Cooling - How it works 

Engine swops or modifications are amongst the top few in the Mini owners list of desires for their car. Little thought or consideration is given to the cooling system when either of these up-grades is carried out. Largely because very few understand just what the cooling system does and how it does it, and that shortfalls in compatibility between the cooling systems capability and the power output of the engine can spell disaster for the new engine. All this is obviously exaggerated in the case of racing engines. Questions along these lines are popular - in most cases too late to be of use, so a little explanation should go a long way... 
Cooling system functions 
The internal combustion engine as used in cars is not particularly efficient. Burning a fuel/air mixture produces energy, but because this method of energy produces high levels of heat, much of the energy produced must be dissipated. This is essential to prevent component failure through thermal fatigue. The components most susceptible to failure in this manner are the pistons, piston rings, cylinder walls, cylinder head, valves and associated parts; although excessive heat will eventually cause more wide spread failures. The energy/heat level is regulated by the cooling system, passing into the coolant from the combustion chamber in the head, and partially via the cylinder walls to the radiator then to atmosphere. 
The combustion chamber area must be cooled sufficiently to prevent pre-ignition and detonation, problems that are exaggerated by current low-octane, unleaded fuels, and the ever-tightening legislation on lower emissions and lean burn engines. Fortunately the latter does not affect the venerable A-series engine, although those seeking to maximise fuel economy should take note. If an inefficient or inadequate cooling system is used, further losses will be experienced. The higher the combustion chamber temperatures are, the more the ignition has to be retarded to avoid the onset of the aforementioned pre-ignition and detonation. This causes a reduction in engine output; particularly torque that is the mainstay of driving the car. Further torque losses are caused when an engine is running too hot by increased inlet temperatures, creating a less dense fuel/air mixture. 
Heat dissipation and temperature control are regulated by the cooling system. A thermostat is fitted to keep the temperature constant and consistent at the required level. Heat dissipation is largely by thermal conductivity. The coolant passes over the hot metal surfaces inside the cylinder head and water jacket around the cylinders where heat is transferred to it as it is at a lower temperature. The coolant then passes into and through the radiator where the heat is passed into the cooler air. 
The coolant explained 
Water is the most common form of coolant used in car engines. It has excellent heat transfer properties in its liquid state and does an extremely good job when properly controlled. It does have one or two shortcomings though. The worst from a cooling point of view when not controlled is it has very high surface tension - the thing that allows bugs to walk around on it without sinking. 
This surface tension limits its ability to ‘wet’ the metal surfaces of the water jacket, forming a sort of barrier. Because of this, hot-spots can be caused - particularly around the combustion chambers where temperatures are highest. These hot-spots form vapour bubbles by boiling the water despite the fact that the bulk of the passing water is well below boiling point. The bubbles formed on the metal surfaces then act as an insulator around this area, greatly impeding heat transfer. This in turn reduces the cooling systems efficiency, thereby increasing the combustion chamber temperature. 
The eventual result is component failure, the piston usually being the first to go, or maybe the spark plug, then the exhaust valve, inlet valve, and so on. The speed at which this happens can be alarmingly quick, and is governed by the severity of the hot-spot and the dynamic loads on the engine (i.e. foot hard down = max. load = blisteringly quick melt down if there is a hot-spot present). 
Anti-freeze is widely used as an additive to water in car cooling systems, and is indeed essential where freezing temperatures are to be experienced. It also raises the boiling point slightly, as well as providing some lubrication for the coolant pump seals and reduces the formation of rust on the iron surfaces. The reduction of corrosion helps prevent blockages in the radiator. It does not, however, increase the cooling capability of the system. Many people are under the false impression that adding more anti-freeze will solve over-heating problems - nothing could be further from the truth. 
No more than is absolutely necessary to provide sufficient protection in the environment in which the car is used should be added. Follow the manufacturers instructions to the letter. Although as standard all road cars have a larger cooling capability than is required to allow for a fairly strong anti-freeze/water mix, bigger or more powerful (tuned) engines will soon render it inadequate. 
