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Buy and install a "super two core radiator".

Absolutely "Wrong Again".

Your fan blades angle in the same direction whether it is on frontwards or backwards. You engine turns in the same direction always. Therefore your fan always pushes air towards your rad and tire. If you don't believe it, start your engine and blow some smoke into the fan. Or reach under the wheel well and feel which direction the air is flowing. 

The only difference in having an 11-blade plastic fan on the right way is that it is about twice as effective at moving air.
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Harvey: absolutely right, with the operative part being "the very short term", as your explanation describes.
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Dan,
the advice about leaving the rad cap off does work for the very short term. I have done this. But.....

Rather than the system cranking up the coolant to too-high temperature and pressure relieving  itself all at once (and/or all the coolant), sometimes you can get away with it by letting the coolant boil away more slowly at 212F, plus shutting off the engine and coasting as much as possible. Obviously this cannot work if the geyser has already spewed. Obviously can only work for a few miles, and low loads, and best if starting off with a problematic engine that has not yet reached "hot."  Obviously only works in certain circumstances. Not a cure-all but may get you to the nearest safe spot or repair station. 

This tip has little to do with the overall issues here. Is it bad advice? Yes and no, depends on circumstances.

Eversince, my mini is known to have a backward fan, the fan pulls air from the tire towards the engine,  it has been that way since i bought the car. and it's the very reason why even at 127F weather, my mini DOES NOT overheat.
it's a blessing in disguise...

Tony, Jeg was not referring to the advice from mur and minimans. He was referring to the bad advice from somebody else to leave the rad-cap loose.

 sir jeg, i think mur and minimans advice were not bad, in fact they are very informative.
 all your advices are good, thanks, i learn a lot too.

All good advice above. Only other thing I can add is that I fill my rad about 1" above the fins, which leaves room for expansion. Folks often fill to the brim, which almost always causes the excess to puke out the overflow tube, which then causes concern, and it is then overfilled once again, and so the cycle contunues....

I believe you got some bad advice - you need the cooling system to be pressurized to raise the boiling point, thus the pressure-type radiator cap.  By running with a loose radiator cap, you kinda defeat the purpose and will overheat sooner...

I'd also suggest testing the temperature gauge/gauge sensor, to get an idea of how hot is 'hot' or in the 'red band'.  

Depending on the internal condition of the radiator, yes, it's possible that it's no longer serviceable and in need of 'rodding'.  Sometimes it's worth having the original radiator rebuilt, but visually appealing high-performance replacement radiators are available.  The stock radiator isn't really a superior item...

Make sure your fan is oriented correctly - you'd be shocked to know how many fit the fan backwards.  It blows outwards through the radiator, towards the tire, not inward.  There are different fans available also, should you decide to upgrade someday.

This is a real nice radiator - it looks stock, too.  C-ARA4442  I've been using one for years...  Should you decide on this one, note that you'll need a new cap also:  GRC111

Use distilled water in the cooling system, add a splash of anti-freeze and a splash of 'Water Wetter' from RedLine (WATER-WETTER).


Your pre-ignition or detonation problem which suggests an issue with ignition timing or combustion chamber hot-spot as indicated in your other thread no doubt contributes to your current temperature issues, so get that sorted first and see what happens.  Most folks use between 30 and 32° BTDC at 3500 or 4000 rpm, vacuum advance disconnected and the hose plugged (yes, you don't have a vacuum advance, but someone else on here might...)

The gauge is telling you nothing of value.

it is not unusual for excess coolant to be pushed out of the rad as the car warms up. A stock mini rad can find its own level just above the top of the tubes, where many people would look into the rad and ink the coolant needs to be topped up.

If if you have a laser thermometer and are seeing 200°+ on the outside surface of the head, then the coolant is probably 195° or so.

I always fit a good expansion tank with a blanking cap. Other carmakers call this a de-gas tank, and it helps the system to deal with small amounts of superheated steam without a bunch of coolant being pushed out, as well as ensuring that the maximum mass of coolant your system can use stays circulating. 1300 engines in MG cars, the larger ADO16 cars with 1300s, the Innocentis, etc. Came with expansion tanks.

You can buy Redline Supercool and use that alone as coolant.

This is an Internet forum, and any number of silly notions, bad ideas, and myths are likely to be added to this thread. My personal preference for minis is to get the coolant HOT, run it through the rad, cool it, and then pump it back into the engine. Oil coolers, aux rads etc. only add heat to the air being pushed through the rad.

Your going to get a lot of guess work and "try this try that" but what you really need is a diagnosis based on looking at the car in front of you. 1st thing is to get a temp reading you can rely on, not the gauge in the car. infra red gun is good. You said it pushed coolant out after you shut off the engine? thats heat soak pushing it over the boiling point. could be as simple as a bad cap. Do you have the correct radiator cap? what year cooper? what engine? the early cars had a "long neck" Rad. cap if your using a short It will overheat and do what your does because it is unable to pressurize the system. was the radiator overfilled? if it's over filled the expanded heated water has nowhere to go except out the overflow.  There are so many different things this could be in the cooling system itself and the of course other factors like engine timing and mixtures? you say you changed needles and now it really goes well? for best performance the engine will like to run very lean hence goes well but get's hot. or timing too far retarded at cruise speed (Vacuum advance broken)

I would suspect you really need to find a good mini mechanic before changing "this and that"....................

PS Just read your previous posts in a different thread, so some of that of which I wrote has already been covered. but my main points are still valid. I would check all the basics 1st then get yourself a booking on a rolling road. to have it set up properly since you  don't know what the engine has had done to it previously this will tell you what it need's...............

Hello.

I just posted an update to a previous post, relative to installing and running my mini with new needles and springs.

The motor is just a hoot!  It starts and runs just great!  We were at British Car Field Days in Sussex WI this past Fathers Day.  The car goes 65 MPH with out issue, (without issue with the engine, see below.)

The car has a heating problem.  When I pulled the head to replace the bypass tube; I also ordered and installed a new high capacity water pump.  I installed a new thermostat, I also flushed the complete cooling system including the radiator.  I thought that would take care of any heating issues.  Well it did not.  With the motor cold; the needle is on C.  The needle gradually climbs all the way to the HOT red band on the gauge.  Today on a 45 minute ride; after stopping and shutting the motor off, coolant was coming out of the radiator overflow.  Someone said just put the radiator cap on a half turn, not completely tight.  So on the return trip home, I did that but it was of no help.

So what is the problem and or course what is the solution? 
I am thinking the radiator may be the problem.  Maybe it needs to be "rodded" or cleaned and or a new core installed or I need to buy a new radiator.  Our host has a few radiators available but quite honestly its a little confusing.  I would like to keep the mini close to factory, at least in visual appeal, as possible.
Any suggestions would be most appreciated.
Regards
Tim