Welcome to our resource for MG Car Information.



MG parts spares and accessories are available for MG T Series (TA, MG TB, MG TC, MG TD, MG TF), Magnette, MGA, Twin cam, MGB, MGBGT, MGC, MGC GT, MG Midget, Sprite and other MG models from British car spares company LBCarCo.

MG MGA - Temp...Interplay between gauge,thermo and fan

Whilst working on the cooling system I pondered on the issue of how the coolant thermostat, the dash gauge reading and the fan controller interplayed. The fan controller probe is in the top hose leading from the thermo housing back to radiator and the dash temp probe is in the block.
Prior to a recent change I had the fans kicking in at 190oF as read on the dash gauge . The thermostat is a 180oF unit so should be fully open at 185to I thought. I set this way because I considered that only when the thermo was open could the fans start to do any useful work as only then the hot water came fully through the radiator.
I have just altered this based on recent experience and have the fan kicking in at a dash gauge temp of 180oF. This is not probably the same temp as the fan controller is seeing in the top hose ( which would probably be lower). The new system well and temp range settled 185/190 under non arduous or transient conditions ( previously 190/190).
It does mean most likely the radiator temp is being cooled before the thermo fully opens and it seems to have settled the system down 5oF lower.... but on an energy in and out basis this does not seem to make sense. Am I kicking the dog and other conditions i.e. ambient, new coolant etc are dominant.. rational thought tells me the latter.
Neil Ferguson


One consideration, are all the sensors and indicators working in unison? i.e. have you calibrated the thermostat, gauge and fan controller against an independent temperature sensor?

Steve Gyles


I've had a good run fitting the temp probe in the bottom hose and setting it low at about 100
With it like this if you have good airflow through the radiator (on the move) the fan doesn't come on ,but as soon as airflow is low like in traffic and hot water circulates straight through the radiator, the fan will operate - It works quite well and doesn't rely on coolant getting over the thermostat temp
might be worth a try willy
William Revit

if your fans start up at only 190 degrees, what temperature do they switch off at?

My similar set up to yours (twin Davies-Craig fans) are operated by a Revotec thermostatic switch in the top hose which has a 15 degree range.
So set up like yours, my coolant temp would have to drop to 175 degrees to switch off.
As my normal running temp is between 175 to 185, my fans would never switch off!

So I have to set it nearer to 200 degrees so that it only comes on when the car is stopped in traffic.

What type of thermostatic switch do you have? I would love to find one on which I could independently adjust both the on AND the off levels.


PS Your twin fan set up worked superbly over in Europe this summer.
c firth

Steve...No I have not but certainly will....want to understand the interplay.

Colyn....You make good points and probably show a huge hole in my thinking about the cooling system on my car. I needed a refresher .
I have a Davies Craig mechanical fan controller ( I attach its instructions below )and as I mentioned I had had it set at 190F and my normal coolant range ( assuming gauge correct ) was 190/195F and given the system it could not be lower. I reset it to 180F and it controlled the normal temp to 185/190. The controller , according to its instructions , should cut out if it reduce the temp 'by the thickness of the gauge needle' and then switch on off on a time basis to control. It therefore makes sense that when it was set at 190F the temp was approx. 190F ( and on a warm day in Oz it could not go lower) as it was constrained. When I set it lower at 180F it controlled lower at 185/190F and also probably the water thermostat interfered with the latter going lower.
I am considering changing the Thermostat to one that opens at a lower temperature ..say 170 to 175F to give the fans a chance to control the temp slightly lower and to have the very important capability for the small capacity cooling system to accommodate energy and therefore transient temp increases on hills etc on a hot day without blowing its lid. I am a little reluctant to do this as I like the engine to work hot as practical for thermal efficiency..but it may be a necessary compromise to accommodate normal and exceptional duties required from the cooling system.
William... Interesting approach ...what fans and controller do you use?
Neil Ferguson

Forgot to attach the Davies Craig controller set up instructions as referenced in last goes...
Neil Ferguson

Neil. You need to read the tech article on cooling systems on my website, before making any changes. It will explain some things you seem to have a mistaken understanding of right now. Thermostats, for instance, begin to open at the rated temperature and are not fully open until +20 deg F. Also, thermostats can make an engine run hotter, but they cannot make it run cooler unless you are using a thermostat with too high an opening point. Please take the time to read the article and, then, you can ask your questions with a better understanding of what is happening and what are the variables.

