Cooling - Controlling coolant temperature 

The introduction of the Cooper S proved to be a testing time for the Mini’s systems, but conveniently provide a guideline as to what the standard cooling system was capable of - that used on the ‘S’ was marginal to say the least! It wasn’t uncommon for many S’s to spew coolant from their overflow pipes when ever it was doing anything other than a steady 70 miles an hour, over-heating eventually caused through coolant loss. Perhaps some deductions can be made from the following… 
There are a number of elements involved in controlling coolant temperature. Some confusion over what to sort first when over-heating occurs leads to wasted time and money, and possibly terminal engine damage. Maximum power is usually generated from A-series engines at 70 – 75 degrees C (160 to 170 degrees F). The main problem with this on a road car is the oil is unlikely to get hot enough for maximum performance – the results outlined previously. Another being that the heater (where needed) will be grossly inefficient. So, excluding race-cars, the optimum temperature to aim for is 85 to 90 degrees C (185 to 194 degrees F). 
Radiators. No amount of tweaking the rest of the cooling system will help if there simply isn’t enough cooling capacity in the radiator. Coolant capacity used to be the answer, hence the production of four-core radiators. It is possible the improvement in cooling was a product of more surface area created by the extra tubes, but the inefficient airflow through the congested radiator area reduced its ultimate effectiveness. In reality, effective surface area is the answer, and why the latest after-market, super-efficient two-core radiators are the best. The standard radiator can just about cope with a standard engine in most cases. The exception appearing to be the fuel injected cars. They will stand the limited modifications that can be made without problems. Perhaps it is the ECU compensating for it somehow in trimming ignition and fuel? If you are significantly increasing the power output, I strongly advise fitting one of the aforementioned two-core radiators. And ALWAYS have coolant flowing out of the heater tap take-off. If no heater or auxiliary matrix is used, plumb it into the top hose. If it is put back into the bottom hose it will not work properly, if at all. The coolant temperature going into the bottom hose at that point MUST be below that in the main hose coming out of the radiator to give effective cooling. However, the rule of 'hot always goes to cold' suggests putting the hot water back in to the bottom hose will actually increase cooling system effectiveness. 
I'd like a pound-sterling for every time I've seen an oil cooler doing duty as an auxiliary coolant radiator. They simply do not work at all efficiently. Their design makes them grossly inefficient as the coolant flows through too quickly, and material spec causes minimal heat transfer. If you need to run an auxiliary radiator, use a heater matrix. See 'Cooling - How it works' for hook-up details. 
An expansion tank could be the answer if your motor runs at the right temperature, but is prone to spewing coolant out the over-flow at odd occasions. Usually when coming to a standstill after tramping-on a bit. Coolant passes into it when over-flow occurs when hot then is drawn back in again when cooling down. Make sure the pressure cap is fitted to the expansion tank and a flat, plain cap on the radiator. Unless the pressure cap is of the latest valved design with the return valve in it, in which case the pressure cap goes on the radiator and the flat cap on the expansion tank. 
Fans. They are there to cool the engine whilst at low speed where air speed is too low to cool efficiently. The air flowing through the radiator is at maximum efficiency at around 35mph. Any faster than this, heat transfer from rad to air is compromised. Having the fan blade there all the time tends to help regulate the air flow at this speed. Electric fans only help up to about 30 mph, so fitting one won’t cure hot running at speed. Trimming the fan blades down in length (NOT removing blades) can sometimes help in climates with high ambient temperatures. Generally the standard plastic fans are the best all-round as they are aerofoil shaped, cutting power consumed, increasing airflow, and quietest running. Two-blade fans are good but noisy, four-blade fans made up of two two-blade fans more so of each. Six-blade ‘export/tropical’ fans better, but noisier. 
Coolant pumps. One good thing that came from ‘S’ development – an improved coolant pump! Unfortunately, the coolant pump has fallen into the oil pump syndrome – biggest is best! True for road cars spending most of their time at low-ish rpm under load, but not for high revving engines. The A-series pump is essentially centrifugal, its pumping capability squaring with engine rpm. The design is such that maximum efficiency is around 2,000rpm, so at low speed it is hardly moving any water. At 2,000 rpm it is pumping all the coolant needed to cool the engine, so higher rpm just means it is sapping power. If your engine spends all its time north of 3,500 rpm or so, a deep impeller pump is costing power, and may be causing cavitation, reducing cooling efficiency. To mediate the A-plus motors got a bigger diameter pump pulley (first seen on the Cooper S), and should be used where possible on modified road engines. 
I would very strongly advise against the use of the after-market coolant pumps that have the 'folded tin' impeller as opposed to the cast iron one on the original equipment types. They are inefficient and have a tendency for the impeller to fall off at the worst moment! There are some about with plastic impellers. They seem OK, but I haven't put one to test on a race motor yet. All I can say is I have not seen a road car with one fitted that has failed yet. 
Recent testing has seen the growing popularity of electric coolant pumps. These have to be the ultimate answer. Their pumping capacity remains constant as they are completely independent of engine speed. Consequently cooling efficiency is far greater. The only two drawbacks being their initial cost, and installation, as adaptors have to be made up to blank-off the coolant pump mounting hole. Both, however, are well worth it - the results are outstanding. Not to mention the fact the coolant pump consumes power to drive it and reduces accelerative power output - to the tune of 4 bhp on a small-bore engine and 2 bhp on the large-bore ones! A further benefit is that the pump can be left running with the engine shut off after a race/hot/long journey to reduce the problems associated with the 'heat-sink' effects of non-circulating coolant at stand-still. For further information on electric pumps, see relevant article. 
Coolant additives. Too many folk seek solace in antifreeze. They keep adding more and more in the hope it will solve their problems. Whilst a small amount of antifreeze does help to reduce corrosion and lubricates the coolant pump seals, in large amounts it dramatically reduces the cooling capability of just water alone (see 'Cooling - How it works' for further information). 
The only additive I have ever tested that actually lives up to expectation/recommendation is Redline's Water-Wetter. This stuff basically breaks down waters surface tension without affecting its cooling capability. This maximises waters wetting capability, getting as much coolant against the metal surfaces of the water jacket as possible. Consequently it prevents the hot-spot syndrome outlined in 'Cooling - How it works'. I always use the liquid product (they do it in crystalised form too, but I am not so keen on that). Temperature reductions in the order of 8-10 degrees C have been experienced. Brilliant stuff. It also acts as a corrosion inhibitor - effective enough to stop ALL corrosion on the block coolant jacket walls, and the coolant pump impeller/housing. Lubricates the coolant pump seals too. Most impressive. For the racers even more good news is it does not make your slicks slippery if it gets out of the cooling system. 
For further cooling information, see 'Lubrication - Temperature critical'. 
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 coolant pump with by-pass hose take-off 
GWP187 Large impeller coolant 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