I agree with tmsmith; if the temperature is increasing at faster speeds, I'd be suspecting an ignition timing issue, a lean mixture issue or combination of both.

Edit:  Speaking of cavitation: New Generation Waterpumps (wow - tested up to 10K rpm!)

I had some overheating issues last year when stopped in traffic, so I pulled my cylinder head last year, suspecting a crack.  When re-assembling the engine, I spoke with KC and decided to replace my water pump while access to it was best.  KC and I discussed the pros and cons of the small vs. large pulley and he sent a small pulley to me for me to try.  I cleaned it up and went to install it on the new pump - the GWP134 I'd received from MSC in July 2014 still had the cast iron impellor, the new ones weren't available yet - , the small pulley didn't fit the waterpump flange.  This prompted a call to KC and he measured a pump he'd received the same day and sure enough, there was a couple of mm difference in diameter.  So, a call to MSC, a new pump sent and the small pulley installed without a hitch.  Just my luck, I guess.

Since I installed the small pulley, the water flow through the system is indeed faster, but the impellor is decent in that there haven't been any issues and the engine does run slightly cooler.  

My conclusion is that it's worth considering using the small pulley also, and I'm tempted to buy one of these new-fangled MSC 'evolution' waterpumps to have on hand in the future.
 

The Van with 1275 that lost a head gasket and cracked both the block & the head after crossing the Mojave Desert in 2012 had an auxillary radiator which I supplied...but it was not mounted in the airflow effectively (not my decision) until AFTER the problems.  Mounted in the airflow in Prescott Valley, we got 1,800 miles to home without further issue (except the following engine build).

Bear in mind this Van was heavy...full length skid plate, roll cage, tools, spares, winch, long distance fuel tank etc. etc.  It needed extra cooling for the conditions we faced.  By comparison, my 1380 powered sedan (with 195F thermostat) had several advantages...aux rad in the airflow, oil cooler...and...an electric fan mounted in the inner wing, which I turned on when we stopped after crossing the desert to help address heat sink...and no problems.

Since Leyland Australia installed a second rad (inline) at the factory on the GTs, we should not be debating whether extra cooling capacity is a good idea for travel in very hot conditions.  By all means check everything then add the insurance.  Choose a better radiator and/or an extra rad or an engine build.  The choice seems simple to me.

In the Northern Territory and crossing Western Australia facing temperatures consistently in the 43-45 degree Celcius range, my loaded Van with 1098 had no overheating problems running on regular or opal fuel.  The GM pickup truck heater core and some hose was all the insurance I needed.  When we saw big diesel utes (mostly Toyota or Nissan) on a flatbed, with the caravans following on a 2nd flatbed, we could hear the sound of the cash register ringing up their tabs.  On a 70 day trip, most often many miles from any help, we could not afford to have a significant failure...and we didn't, with both Minis running extra rads.

No question parts are not what they used to be and even the major suppliers get caught out at some point.

But back to the issue:

My mini has the side mounted radiator, and the Texas heat is a little much for it, especially during highway cruising (first car I've had which gets hotter the faster you go).  So, I'm thinking of adding a small front mounted auxiliary radiator by redirecting the coolant flow from the cabin heater core into a smaller front mounted radiator that would help out at highway speeds (and hopefully avoid me having to add a fan).  Problem is I can't find a good small antifreeze radiator that I can easily fit up front.  I can easily find small oil coolers however.  Question is, can I run antifreeze through oil coolers, or would there be corrosion issues, etc. 

I would check to make sure everything is in order.

If you are increasing temps at faster speeds, it seems something is not right. At speed there should be plenty of cooling capability. An aux fan will not help at speed as has already been mentioned. How many RPM at speed? The cooling system should ideally reach a steady-state at some temperature and speed combination.

If you turn on the heater does it resolve the issue? or does it start to creep back up again? I am not adverse to installing an aux radiator to manage cooling in extreme heat. I would think in texas you could get away with minimal anti-freeze and watter wetter, but that would depend on where in Texas you are.

True words but you would not believe how many radiators i have run across that are cheap knock offs and weigh about 1/2 what a good one should, you can hold them up and look throgh them and tell the difference by the amount of cooling fins. I did it myself many years ago and bought a cheap radiator that always ran hot on the highway, replaced it with a good one and no problems.