Les Bengtson

Les..will read and thanks. I do know that thermostats do not control temp..just set the low boundary for engine temp. before circulating water to radiator but I always recollected that that they took 5 to 10F to fully open ..not I stand corrected. The temp. control I refer to is always by the fans
above the thermostat set point and the characteristics of the fan controller itself. Will read the referenced article always amazes me that when you scratch the surface of any issue in cars ( and life?) there are layers of complication..but good stuff!
Neil Ferguson

Les ..just read your excellent article on Cooling System Inspection. I don't live in an Arizona type climate but temps around can get above 40oC (105oF) quite regularly in a hot summer... but we never get the freeze. I have put a 25% coolant mix in the radiator
Two things I have decided based on above..
....I will remove existing thermostat and check its opening characteristic v temp.
....Based on results of above I will probably buy a 170oF thermostat ( if available ) and set the fan to come on at somewhere between 180 to 185oF so that cooling starts when thermostat is partially opened .
I would then expect coolant temp. under steady warm weather condition to be somewhere around 180 to 190oF leaving a good margin for transient upload blips...
Neil Ferguson

when you next do any work on your cooling system would you make a note of both the coolant temperatures on the temp gauge at which the fan switches on and off at and let me know?
It would be much appreciated.

As I mentioned I am looking for a thermostatic switch with approx a 10 degree F range (or about 5 degrees C) between on and off and it does sound as if your Davies Craig switch may be nearer to that than my switch.

c firth

Colyn..will do ..from the installation instructions it looks like a very narrow range..will advise next week .
Neil Ferguson the interim before I check out my mechanical fan controller here is some bumf on the latest Davies Craig digital controller ..From what I read it cuts in 3 dg above set temp and runs on until 1 deg below..can control the water pump as well..the latter is very useful on the MGA!!
Neil Ferguson

Been running the 1500 with just the single Revotec fan (no water pump fan) for three years now. I set the fan to cut in at 210F and it cuts out at 190F. I've done about 9000 miles with this set-up, thrashed the car for miles on motorways with the gauge showing 200+, and it has never complained. These engines thrive on heat, and as long as they do not boil over, you're not going to have any problems. As I have said before, if the temp gauge were marked C-N-H, no one would worry until the needle got close to 'H', and that is 230F. Even with the needle on 230, if you are running with 50% antifreeze and a 7lb cap, it is not going to boil. Your engine will still be running sweetly providing you are moving and the carbs are getting a supply of cool air. The only thing that will be uncomfortable will be the driver, because he keeps looking at that stupid temperature gauge!
Lindsay Sampford

Lindsay...Sounds excellent for operations in the cold Northern part of the Northern Hemi ...Conditions in Oz ( and Arizona ) are just a little different. Driving in an Summer ambient of 36oc , and sometime 40oC, and steep drives combined ( there are hills in Oz..(the great dividing range runs along he coast where I live ). I do believe the characteristics just may be a little different for a small capacity cooling system. As I mentioned I am after some transient capacity to cope with exceptional an aside I note that the new Davies Craig controller ( not the one I have ) can be set to different temps from the cockpit...very useful in this climate .
I agree re the engine and high temp. it is , I recollect. one of the Laws of Thermodynamics.
Neil Ferguson

Colyn..just ran the car and reset the fan controller as close to 190 as I could ( decided 185 too close to the water thermo setting ..need it open ..even partially )..on rechecking I found it was about 192 as seen on the gauge with a fat needle. The fan then ran until it reduced the gauge reading to about 188. Did it twice and got about the same gap on/off is about 4oF on my Davie Craig mechanical knob dial controller with thermometer in top hose.
Neil Ferguson

But Neil, have you ever had your MGA boil over, or run badly due to heat? Sure there is a problem with ethanol in fuel these days, but that usually only causes bad running when in slow moving traffic or when restarting with a hot engine bay. I have eliminated that problem (in our temperate climate) by having a 4" bilge fan; which I can turn on when required, in the cool air duct to the carbs. Admittedly, the standard cooling system does need frequent topping up to the recommended level, but that can be sorted by fitting an expansion/recovery tank.
Lindsay Sampford