Water, as previously mentioned, has amazing heat transfer properties, far better than almost any other liquid cooling medium within a vast majority of spheres. It is certainly superior to a mix of anti-freeze (usually glycol based) and water. In fact water has up to two and a half times greater thermal conductivity to, say, a glycol-type coolant given the same operating capacity. As the cooling system works by conductivity - from hot metal to a cooler liquid (as in the engine coolant jacket) then from hot liquid to cooler metal surfaces (as in the radiator), the coolants thermal conductivity is of ultra importance. Tests carried out by major motor manufacturers have concluded that the improvement of glycol’s thermal conductivity is practically directly proportional to the amount of water added to it. Just to illustrate this, a 50/50 water and glycol mix has about 70% of the thermal conductivity of water on its own. 
To labour the point so that you are left in no doubt about this, other factors such as the viscosity of the coolant, and the convection coefficient of the coolant in a tube (a complex relationship between the thermal conductivity, viscosity, tube diameter - as in radiator core tube - and turbulent flow of the system) influence the effectiveness of the system. A 50/50 glycol/water mix has roughly four times the viscosity (thickness) of water alone and, as previously mentioned, about 70% of the thermal conductivity. A trial using these factors established that this mix had approximately 50% of the convection coefficient of water only. Or to put it in English, water on its own as a coolant is capable of TWICE as much heat transfer as the 50/50 mix. Hopefully this has exploded the ‘more anti-freeze will help’ myth once and for all. 
Capability Improvement Options 
So what can be done, and when is it needed? The last thing you need to do is install your mega-hyperpower engine with a cooling system that is a wild guess at best, to find that it is woefully inadequate, causing the early demise of your pride and joy. To give an illustration of the standard systems capability, even the Cooper S having its radiator with increased ‘gills per inch’ would over-heat at anything but a steady 70 mph. To all intents and purposes, if you put a 1275 engine in where there used to be a 998, put an up-rated radiator in as well. The standard 998 radiator will cope with the application of a stage one kit, but going to a decent modified head and fast road cam will sorely test it if it is in any other condition than A1 perfect. 
Old thinking used to be that more coolant is the way to go, hence the appearance many moons ago of the special four-core radiator. It certainly helped, giving about 23% more cooling capability over the standard item. But technology moves on a-pace, and the ‘more coolant’ theory soon bit the dust. The latest breed of radiators only have two cores, but a vastly superior core and fin arrangement, giving around 37% more cooling capability than standard. In fact this type of radiator has been sufficient to cool engines fitted with turbos and having outputs of around 160 bhp. Fitting one of these to any normally aspirated A-series should kill the problem dead. After all, it is better to have too much cooling than too little - you can always blank some off, or fit a different thermostat. A bonus here is that it actually weighs less than the standard set-up, every little saving in weight helps - especially when racing. If it were for any kind of racing in the dirt, I would not fit the two-core radiator. Its gills are easily damaged and clogged by clods of mud. Use the four-core. 
Back to thermostats for a moment. It is common practice to remove this and fit a blanking sleeve in a bid to improve cooling. If this is done, you must blank off the by-pass hose, otherwise stagnant areas of water will occur causing the dreaded hot-spots. However, the danger with fitting a blanking sleeve is that the engine may not reach proper operating temperatures, and this can be every bit as bad as running a little too hot. And the old style blanking sleeve (11G176) is not at all efficient. I would strongly advise using a thermostat in ALL road cars, of at least 82 degrees to make sure the correct running temperatures are achieved. A blanking sleeve is not the answer to over-heating problems. I always run a thermostat in my race engines unless bound for foreign shores where high ambient temperatures are experienced. Many folk think that they have to fit a blanking sleeve if they are blanking off the by-pass hose. Not so. Blank off the troublesome by-pass hose then fit a thermostat that has had six or eight eighth-inch holes drilled around the periphery. These holes allow coolant to circulate before the engine is up to temperature and the thermostat opens. 
Fitment of an auxiliary radiator will help if the two-core is not enough - say on a race or rally car. Use the matrix out of the heater box, and plumb this in going from the heater tap take-off, into the back fitting of the matrix, then out of the front fitting and into the bottom hose. Mount the matrix behind the grill for maximum benefit - around fifteen degrees temperature drop can be expected. If you pass the water coming out of the heater tap take-off down the front of the matrix first, you will be blowing hot air across the water going back out of the matrix and into the engine. It is important to know that not taking coolant out the heater tap take-off will increase the temperature that the number four cylinder runs at substantially due to reduced flow around that chamber. Some folk make the mistake of taking the coolant out of here and connecting it back to the bottom hose. This is putting un-cooled coolant straight back into the engine. If you do not want to run an auxiliary radiator or internal heater, plumb the hose from the heater take-off into the top hose. This is the least that should be done. 
