn't come out there it will push its way into the inside of the engine and mix with the oil.

The camshaft runs through a oil seal on the rear case. Next in line moving towards the rear of the bike is a open space. It is about 1/8" - 1/4" long and contains a hole to the outside of the engine case. This is the weep hole. Next in line is the mech seal outer cup that is also sealed into the rear case. Inside this cup is where the actual seals mate to each other. The first one is a rubber bellows with a spring inside that is sealed to the inside of the metal cup. On its rear end is a ceramic or carbon flat seal. The next item is a white ceramic disk (polo mint). It is encased on 3 sides by a rubber sleeve, with the open end mating to the carbon seal. It is essentially sealed to the inside of the impeller and turns with the impeller.

The impeller is sealed from the camshaft by the copper washer. That prevents coolant from migrating along the cam shaft and into the inside of the seal. The cup is designed with two ridges that will allow for any coolant that would pass thru either the seal itself, or the copper washer to enter into the space between the oil seal and the metal cup. From there it will dribble out the weep hole.

If the weep hole is plugged, the pressure of the cooling system will be great enough to force coolant into the open space and without a outlet to the outside, go past the oil seal and into the engine interior. So never, never seal up the weep hole, or you will guarantee coolant in your oil if there is any problem with the mech seal.

Removing the mechanical seal - engine out method

Installing the mechanical seal - engine out method


The thermostat is a temperature sensitive device that acts as a sealed blockage between the water pump and the radiator inlet, until the coolant temperature reaches a preset temperature. At this point the thermostat opens and allows coolant to flow to the radiator for cooling. The hotter the coolant is above this temperature point, the wider the thermostat opens.

The thermostat is important because the engine likes to operate at a specific temperature range (which is what we would consider hot, as engines don’t like running at body temperature). Not only does it allow water to be cooled by the radiator when it gets too hot, but prevents it from being cooled if it’s within the engines optimum temperature range. It also allows water to be held in the radiator for enough time to be cooled.

A faulty thermostat can cause an engine to overheat in two ways.

  • First, by sticking closed and not opening (or opening for too high above the desired point), causing the engine temp to increase too high.
  • Secondly, the thermostat can stay “open” allowing water to continuously flow through the rad.

So, what’s so bad about that? Well, what happens is the water doesn’t remain in the radiator long enough to be effectively cooled, and circulates back into the engine with only a fraction of the heat dissipated. After a while, the engine will overheat because the radiator has no time to cool down the coolant before it is pumped back into the engine. It’s a BAD idea to run without a thermostat.

Usable Thermostats

Radiator and Fan

The radiator is used to transfer heat from the coolant to the ambient air with the aid of a fan. It is a housing for the coolant consisting of many small tubes (cores) that are connected to an “accordion string” of thin metal strips (fins). As the coolant enters the top, it must pass through the many small cores to the bottom of the radiator. The fins which are attached to the cores do the heat transfer by radiation (heat from the metal surface will radiate into the air, heating the air and cooling the metal surface).

The reason the fins look as they do is they have a huge surface area, which speeds the heat transfer. Since heat transfer to air is poor at best (heat transfer coefficient), a fan is used to move the air over the fins at a high rate, keeping the air at the radiating site the same temp as the ambient temperature. This is the same reasons computers use heat sinks with fins (greater surface area) and a cooling fan (move the heated air out fast). Problems that can affect the efficiency of the radiator are many;

  • Scaling of the core (as previously explained)
  • Poor air movement over the fins (bad fan, restricted air flow)
  • System leak (for proper operation, the system must stay pressurized)
  • Clogged/missing/bent fins (reduces surface area for radiating heat)

Radiator Hoses

Here's a list of compatible aftermarket hoses that are readily available

Radiator cap

The radiator cap performs two functions.

  1. It provides an opening to allow the user to add and check coolant in the system.
  2. Controls pressure in the system.

When the pressure in the system gets too great, the radiator cap will open and “bleed off” this pressure (vapor and/or fluid) into a hose connected to the reserve bottle. This tube enters the bottle at its base and BELOW the level of the fluid in the bottle (bottle has min max fluid levels), which “seals” the system from the outside air. When the pressure in the radiator drops and creates a partial vacuum, the caps secondary valve opens and allows coolant to be drawn back into the radiator, which keeps the level constant.

Pressure Testing Your Radiator

Reserve bottle

The reserve bottle (also called overflow bottle) is used to keep the fluid level constant within the closed system. It serves a dual purpose dependent on the state of the system.

  1. During engine operation, this bottle collects fluid and vapor released by the radiator when the pressure exceeds that of the cap's main element. This keeps the fluid in the system so it can be reclaimed during cool down.
  2. When the engine is shut off and the radiator cools, the drop in temperature will generate an equal drop in pressure. If the radiator cap had previously vented some pressure, this cooling drop in pressure will pass the “normal mark” (atmospheric pressure) and create a partial vacuum. At this point, the secondary element in the radiator cap will be opened allowing coolant to be pulled back into the radiator.


There are many types of coolant, but what is required for these bikes is a mix of distilled water and Ethylene Glycol. Distilled water is prefered as it is free on minerals and salts that can contribute to scale buildup withing the cooling system. There has been debate if it should be silicate free coolant, with some saying it doesn't matter. Since the water pumps on these bikes are metalic non contact centrifigal impeller type pumps, there is no point in having a silicate additive. When it comes to coolant concentration however, it DOES make a big difference.

Ethylene Glycol itself has a poor heat transfer rate, but a high boiling point (197.3 °C), as well as low freezing point (-12°C, why its refered to as antifreeze). Water has a good heat transfer rate, but a lower boiling point (100 °C) and higher freezing point (0 °C). Things change when water and Ethylene Glycol are mixed, as can bee seen by the values below.

Weight Percent EG (%) Boiling Point (deg C) Freezing Point (deg C)
0 100 0
10 102 -4
20 102 -7
30 104 -15
40 104 -23
50 107 -34
60 110 -48
70 116 -51
80 124 -45
90 140 -29
100 197 -12

As can be seen above, the combination changes the freezing and boiling values of both, and this allows us to pick perfect mix that will best perform the cooling function. While higher values of Ethylene Glycol increase the boiling temperature, they do reduce the heat transfer rate, so a balance is picked at around 50% or 60% (with 60% being the standard recommended value). At this value, the coolant provides "antifreeze" levels sufficient for the coldest climates, while raising the boiling point enough to help in keeping the coolant in a liquid state. Because the system's normal operating pressure is higher than standard atmospheric pressure, the coolant can be kept from boiling up to values between 120 °C and 130 °C. The Ethylene Glycol also adds the extra benefit of acting as a lubricant and anti-corrosion compound within the system.

Changing Your Coolant

Pressure testing a radiator

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