Lindsay.....On a long up to 36C temp. I was driving up a long hill at a reasonable lick and looked down and my gauge was 230F. From then on the gauge temp was 200 to 210 ( previously 190 to 210). At the next convenient stop I allowed the car to cool and topped took 1.5lt.. I have been using just water and corrosion inhibitor for a number of years and all seemed fine.. After the drive I decided to understand and rework the system to avoid a repeat incl. flush out , using a coolant premix, and resetting controller and maybe water thermostat .ie a rework to suit Australian Summer conditions.
I also have a bilge pump to blow cool air to the Judson well..but I put it in to get cooler air to the filter ..not to stop fuel boiling.
I have an atmospheric expansion tank...

Neil Ferguson

Thanks for the information Neil,
Your mechanical thermostatic switch has a surprisingly narrow temperature range, I think it would be a bit too narrow for my setup.

I checked out the Davies-Craig digital fan controller that you mentioned and according to the suppliers, the nearest setting to what I need is for it to switch on at 208 degreesF. It would then switch off at 194 degreesF, so it has a range of 14 degrees which is pretty much the same as I have now with the Revotec controller.

On the internet, I have just found another Fan Control Thermostat that is made by Derale in the USA which apparently has the 10 degree Farenheit range that I would prefer. All I have to do now is to find out how to get hold of one in the UK.

Back to your engine Neil, why not remove the thermostat altogether? It only speeds up the engine warm-up time so that the heater works quicker and I presume that you dont use the heater too much in Oz. You could fit one of those blanking sleeves that prevents any coolant from bypassing the radiator.

It would simplify the cooling system a little for you and probably work even better.
c firth

Neil, is your expansion tank a coolant recovery system? It's a very useful mod because it allows the coolant to expand when things get extra hot but instead of pouring it all over the road, it is stored in the expansion tank until things get cooler, when it is drawn back into the rad. I've seen over 230 on the gauge when I've stopped for fuel on a hot day, but I just jumped back in and carried on. I've never stopped to let it cool down, and since I've had the coolant recovery system, never lost any coolant due to high coolant temperature. When your gauge hit the highs, was the engine still running sweetly? Have a look at this picture on Barney's site Check the temperature and the speed and the truck he is overtaking!
Lindsay Sampford

Lyndsay...have had one of these..a non pressurised push and suck system for blue well...but did not help on the recent drive. I lost all that water through the bottle on the recent drive.

Colyn....What you say makes sense and is an extrapolation of my reasoning for putting a lower 170 or 175 water thermo get it well below fan operating range and have the thermo near or better still fully open during normal running ( I am very aware of the comment about 20oF thermo opening range as per Les's comments ) The objective for me is to have the radiator and fan doing their job at normal operating steady state conditions with all engine water passing through.
Will probably still pursue the 170 as despite the high ambient temps you get in summer it can still get down to a dreadful 6 C brrrrr!!..also I like to get the choke pushed in asap after start up....

Still have to pull out thermo and check operation and calibrate the gauge as per Steve's comment.

Neil Ferguson

Thermostat ''should'' be 170

any higher and the whole system will be too hot to start with and be more prone to overheating easily
Well I'm sort of talking MGB here but an A should be the same
Spec for MGB --170 but higher 180-185 for cold weather
That's cold - not Aust.
cheers willy
William Revit

Neil, sounds like you are boiling then. If you are running with a 7lb rad cap, with plain water it shouldn't boil below 230F. With 50% anti-freeze and a 7lb pressure cap, it shouldn't boil until it gets to 245F. Is your rad cap OK and if you have the standard type radiator, is it a long-reach cap?
Lindsay Sampford

Lindsay..agree was boiling in order to kick out that much water.It was a steep long hill and I had the hoof down and the A cooling system has little transient heat storage capacity . I need to get further away max.. I am now on a glycol premix. the cap works ok and is the long type . Checking everything again though. thoughts exactly ..looking for a 170f thermo with a sleeve which should be fully open at 190f and all water going through radiator but no luck so far. Do you have one in your B?
Ay ideas about where I can get one?
Neil Ferguson

Just had a look in an old w/shop book I happen to have and it lists 160deg F as the std thermostat for an A and 170 for a B