It seems that there are always a number of cooling threads during the summer.

If you approach it from a system point of view, you need to manage all the variables. Unfortunately the variables are different for different systems(engines).

As with any engine component, all the parts must be installed properly and working properly.

I did stupid things in my younger days like removing a thermostat and not installing a blanking plate/disc. I also stuck an Awful American radiator in a car and clipped the front shroud to make it fit, all for 8-16 oz more coolant. If I had made sure all the components were installed properly and working properly I would have saved myself some time and an engine rebuild.

The only time I really had an issue was after a several hour driving session in heat through the California central valley and then getting slowed down in Sacaramento or Bay Area traffic. An aux electric fan would have helped that.

Terry

X2 Mini14. The only thing i would add is that a 180 thermostat would be more suitable for road use and there is no need to drill the hole in it if you have the spi sandwich plate or the small bore by pass hose.

Terry if the thermostat is not fitted the flow of the water becomes too fast and does not cool as efficiently so by having the thermostat installed it actually slows the flow down to aid in the cooling. Some folks who do not run a thermostat just run the outer ring of it for "somewhat" the same effect.

Mur agreed always run an overflow tank.

oil coolers and aux rads just heat up the air you are going to push through the radiator.

a 160°F thermostat is not any more open at 200° than it was at 160°.

An expansion tank is essential.

cutting the plastic fan blades shorter will result in overheating at highway speeds in heavy winds. The area between the shroud and the blade end becomes a huge mess of air vortices.

jwakil, I am going to come at this from different angle. While the advice and theoretical explanations of the mini cooling system and coolant in general has been explained here and appreciated, it has been my experience that most cooling issues with minis are related to either bad components, or improper assembly. I have run my race car in 100 + heat at AAA speedway several times and it does not get over 205. And it takes time to get there. I run a super two core radiator, water with redline water additive, a cooper s pulley, stock water pump, plastic multi-blade yellow fan, a 160 T-stat with a 3/16 hole drilled in it, and an overflow bottle. Thats it. No exta radiators, no electric water pumps, ect.

All I am saying is check all of your components first to make sure there is not some issue like the fan being on backwards, bad T-stat, crap radiator, ect. Also, for the stock mini cooling system to work properly the inner fenders need to be in place so the air coming through the grill gets pushed through the radiator.

I don't mean any offence to anyone. Its just that I have spent lots of money over the years with various cars trying to cure problems. Most of which could have been solved by starting at the beginning and making sure what I had was working properly in the first place. 

Good luck. 

If people don't want to check the basics like if a thermostat is working or not, they will probably have bigger problems than overheating.

so I will amend this:

Once a system hits operating temperature, a properly functioning thermostat has no impact on cooling or flow.

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 water from their overflow pipes when ever it was doing anything other than a steady 70 miles an hour, over-heating eventually caused through water loss. Perhaps some deductions can be made from the following…

There are a number of elements involved in controlling water 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's 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. Water capacity used to be the answer, hence the production of four-core radiators. It's 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's 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'll stand the limited modifications that can be made without problems. Perhaps it's the 'brain' compensating for it somehow in trimming ignition and fuel? If you're significantly increasing the power output, I strongly advise fitting one of the aforementioned two-core radiators. And ALWAYS have water flowing out of the heater tap take-off. If no heater or auxiliary matrix is used, plumb it into the top hose. If it's put back into the bottom hose it won't work properly, if at all. The water going into the bottom hose at that point MUST be below that in the main hose coming out of the radiator.

I'd like a pound-sterling for every time I've seen an oil cooler doing duty as an auxiliary water radiator. They simply don't work. Well, they do a little. Their design makes them grossly inefficient as the water 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 water out the over-flow at odd occasions. Usually when come to a standstill after tramping-on a bit. Water passes into it when over-flow occurs when hot then is drawn back in again when cooling down. Make sure the pressure cap's fitted to the expansion tank and a flat, plain cap on the radiator.

