The MGA, B and Midget are by far the most common classic MGs and all have very basic format cooling systems for their A, B series and 1500 cc engines, with similar patterns found with MGC, V8 and MG saloons. Importantly, none have historically suffered from any major cooling problems from any shortcoming in design, but many issues have arisen from lack of maintenance, original component degradation and sometimes less than ideal replacement components.
These basic systems consist of the engine that has internal passageways for the liquid coolant (usually made up of water and antifreeze) to flow through. A thermostat is present to control the coolant temperature at a predetermined lower level to maintain the best engine efficiency, and it is mounted in the cylinder head, opening and closing to control the coolant flow leaving the engine.
The engine is connected to a radiator mounted at the front of the engine bay so it can exchange heat the coolant has absorbed from the engine and transferred into the radiator with outside air that passes through it, before the coolant returns to the engine by the pumping action of the water pump, which is mounted on the front of the engine and driven by a belt that in turn is driven by the crankshaft pulley.
Most, but not all MGs also have a heater that has a separate coolant take off from the top of the engine where coolant will be hottest and this is directed through smaller bore pipes and hoses to a heater radiator (matrix). Coolant flow is controlled by a separate valve to provide heated coolant only when the driver wants it, and when that is demanded a separate electric fan boosts the airflow through the matrix to increase the heated air volume entering the cabin. Coolant leaves the matrix via separate pipes and hoses to return into the main coolant flow run returning to the engine from the main radiator.
When it comes to coolant, it’s worth noting that pure water has the best heat transfer efficiency, but using pure water will allow corrosion to occur quite readily, so engine coolant is ideally a mix of water and antifreeze between a ratio of 33% to 50% antifreeze. Interestingly, the main purpose of antifreeze is not to deter the coolant from freezing, at least not in any hot or temperate climate, but to provide anti-corrosion properties, which applies in all climatic conditions.
System Operation
When the engine is started from cold, then the coolant will be at the surrounding ambient temperature, whatever that is, which will always be below the opening temperature of the thermostat. MG thermostats will commonly have settings of 74 degrees C, 82 degrees C or 88 degrees C. Coolant flow will be restricted until the coolant temp reaches the thermostat's opening temp and from there it gradually opens, taking about another 3 to 4 degrees C before it reaches the fully open position. This delayed opening is simply to enable the internal engine temperature to be raised quickly to a level where it is working more efficiently. As far as the driver is concerned, they will see this via the temperature gauge in how much choke is required to keep the engine running smoothly.
Once the thermostat starts to open, even a small amount, quite a significant volume of coolant will be allowed to pass into the top hose and on into the radiator and during warm up the thermostat tends to fluctuate between open and closed until all the coolant has warmed. Most classic MGs have coolant flow that runs from top to bottom of the radiator, although post 1968 Midgets have what is known as a crossflow radiator that flows from one side to the other. The heater flow, incidentally, is completely separate from this main flow with its own separate control valve and heater radiator.
ARA318 - Q Parts, MG Midget 1275 Cross Flow Radiator
Once coolant flow enters the radiator, it will then feed into the ‘core’, which is the central part with the very many thin coolant tubes and very thin vanes to conduct as much heat from the coolant to be exchanged with space for air to flow through the radiator and pick up heat to reduce the coolant temperature. Radiators may have two, three or more rows of these tubes, depending on its designed capacity, and once the coolant has passed through these tubes, it collects in the bottom tank (opposite tank for crossflow) from where it moves into the bottom hose that connects to the water pump. The water pump is driven by the engine via the fan belt and its internal impeller pumps the coolant through the engine and continues to circulate it.
Q Parts Waterpumps with Cast Iron Impellors
One restrictive characteristic, rather than a fault, with many MGs up until around 1968 is the way in which the radiators are filled and the positioning of the fill point which is also the mounting position for the pressure cap. These earlier cars' fill point is offset (dog legged) to the rear of the radiator, described as a rear fill style. The issue that arises with these is that there is no view into the radiator core when filling, so there was no way of assessing how far down the radiator the coolant level was until it reaches the neck. Later radiator designs changed this to a central fill point that gives that view.
Many earlier MGs had radiators with rear fill points (circled) mentioned in the text regarding topping up.
Post 1968 MGB radiators were changed to have a central fill point (circled) allowing a view into the radiator to see coolant level.
The potential problem here is that both designs of radiator do not have any specific expansion room for the coolant as its temperature rises from ambient to normal running temp. With temperature rise comes expansion, so the heated coolant needs more space, and with no extra internal space for that expansion coolant is forced out of the system past the pressure cap and lost. When all is working well, only a small volume of coolant will be ejected and the coolant level will settle and remain constant.