I'm thinking that the modern day shopkeeper doesn't really remember farenheight and if you ask for celsius deg. you will have better luck
70deg C --normal
72deg C --cold weather
76deg C --normal
82deg C --winter

cheers willy

DAYCO make them and Repco or Supercheap stock them

I've just been for a run in a mate's B which we fitted a75C/170F thermostat to and it runs dead on 180 on the gauge if that helps BUT remember not all gauges are the same
I'd be going for a 70c if you can remembering that these cars are 50/60 years old and the metal isn't quite as capable of hacking the higher temps that modern cars cope with
Most of the BMC cars like mini's etc and Triumph Stags and probably heaps of others ran these thermostats Cortinas probably there shouldn't be a problem getting one over there They've got the 75deg on the shelf here
William Revit

Found this on the Dayco site

DT14A 82C
DT14B 91C
DT14C 71C
DT14E 77C
DT14G 89C

They have listed the DT14A for all the four cyl MG's
BUT as mentioned before MGA's are supposed to have the 70deg unit according to the w/shop book
I'd be trying the -DT14C- 71degC if it were my car
William Revit

Also if you don't really have a need for an electric fan I'd be going back to standard fan - it's a lot more reliable
William Revit

William, true, the yellow fan is a lot more reliable at cooling the rad, but if one of those blades came loose while you were leaning over the front of the engine compartment it could kill you in a very messy way. And it has happened. There are also reports of broken fan blades liberating themselves straight through the bonnet (hood if you are American). That's one reason I removed my yellow metal fan, the other reason for removing it is that it is said to rob you of 5 or 6 bhp. I certainly found removing it made my 1500 a lot more responsive, an 1800 could be different. But seriously, a fifty year old steel riveted fan has spun round billions of times and has been subjected to g-forces and vibration. It was probably only designed to be good for ten years, so treat it like a knife wielding maniac or change it for a plastic one. Not sure I would trust a modern reproduction steel fan; probably more dangerous than the original one if other modern replacement parts are anything to go by!
Lindsay Sampford

I find it hard to believe the stories of the fan blade coming through the radiator fan guard and then the aluminium bonnet. The fan guard on the radiator is quite heavy gauge and would deflect almost anything.
dominic clancy

William..great advice on thermostats. I note that Supercheap ( horrible name .. makes you cringe at buying there. I sneak in and out.. ) stocks Tridon thermo TTI 160 that seem to parallel the dayco dt14c...relabelling ?
Will stay with my twin electric fans..quite apart from Lyndsays warnings of getting the Charles 1st treatment.. I have had them in for about 4 or 5 years now and they work very well..especially in stagnant traffic on a hot day. Do you have latter in Tasmania ?
Neil Ferguson

hahahahaha -- yes it gets warm here and We've got the highest UV on the planet to go with it
On a clear summers day here sunburn is a real problem
15 mins in the middle of summer unprotected and the sizzle gets you It catches a lot of tourists out as the air is cool and the ambient temp is lower than on the big island but mate cooking time is a lot shorter
We see quite a lot of bright red visitors-most heading to the chemist -- too late
With the Tridon/Dayco part nos. A lot of auto parts suppliers are finally starting to use similar part nos now --at last-- belts hoses thermostats water pumps some filters. I don't think it's rebadging it just took a while for common sense to float to the top

On the fan yes naturally it is your choice and if you have something that works------ but isn't this thread about your car overheating
The plastic fans off the earlier rubber bumper B's are safe and suck well especially on those long uphill runs where most thermo fans just don't seem to hack it on them hot days
I think the horsepower gain thing is a mindset thing
You get nothing for free the alternator has to drive the fans but I guess they(electric) can be tailored to needs more acurately if the rest of the cooling system is up to the job
Years ago I had done some work on an old International truck and after the job was done, it was sitting there idling away and I closed the bonnet and got down off my step and there was a bang and rattle and a fan blade had snapped off AT IDLE cut straight through the steel bonnet and went off rattling round the workshop
It was a real eye opener to me and I NEVER stand in line with a fan when working anymore or a flywheel
I had the misfortune of a flywheel letting go on a Mustang in the shed once at only about 5000 revs, what a mess I think it was the pressure plate that went first but thu whole clutch ,in bits, the flywheel in bits the front shaft of the gearbox and a shattered bellhousing went everywhere Luckily it was a race car and had a scattershield and most bits went downwards but the mess was incredible. The motor was still going and then a 30cm gap back to a shaftless gearbox hanging down on it's own and the a pile of bits including the scattershield scattered on the floor. We didn't find some of the pressure plate till about two years later when we pulled the fridge out from the wall to paint the floor and there jammed down the back of the fridge was a piece, probably about 1/4 of the cover off the p/plate I'm far from being a scaredy cat type but man I'm a lot more carefull about sutch things after seeing what can happen
I've been off work damaged since February and have taken on a new respect for the possibility of what can easily happen - hoping to get back to work next year
Safety Fast
William Revit