Fans. They're there to cool the engine whilst at low speed. Fact. Once above 35 mph or so, it's airflow through the radiator that does the cooling. Electric fans only help up to about 30 mph, so fitting one won't cure hot running at speed. The fan creates a barrier to airflow at speed; trimming the blades down in length (NOT removing blades) can often help. 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! Both eat horsepower.

Water pumps. One good thing that came from 'S' development - an improved water pump! Unfortunately, the water 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; it's pumping capability squaring with engine rpm. The design's such that maximum efficiency's around 2,000rpm, so at low speed it's hardly moving any water. At 2,000 rpm it's pumping all the water needed to cool the engine, so higher rpm just means it's sapping power. If your engine spends all it's 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 Ss'), and should be used where possible on modified road engines.

I would very strongly advise against the use of the after-market water pumps that have the 'folded tin' impellor as opposed to the cast iron one on the original equipment types. They are grossly inefficient and have a tendency for the impellor to fall off at the worst moments! There are some about with plastic impellors. They seem OK, but I haven't put one to test on a race motor yet. All I can say is I haven't seen a road car with one fitted that has failed yet.

Recent testing has seen the growing popularity of electric water pumps. These have to be the ultimate answer, as their pumping capacity remains constant, as they're 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 water pump mounting hole. Both, however, are well worth it - the results are outstanding. Not to mention the fact the water 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 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'll solve their problems. Whilst a small amount of antifreeze does help marginally as it breaks down the water's surface tension (waters only real drawback as a major coolant), in large amounts it actually makes matters worse (see 'Cooling - How it works' for further information).

The only additive I've ever tested that actually lives up to expectation/recommendation is Redline's Water-Wetter. This stuff basically breaks down water's surface tension without affecting its cooling capability. This maximises water's wetting capability, getting as much water 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'm not so keen on that). Temperature reductions in the order of 8-10 degrees have been experienced. Brilliant stuff. It also acts as a corrosion inhibitor - effective enough to stop ALL corrosion on the block water jacket walls, and the water pump impellor/housing. Lubricates the water pump seals too. Most impressive. For the racers even more good news is it doesn't make your slicks slippery if it gets out of the cooling system.

TEMPERATURE RISING

Engine swaps 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’s 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 can happen 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 water 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 water 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 it’s 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 it’s 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 water 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 water’ 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. 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 water 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 water 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 water out of here and connecting it back to the bottom hose. This is putting un-cooled water 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 water pump with the deep impellor. These are fitted to everything as standard these days, but 850/998/1098 engines before about 1975-ish had the old shallow impellor type. The shallow impellor protrudes from the gasket face by 7.9mm (5/16&rdquo and the deep impellor by 15.75mm (5/8&rdquo. 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 impellor. To help engines that will be run mainly at high rpm, use the Metro 1275 large diameter water 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.

C'mon Willie - let it out, tell us how you feel ~

                                           what????  send some of that Ca. grass my way will ...

                               what if the t-stat is stuck and can't open all the way ....

There certainly is alot of confusion over what a thermostat does.

Once a system hits operating temperature, the thermostat has no impact on cooling or flow.

Terry

you guys need to read intelligently because i will only explain this once...

In essence, the rate of coolant flow really does not matter in an enclosed cooling system.  why? because thermostat regulates the coolant flow. all of you might be thinking that the thermostat only regulates the temperature, NO!  it primarily regulates the coolant flow then cooling of the fluid comes secondary.  so no matter how fast your water pump push water around the system, the flow is still regulated. it will only flow on a standard constant speed ALL THE TIME!! water will rush through as the thermostat valve opens in a few seconds then closes back when cooled down a bit.

volume of coolant flow only matters if you don't have a thermostat installed.

 //youtu.be/WOdZkSdNKSM

On the pulley size thing: yes. A hgh rpm engine could spin the pump impeller too fast, resulting in cavitation and sudden loss of pump efficiency. If you look at the impeller, it isn't very hydro-dynamic at all (not much in the cooling system is). Cavitation is where the impeller (or a propeller) spins too fast and the water behind it doesn't keep up and forms a small vacuum space or "cavity" behind it, that really messes up flow. With a small outboard motor, say 5 -10 HP, if you try to turn the motor sideways too quickly or run it too fast for the sped of the boat, the propeller will churn the water, thrsut is lost and you sometimes can actually see the 'bubble' of vacuum behind the blades.

