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MG MGA - Cooling; thoughts on thermostat options

I hate to open another thread on cooling but would appreciate some advice on thermostat options. I am still chasing the overheating gremlin. This past summer I was running a Moss moving sleeve 160 F thermostat. I was experiencing the same kinds of issues Barney mentions on his site, . On a hot day I would see the temperature peg as high as 230 F on long uphill grades. I was also loosing coolant through overflow just driving around town.

So far I have applied all the usual measures, including a re-cored radiator, added the felt pad on the hood, changed to a 7 lb cap. Over the weekend I pulled the radiator to install the Moss radiator shroud. While I had the radiator out I also installed the front 4 inch ducting hoses. Also added a 1/8 inch rubber washer to the bottom of the radiator cap (not under the cap itself, but at the sealing end) as it was only measuring 1 inch in depth.

So should I go back to a standard 195F thermostat, or go with the standard 195F thermostat with a modified blanking sleeve as described by Barney here, . One minor concern with the latter option is that between the thermostat and the blanking sleeve, the gap for the gasket is larger. Any difficulty getting a good seal?

Side note on the Moss blanking sleeve, it was approximately .02 inches too large in diameter to seat correctly. Only took a few minutes to file it down the correct size.

jjb Backman

Time for a progress report.

A few years ago I used an original style 160dF moving ring thermostat for summer driving. Of course it only controls minimum coolant temperature, so it was running 200dF in warm weather regardless. Come winter, very little heat from the heater, and had to run a little choke constantly, as it was running too cold in cool weather. Lesson learned, do not run 160dF thermostat in cold weather.

I ran a 195dF thermostat through winter a year ago, quick warm up and great heat from the heater. For highway cruising in warm weather it hovered consistently between 210-220d with very little variation. I could have turned the needle around on the gauge to point to 190-200, and the world might have been happy. Problem was this may peg the gauge quickly with quick vapor lock issues in the carbs when you stand still for a few minutes. Lesson learned, do not use 195dF thermostat in warm weather.

Last summer I installed a "Chevy" style 180dF thermostat along with a blanking sleeve to cover the bypass port. I don't think you need to drill a hole in the thermostat flange, as the blanking sleeve will still allow a small (tiny) amount of bypass during warm up. In warm weather this ran in the 190-200 range most of the time, occasionally pushing 210-220 with expressway speed trailer towing and hills. Ease off the trailering speed a bit, and it is generally back below 210, and no vapor lock issue (very seldom anyway). This winter it would run 190 with sub freezing ambient temperatures, 185 with 0dF-10dF ambient, and acceptably good heat from the heater.

Maybe this particular thermostat runs 5dF higher than expected, but I like it, and it's a keeper combination for year round driving. Note that the fan shroud makes a HUGE difference in keeping the needle below 220dF if you get caught in stop and stop "rush" hour traffic in hot weather.

Do not expect the MGA to run below 190dF in warm weather with standard type radiator (even with high density core). The only ones I've seen run cooler were modified with a significantly oversize aluminum radiator and one or two electric fans. Since copper transfers heat better than aluminum, I'm sure the only advantage of the alloy unit is being notably larger (usually much thicker with sheet metal mod to the air pan). The comments I hear are very often, "If it starts to run hot, switch on the fan(s) and it cools right down". This tells me the electric fan makes a greater difference than the alloy radiator (and the alloy radiator might not be much of an improvement without the electric fan).

FWIW, I run a high density copper core (standard dimensions), 7-psi pressure cap, fan shroud, and no electric fan. Under the worst hot conditions it will not run off the end of the gauge (230dF max), and it will never boil. The engine is always happy, although the carbs may fuss a little with the highest temperatures at stand still or very slow ground speed. Under those conditions an electric fan may be of some benefit, but not enough to cause me to endure the pains of using one (noise and maintenance issues). I do run a coolant recovery bottle so I can keep an eye on fluid level without removing the pressure cap. Ran all last summer (several thousand miles) without losing or adding any fluid. This included trailer towing from Chicago to Hot Springs, Arkansas and back (1700 miles in 4 days) in July heat.
Barney Gaylord

John, the 7 lb cap is good if it's the correct length. It may be too short allowing premature overflow. Might I suggest using ACME parts Master #46004, it's a STANT R-6, I believe, available at NAPA. Marvin
Marvin Stuart