Dominic, from the archive:
"Dave Swinburne, Kent UK
This happened on the MG80th (SE Car Club)run at the weekend, gave the owner quite a fright and an unexpected power bulge, also damage to the rad top plate.

MGA 1500"

"Ralph, Ontario, Canada
Oh, wow! That's frightening. A friend of mine with an early MGB had a blade slice right through the bonnet at about 50 mph.
He had heard a clicking sound that he couldn't diagnose on the side of the road.
He found out what it was:>( In that case the rivets had failed.
I try to check mine occasionally for loose rivets etc by trying to move the blades relative to the fan. I don't know if I would see a crack on the main fan structure though, probably not.


I rest my case!

Lindsay Sampford

I am intrigued why broken blades always fly upwards. Or is it simply that these are the breakages that hit the headlines? If this is the case are there far more broken blade incidents than we realise?

Steve Gyles

Steve, there's an explanation on Barney's site about why they usually come out through the top.
Lindsay Sampford

Lindsay. To me it read more as a statement than a true explanation. I am just wondering if the top shroud/guard causes localised turbulence around the fan tips between the 10-O-clock and 2-O-clock. When the cracked blade is on its very, very last legs it hits the localised disturbance and bang, it goes. Food for thought.

Steve Gyles

I ran out of editing time in my last post. I would just add that if there is any credence to my theory (lunchtime lateral thinking) then the radiator blade guard may, unwittingly, be the root cause of fan blade fatigue. It could actually be causing the fan blade flexing in the first instance. Remember, these things were all built before we had a true understanding of fatigue cracking that eventually became apparent with the likes of the Comet airliner disasters in that period.

Steve Gyles

Steve, it's either going to be that or good old sod's law.
I've just had another thought, if the blade doesn't necessarily come out through the top, ought we be wary of standing in the front wing area of a running MGA? Should all MGAs with the original yellow metal fans be forced to bear a warning notice that says 'DANGER AREA,DO NOT APPROACH WHEN ENGINE IS RUNNING' on all areas where an errant fan blade is likely to emerge?
Sorry, I am getting silly now!
Lindsay Sampford

Lindsay. Who knows? I painted my red fan blades yellow as a safety measure.

Something I have learned through life is that there is a reason for everything, they don't just happen. For blades to break they have either been bent by owners or they have had other forces impacted on them. Airflow disturbance is the most likely in my opinion. I just recall many of my low flying sorties in the hills. You could orbit an isolated hill/mountain at a couple of hundred feet below the summit. For 300 degrees it would be totally smooth. Then you would hit the down draughts on the lee side and the aircraft would suffer the most severe thumps and jars. makes you wonder!

Steve Gyles

Could be the yellow paint, you ought to paint them red again!
Lindsay Sampford

I'd be interested in knowing whether anybody has had a fan blade failure on a shroud-equipped MGA? If the plastic radiator shroud can prevent or even reduce damage to the bonnet, it would be worth having for that reason alone. If the plastic alone is not strong enough, a "containment ring" of metal could be riveted to it.

D Rawlins

With apologies to the bonnet destruction posts and Reverting to the original thread ....I have found the culprit in my cooling system and show the evil sod below..
It was the water thermo and it has a stamp 180deg on its bum. It is the type where the outside sleeve moves downwards and begins to partially blank off the internal engine water recirc. system when the temp reaches its set this case 180f.
I immersed this unit in water and using a thermometer I twice cycled it to find out when the sleeve started to move ( and hence open up the passage straight to the radiator). First time it perceptibly moved at 195F and seemed to have maxed out at about boiling point. The sleeve moved only about 1/8in . I let it cool and tried again and it moved about 196f ( ie effectively the same ) and with the same movement. Obviously it may have moved more under pressure conditions but...
It is now replaced with a 160f unit without sleeve and I am going to try during the summer months and maybe if needed move to a 170f unit for the harsh winter here.
Interesting to note when I re-read the instructions on the twin fan controller it advises setting the fans to cut in half way between min and max normal temps and if fans operate longer than 2 mins controlling reducing coolant temp.then adjust it up slightly until it does. On this basis I have set it to 190f...and see how it goes..