Cavitation Video

I don't know much about rate of flow through radiators, but it seem logical that if coolant flows too fast, it won't have time to release its heat.

//www.calverst.com/articles/Cooling-Controlling_water_temp.htm

"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."

In another thread about cooling systems, I tried to explain to Boison the fundamentals of the Mini cooling system. (See below in blue.) To answer your question, yes the main rad and hoses have a parallel path conisting of the heater core and its hoses. As Alex pointed out in that other thread, it is better to have the heater circuit open to ensure some flow through the back end of the head and block. But since the heater system is carries about 1/12 the total flow, on typical cars in normal temperatures, nothing untoward happens to the engine if you drive with the heater circuit closed. If it did, Sir Alec Issigonis and British leyland would not have fitted the shut-off valve. As our Alex and others have mentioned, it you are stuck in traffic and your engine appears to be runing excessively hot, you can shed some of he heat by opening the heater circuit AND turing on the heater fan. If your car has this issue consistently, and you've emiminated all the other issues that cause excessive heat,  then consider adding an auxiliary cooling rad and/or an auxiliary electric fan.

A bit of clarification: you refer to "a sufficiently low pressure drop aux radiator". Engine cooling systems are only heat transfer systems - the engine puts heat into the coolant and the rad takes the heat out of the coolant. The is NO pressure drop as in a refrigeration system. The engine's water pump only moves the coolant around. It does not pressurize it in any way. The cooling system as a whole is very slightly pressurized by the rad cap, but that only serves to raise the boiling point of the coolant.  Pure water boils at 212F/100C at standard atmosphereic pressure. If you boil water for tea at higher altitude, your tea may not be as good because the water will boil slightly below the ideal temperature. In an engine, the netal around the cylinders is much higher than 212F/100C, so water would tend to boil there. But if the pump keeps it moving, the water doesn't get too hot. [Shut off a really hot engine and listen to it... you may hear the water beginning to boil inside the block, which is why some cars burp coolant after they are shut off.]

More clarification: As described below, the parallel system won't "rob" flow or coolong efficiency from the main system. It is relatively small and was taken into account when the car was designed. An auxiliary rad substitutes for the heater core to dump the heat outside the cabin. The net system has the same cooling capcity.

I hope this answers your questions.

You [Boison] are over-thinking it a little. The small hose (probably 1/2" = area 0.196 sq. inches) and the even smaller opening where it connects to the engine is much smaller than the main top hose (approx 1. 25" = 1.227 sq. inches) connected at the thermostat. The cross-sectional area of the rad hose is about 6 times the heater hose. Therfore the proportion of coolant flow through the heater hose is 1/6 that of the rad hose. In a typical car with a heater, the longer 1/2" hoses and the heater core also add resistance to flow, so the effective flow rate is maybe 1/12.  Add into that the fact that the engine designers would have taken into accound this alternate coolant circuit when figuring out the thermostat selection for opimum operating temperature. They also wanted flow across the the head through its coolant passages to ensure that the head heated up evenly, so placed the heater take-off at the far end of the head. To sum up, the thermostat controls about 11/12 of the engine cooling system, and the heater circuit helps keep the heat even across the engine. In the case of Zippy's Moke, I am sure all is well. Besides, if he found the engine temperature to be off a little, he could replace the thermostat with one of a slightly higher or lower rating to suit.

To back-track to the problem of not having any thermostat, there are two (at least) basic concepts:

1. Without one, an engine does not maintain it optimum operating temerature - the thermostat modulates the temperature by opening and closing to suit. 
2. Without one, the flow characteristics of the cooling system are thrown off and you risk uneven head temperatures (hot/cool spots) and the risk of premature gasket failure and uneven cylinder temperatures leading to uneven combustion (one or more cylinders not being as efficient as the others).

I used a Toyota heater core in the front, modified with the intake and exit tubes pointing upward eliminating the flow through the heater, a 2 core radiator replacing the stock one and an electric fan to help the engine fan. No more heating issues ever, even up the long mountain roads we drive here in the east Tenn. in the 90+ summer. A 10" fan blowing on the driver helps too.