The coolant recovery system that Barney mentions is highly reccomended. My MGA would continually lose coolant to the point where the top hose was only half full. That has got to compromise coolant circulation an cosequently cooling. Since fitting the coolant recovery system my rad is always full to the brim. My temp gauge does hit the heights from time to time but I don't worry about it anymore! Incidently my thermostat is the original sleeve type.
Lindsay Sampford

Quick question Barney;

My Car runs consistently at about 160-190F for less an 1h drives in 20degC weather.
I doubt the PO has done any 'upgrades', everything seems stock.
No overheating problems as far as I can see, however should I be worried my needle is not reading right?

I don't want to get over-concerned, cos I am firm believer in 'if it ain't broke don't fix it'.
G Ramos

I wonder if removing the temperature gauge would cure all MGAs of overheating!
Lindsay Sampford

You have a point there.
Some years ago My MGA started overheating more and more.
I turned up waiting along the roadside to let it cool down, etc.
I tried about everything possible, mailing with Barney, who gave good tips, flushing the engine, radiator shroud, electric fan, tested the gauge in boiling water, where it seemed ok, and probably more things I cant remember anymore.
At some day (22 centigrade) when the gauge was again pegging after something like mile, I decided to go for a good drive, an see what would happen.
Nothing. At home with the gauge still I (very carefully) removed the radiator cap, and not even a bit of steam. That was strange.
Finally I got myself a new gauge, and my MGA is now running at max. 200F in every situation, without electric fan, shroud etc.
The gauge seemed to be behaving different depending of the form of the copper pipe to the sensor.

R de Krom

I struggle to understand the MGA cooling system. These are my thoughts, right or wrong:

I appreciate you guys over stateside and down under have generally higher temps than over here. That said, the only time my temperature gauge has ever gone the high side of 200 has been with a sleeved thermostat. I struggle nowadays, come snow come sun, to get above 160 with a bog standard tuupenny halfpenny stat and an original spec radiator. I know that Dominic even struggles to get up to my temperatures and he has fitted a rad blind to get the temp up.

There must be underlying problems here. We see time and time again guys talking about new radiators being installed and still no improvement; but is that not the problem? Many of these replacement 'modern spec' radiators are not suitable for the unusual MGA airflow system. By accident or design, the MG Car Company got the original radiator correct. Many modern radiator cores are just no goers with the MGA.

Sorry to be controversial but it's 3rd glass of wine and nothing on the box.

Steve Gyles

Do you have a photo of your bottle and its mounting position? The ones I have seen on cars here are sooo ugly, and mounted in front of the heater on the shelf.
I had one fitted temporarily when I slalomed the car, but removed it some years ago as it looked temporary.
Anyone else with smart suggestions?
P. Tilbury

I installed a windscreen wash bottle just aft of the radiaor as shown on a new double seal cap.It does not interfere with anything and looks reasonably neat. The overflow runs down the same route as standad overflow. Put it in a number of months ago and never had to top up since...and daytime temps have been up to 36c ( 96f ). the radiator temp goes up to max 205f when beating hard up long hills..but as you can also see I have fitted twin fans with thermo control and discarded the old belt driven unit. Very pleased with performance of the cooling system now.

Neil Ferguson

Peter, I don't have a picture of my recovery system, but it is exactly the same as Steve Gyles' setup (I copied it cos it looked so good!). It is the third picture down on Steve's site. Lindsay.
Incidently, I run mine as an un-pressurized catch tank but you can work it either way if you use the MGB overflow tank.
Lindsay Sampford


Mine is a pressurised expansion system, as per modern cars, as opposed to a coolant recovery system. In simple terms it is where you put the the pressure cap. i.e. my understanding is that a coolant recovery system keeps the pressure cap on the radiator whereas a pressurised system transfers the pressure cap to the overflow tank.

Steve Gyles

Steve, I can make mine pressurised in seconds by switching caps but I prefer it in the Un-pressurised configuration because it seems to me to be more "failsafe". A leak on the pipe to the tank in the pressurised mode would be more serious than the other way round, it's just me! I don't think it makes any difference either way, the end result is a full radiator no matter what. Thanks again for your idea Steve.
Lindsay Sampford

This thread was discussed on 16/03/2010

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