The obvious problem with a rear fill if you want to confirm that the coolant level has reached a settled point, you can’t see that level, so owners will have to repeatedly top up, noting how much is added and only after several top-ups will they be able to judge if the level has reached a consistent level.
MG were obviously aware of this as the 1968 Mk2 MGB moved to a central fill radiator, but at the same time, they also introduced a much better solution for the MGC by adding an expansion tank. This simple solution sees the expansion tank with approximately a one pint capacity mounted near the radiator and connected to it via a small bore hose.
Later MGs moved to a sealed radiator along with an external expansion tank that made the cooling system a ‘no loss’ one that was more efficient.
The pressure cap on non-expansion tank systems will be mounted on the radiator, but with an expansion tank, it is now mounted on the expansion tank. The previous mounting point on the radiator is sealed with an unpressurised cap or the rad is slightly redesigned with a screw in filler plug that is now only used for topping up; but note only when the engine is cold. Now the radiator is fully filled along with a small amount of coolant in the expansion tank, leaving more than enough room in the tank for the expanding coolant to be pushed into without being lost, and crucially, it is automatically drawn back into the radiator when it cools so making the system a ‘no loss’ one.
Pressure caps are used to slightly raise the natural boiling point of the coolant to provide a wider margin from the normal range of temps that can be found in a fully working cooling system to that raised boiling point. Why this is important is that water at sea level boils at 100 degrees C, but for every 1000 feet of elevation that temperature drops by approx. 1.1 degrees C, so as driving at higher elevations is quite common, and reaching 5000 feet above sea level is a reasonable possibility then the boiling point will drop to 95.4 degrees C. Bearing in mind that for the car to climb the engine has to work harder and it generates much more heat so its operating temperature will be uncomfortably close to the lowered boiling point!
GRC111SS - Radiator "pressure" Cap 13lbs MGB R/B - Stainless Steel
Increasing the coolant system pressure seals the system at the increased operating pressure and adding a 5psi cap increases the boiling point by 8.5 degrees C to 108.5 degrees, and with a 15psi cap the boiling point is now approximately 125 degrees C. Older cars tend to use lower pressures, 4 to 7psi, whilst later systems (often the ones with expansion tanks) will be in the range of 10 to 15psi. It isn’t uncommon for owners to try and increase the system pressure to raise the boiling point of the coolant by simply replacing the cap only to find the replacement cap either doesn’t seal or doesn’t fit.
The reason is usually because there are two cap depths, a short reach and a long reach, with if a short reach is fitted to a radiator that needs long reach won’t seal, or conversely a long reach cap simply is too long to fit. Increasing the internal pressure of a cooling system will increase the stress on all components and especially hoses and today its worth noting that a greater increase in the coolant boiling point can be achieved without pressure increases by using special coolant such as 4Life.
Cooling fans on classic MGs up to the introduction of the MGB GTV8 in 1973 then the four cylinder MGBs from the start of the 1977 model year were mechanical items fitted to the nose of the water pump and driven by the fan belt. Depending on the market would also depend on the number of blades the fan would have, three up to seven. Usually these were made from steel, although plastic started to appear from the late 1960’s.
12H4744 - Seven Bladed Plastic Cooling Fan - MGB GHN5 & GHD5 (to Sept 1975)
The V8 had the forward-mounted radiator that left no room for an engine-driven fan, so introduced two smaller-diameter electric fans mounted in front of the radiator that were thermostatically controlled. The same position for the radiator was adopted for the 1977 model year MGB so for temperate climate markets, a single electric fan was offered whilst the same twin fan set-up as the V8 was the spec for hotter climate markets.
The water pump has a simple impeller on a shaft that sits in a chamber against the engine block, with coolant entering from the bottom hose. The other end of the shaft is connected to a pulley that is driven by the fan belt that gets its drive from the engine's bottom pulley. The operation of the water pump generates sufficient coolant flow to enable the engine's temperature to be maintained within the range of temps just above the thermostat's opening setting, irrespective of how much power the driver demands and what the outside ambient conditions are; that is, unless there are faults.
A ‘B’ series water pump with the fan belt drive pulley removed…
…that mounts onto the front of the ‘B’ Series engine block.
A removed ‘B’ Series water pump that has over time suffered corrosion eating away at the impeller (arrowed) that reduced the pumps efficiency.