Further assistance 
Ensure you always use the coolant pump with the deep impeller. These are fitted to everything as standard these days, but 850/998/1098 engines before about 1975-ish had the old shallow impeller type. The shallow impeller protrudes from the gasket face by 7.9mm (5/16”) and the deep impeller by 15.75mm (5/8”). All Metros also have the by-pass hose blanked off in the casting, as do the very late Minis. The exception to the rule here is the 850, there is rarely enough material in the block to be able to run these. If fitting to an old 998/1098 block, it may be necessary to grind some of the cylinder wall away to clear the deep impeller. To help engines that will be run mainly at high rpm, use the Metro 1275 large diameter coolant pump pulley (4.725” diameter), as this will slow the pump speed down, reducing the onset of cavitation. 
There are a couple of alternative fans available. The old two-blade type (that is usually run doubled up to make a four-blade), or the six-blade export type fan. I am ignoring the old metal multi-blade type, as they are not generally available and not that good. The four-blade is very noisy but very good, the six-blade much better than the standard plastic one, but a little noisier. 
Apart from this, make sure your hoses are in good condition, and you have the right hose for the right engine, particularly when going from a 998 to a 1275 based engine. The top hose is very much different - the 998 looking like a boomerang, the 1275 one shaped like a question mark. Using the 998 one on a 1275 will put a kink in the hose that will cause a severe restriction. It will also be necessary to change the top radiator bracket. This is caused by the thermostat housing pointing sideways on the 998 and forwards on the 1275. The Cooper S top hose and bracket, or 1275GT versions, are the ones to use. 
This is a basic introduction to cooling systems and various well-tried solutions. For further information, see 'Cooling - Controlling water temperature' under the cooling section. 
Useful part numbers: 
12G617 Cooper S top rad bracket - 1275 engine in round-front Mini 
12G617S Stainless steel version of above 
11G227 Grommet for above - 2 needed 
11G228 Shouldered bolt for above - 2 needed 
12G2453 1275GT top rad bracket - longer than S type, uses above 
grommets and bolts 
11G176 Thermostat blanking sleeve 
GTS102 74 deg-C/165 deg-F thermostat 
GTS104 82 deg-C/180 deg-F thermostat 
GTS106 88 deg-C/192 deg-F thermostat 
GTG101 Thermostat gasket 
GWP134 Large impeller water pump with by-pass hose take-off 
GWP187 Large impeller water pump, blanked off by-pass hose take-off 
GUG705555GM Coolant pump gasket 
CAM6239 Standard coolant pump pulley - 3.875-in dia. 
CAM116 Coolant pump pulley - 4.265-in dia. 
CAM6408 Coolant pump pulley - 4.725-in dia. Latest A+ large diameter version 
of S iron pulley (12A667) 
GCB10813 Fan belt for CAM6239, up to 1985 
GCB10825 Fan belt for CAM6408 with latest A127 alternator 
GCB10838 Fan belt for CAM6408/12A667, dynamo or alternator 
2A997 Two-blade fan - use two for four-blade 
2A998 Six-blade export/tropical fan 
12G1305 Eleven-blade plastic fan. 1.100-in wide at tip, up to 1968 and 1991 on 
12G2129 Eleven-blade plastic fan. 1.500-in wide at tip, 1969 to 1990 
12A312 Fan blade spacer shim - as required 
ARP2000 Standard, modern 3-core radiator 
GRD974 Latest aluminium and plastic made front mounted radiator from Tpi 
Coopers. 3-in thick, 11-in tall, 178-in wide. Side outlets and mounting 
Lugs top and bottom. Light weight, small, but very efficient. 
C-ARA4444 4-core radiator 
C-ARA4442 Super-cool 2-core radiator 
C-ARA4443 Super-cool 2-core radiator with temp sensor fitting