Neil Ferguson

Hopefully you are on a winner
Earlier in the thread I mentioned about fitting an early rubber bumper B plastic fan as an option
Have a look on the MGB general section under spot the mistake heading and there is a very clear pic. of said item It is fitted incorectly in the pic as it is a later car with the radiator mounted forward as they were and would normally have electric fans but the fan blade itself is quite a good unit when fitted up properly
Strangely the newer thermostats now appear to fail by opening too soon rather than too late which in itself saves a lot of engines from being ruined. I haven't seen one stuck shut for ages, the units that fail seem to stay open a bit even when cold
Please let us know how you get on with the new thermostat and fan controller setting
cheers willy
William Revit

Neil, I have never seen a 'modern type' thermo with the blanking sleeve before. I tried an old 'bellows type' thermo with a sleeve in my car; it worked OK for a while, but then I started to get overheating problems, so I switched to a modern type without the moving sleeve and everything was OK again. Little wonder the old bellows one failed, it was probably 50 years old! What engine does your MGA have Neil and I am intrigued my your mention of a "harsh winter" Australia?
Lindsay Sampford

Lyndsay..the engine is a standard 1600 but with a Judson fitted. I was being a little shallow by refering to the harsh winter as all things are relative and min temp is about 6c normally in winter ...nothing compared to the icy north of northern hemi....but I did recollect Barney saying that 160 worked well for him in Summer when he tried but he needed to change to 170 for winter...

Neil Ferguson

Not me. I would never install a 160dF thermostat in the MGA. It runs too cold in cooler weather, requiring enrichment of fuel mixture (similar to partial choke). In which case the excess fuel washes oil off the cylinder walls and dilutes oil in the sump (very bad for the engine). When driving in warm weather it runs 200+dF, meaning it would see wide temperature swings, which bothers the carburetors.

So I use a 190dF thermostat, which runs 195-200dF base temperature (good heater in cool weather), and has minimal temperature swing. I recommend 180dF thermostat for most people for "normal" street driving in warm climate.
Barney Gaylord

Barney...I looked at where I could have picked up my ref. to a 160f thermostat and your fine self and it was from your bible.article CO-103E . You used one for a limited time in good summer weather but found it evil in cold weather and went back to a higher temp unit ( I was wrong about the 170f). In any case my engine will find its natural heat balance in/out for the Oz summer at about 190f irrespective of the thermostat ( I have set the fan at 190f as well and will adjust as necessary )so I do not envisage any issues with the 160f here but will change to 170f when I can get find one.
Neil Ferguson

All ..hottish day in Melbourne, 31c (88f) , and took car for a drive . Engine temp climbed to 195f with fan running almost continuously ( set at 190f) so adjusted up set point until cutin/ cutout time was approx. 2min as per Davies Craig bumf. Took for another drive and engine temp. range approx. 190 to 195 max... problem sorted I do now believe .

NB Interesting to note that when warming up the engine temp lifts to 195f and then came sharply down to the thermo is very sticky but once open seemed ok this usual ?
Neil Ferguson

Well done
With the temp fluctuating sharply during warmup like that I suspect that you need to drill a small hole about 1/16" in the flat face of the thermostat just to allow a very
small flow of coolant so that it doesn't get the sudden temp shock
Just next to the jiggle pin is a good spot
Don't take the jiggle pin out though as that hole is too big
Thermostats used to have a little nick in the side of the operating door to allow flow but they don't seem to have them anymore
Happy motoring - willy
William Revit

Willy ..will do and thanks for all the very useful comments and Stay out of that intense UV down there on the Southern Island.
Neil Ferguson

This thread was discussed between 02/12/2014 and 17/12/2014

MG MGA index

This thread is from the archive. The Live MG MGA BBS is active now.