The brass thermostat in its fitted position on a ‘B’ Series engine.
Typical failure found on a thermostat that doesn’t close fully, so lets too much coolant pass. See the arrowed gap.
New thermostats are widely available and for older engines, it is better to have one that has a built-in jiggle valve (arrowed) that allows air and steam to pass rather than be trapped behind the thermostat.
The heater is part of the cooling system and consists of a small radiator (matrix) within the car that is fed hot coolant that circulates from the top of the engine via a separate control valve. Separately, air is drawn in by an electrically driven fan that is blown through the matrix to transfer heat from it and move it into the car or against the windscreen. Coolant will exit the heater and return to the engine on the return side of coolant flow from the main radiator back to the engine.
Coolant choice is important and a base fact is that the best medium for transferring heat is pure water, but of course that comes with some associated negatives, primarily corrosion. To counter this and reach a balanced position which serves the needs of efficient cooling and minimal corrosion sees the addition of anti-freeze that has a much more important function beyond preventing freezing. This is because it contains anti-corrosion inhibitors, but over time the effect of these will diminish and why there are specific service change intervals for draining, flushing and renewal of the coolant that sees different time limits depending on the type of coolant.
Different types of coolant exist and today starts with two fundamental standard types that should never be mixed as the result is generation of sludge that can create internal blockages. Firstly the traditional Ethylene Glycol coolant often green or blue long associated with classic cars. However, around the turn of the century mainstream car makers changed from the traditional coolant to OAT coolants. OAT means Organic Acid Technology and this is not recommended for classic engines as some of the materials in our classic engines and the parts where coolant touches may suffer damage over time. So the first clear recommendation is to use a NON OAT coolant in our classics.
It is also very important to realise that both types usually use Ethylene Glycol as the antifreeze element, so you have to specifically read the containers labelling to note the coolant type. (The author has very good experience with 4Life coolant in his classic MGs for nearly thirty years now and it should be noted that 4Life is a NON OAT long life, high boiling point coolant developed for classic engines.)
4Life Radiator Coolant available in 1L, 5L and 20L containers
This author also remains wary of waterless coolant (Note; 4Life is water based) as its viscosity is greater than a water based coolant and this slows the rate of flow and also slows heat exchange from engine to coolant and coolant to radiator, resulting in the engine taking longer to remove heat and so run slightly hotter. It is also noteworthy that some engine rebuilders/tuners have had bad experiences with some of their customer engines repeatedly suffering overheating issues when using waterless coolants and as a result they will no longer offer warranty if waterless coolants are used!
The other issue is the high cost of waterless coolants and the need for the whole cooling system to have all existing water based coolant removed before adding the waterless coolant, plus, if there is a leak whilst driving and spare waterless coolant is not available for top up and water is added to get you home, then this will demand another expensive exercise in draining, drying and refilling with fresh waterless coolant.
To enable the driver to monitor the engine's temperature, there is a coolant temperature gauge on the dashboard. Up to around 1976 (except oddly for the North American markets MGC) these gauges operated using a sealed gas within a metal bulb that screwed into the engines cylinder head to be in contact with the coolant. It was then connected to the gauge via a very thin capillary tube where expansion and contraction of the gas as it was subject to the changing coolant temps was reflected in the gauge needle. These types have been very reliable.
Most earlier MGs coolant gauges are driven by an inert gas sealing into a capillary line between the bulb in the engine and gauge.
The capillary types were replaced in late MGB and Midgets with an electrically driven gauge that uses a sender unit in place of the bulb that provides an earth path for the electrically driven gauge, with coolant temp altering the resistance within the sender that was reflected in the position of the gauge needle. Unfortunately these have not been as consistently accurate as the capillary type but provide adequate coolant temp information.
The capillary type of temp gauge bulb (arrowed) sits below the thermostat so it ‘reads’ actual coolant temps within the engine.
Part 2 covers common cooling system faults
Please Note: The information provided in this blog is intended as a general guide only. While we aim to ensure accuracy, classic vehicles can vary, and procedures may differ depending on model and condition. Always consult your vehicle’s official workshop manual before carrying out any work. If in doubt, seek the services of a qualified professional.
Who are MGOC Spares?
At MGOC Spares, we provide dedicated parts and accessories for classic MG vehicles, including the MGB, Midget, MGC and many more.
With thousands of products in stock, we are your one-stop shop for all your classic car needs. Our wealth of expertise means that you can trust us to find the right part for your classic MG vehicle – whatever it may be.
Need support finding the right parts for your MG? Contact us today, we're happy to help!