Welcome to our Site for MG, Triumph and Austin-Healey 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 MGB Technical - Overheating Solutions?

Today driving my '67 GT, I got caught in stop-and-go traffic (mostly stop) for about 15 minutes. With the ambient temperature about 95°F, I watched with trepidation as the engine temperature gauge reached 230° before I got moving again. There was no boilover, but I think there would have been if I'd been stuck any longer.

I'd be really interested to hear if any other MGB drivers have experienced similar conditions and what, if anything, they did to keep the engine temperature in the normal range thereafter. Thanks. -G.
Glenn G

Flush the system to get rid of any old deposits that prevent heat from getting out of the system.

Air pressure tends to take the path of least resistance, moving through any open spaces in and around the radiator-mounting diaphragm rather than through the radiator matrix. Therefore, if you want the coolant system to function to maximum effect, be sure that all of the spaces around it and above it are well sealed. However, do not seal the circular apertures in the radiator diaphragm as they are present in order to allow needed cooling air to vent into the engine compartment. If the rest of the spaces through which air can flow are properly sealed, the air pressure in front of the radiator diaphragm at highway speeds will force air through these circular apertures fast enough to substantially assist in removing hot air from the engine compartment.

Install one of the two versions of the plastic cooling fans for more effective cooling. These molded plastic fans have more efficient aerodynamically shaped blades than the earlier steel three-blade (BMC Part # 12H 1058) and steel six-blade (BMC Part # BHH 1604) fans, thus they move an increased amount of air with less noise and parasitic power loss. The early version (BMC Part # 12H 4230) found on 18GD, 18GF, 18GH and 18GJ engines, was originally introduced on the 18V engine used in the Austin Marina, a single-carburetor engine that was intended to have a lower maximum engine speed. It is of smaller diameter with unreinforced, coarse-pitch blades, which, while producing more noise than the later version, does an excellent job of drawing air through the radiator matrix at low engine speeds, but tends to “stall out” at the higher engine speeds attainable with the dual-carbureted versions of the 18V engine used in the MGB, resulting in little movement of air. The later version (BMC Part #12H 4744) which is commonly found on 18GK, 18V-584, 18V-585, 18V-672, and 18V-673 engines, although it was not standardized on North American Market models until November of 1972 on the 18V-672-Z-L and 18V-673-Z-L engines, is of a larger diameter with seven steel-reinforced finer-pitch blades that does a much better job at high engine speeds. Due to its higher aerodynamic efficiency than that of the old paddle-bladed steel fans, this fan draws more air through the radiator matrix rather than expending most of its energy just stirring it around inside of the engine compartment as the older paddle-bladed metal fans did, requires less power to perform its function, and is actually quieter due to the uneven spacing of its seven blades. Because these fans are lighter, they have less inertia and thus absorb slightly less power and put less strain on the pulley belt whenever a change in engine speed occurs, thus prolonging belt life. Note that the blades are not flat, but have a concave and a convex side to them. The fan should always be fitted with its concave side facing the engine in order to perform with maximum efficiency. The mounting of either of the plastic fans onto an engine that was originally equipped with the earlier paddle-bladed steel fan is a simple matter of removing the fan pulley wheel from the coolant pump and using it as a jig in order to drill four holes through the boss of the plastic fan so that they will align with those of the fan pulley. To install these fans on a MKI model it will necessary to either mount the short-nosed coolant pump of the 18V engines, or to install the Morris downflow radiator of the 1972 through 1975 MKII models along with the complimentary thermostat housing and coolant hoses (flexible pipes) in order to provide proper clearance for the fan. In either case, you will need to mount a shorter pulley in order to maintain proper alignment with the alternator.

A switch to a four row aluminum core with louvered fins will help a lot. A fan shroud will maximize the effectiveness of the fan.
Stephen Strange

Pressure, or the lack of it, will contribute to rising engine temperature under the circumstances you describe. A common source of pressure loss is deterioration of the heater core. Try turning your heater temperature on, turn the fan on and see if mist forms on the inside of the windscreen and you smell antifreeze. Likewise, next time you're stuck in traffic watching the temperature rise, turn the heat on and use it as a second engine cooling system and smile while you sweat. If you find your core is leaking, replacement appears to be quite a task per some threads I've read here. If you wish to get by during a Fort Worth summer, you can "loop" the coolant lines and isolate the heater core until you're up to the job. As Steve mentions, a complete system flush is absolutely needed and not difficult to do. However, a 40 plus year old cooling system might have deposits in it that don't easily give way and flush out with only garden hose water pressure. Check your radiator cap pressure as well, replace your thermostat with a good quality one and replace coolant with a proper 50/50 mix. I've driven a few MGBs in summer heat and it's doubtful that any with perfectly maintained factory cooling systems won't have a temperature rise in the circumstances you described, but not to the temperature you mentioned. MGBs were built for England's climate, not Texas.
Rick Penland

"MGBs were built for England's climate, not Texas."

Hardly, they were built precisely with America in mind with all that implies, and run in desert states there and elsewhere without problems. After 40/50 years there can be all sorts of reasons why any one car runs hotter than it should, I saw my V8 just under the red (about 220F) several times even in the UK until I sorted out the cooling system, since then it didn't go above about 212F under test at an ambient in the garage of 105F. If it isn't steaming or losing coolant it isn't overheating, a 33% Glycol solution should resist boiling up to 240F with an 7lb cap, a 15lb cap as used on later cars almost 260F.
PaulH Solihull

I have much better results with cooling when I reduce the concentration of antifreeze to less than 1/3. Water has a much greater ability to dissipate heat than antifreeze and a 50/50 mixture is only required in areas where temperatures below 0 degrees F are encountered. RAY
rjm RAY

Thank you for all the great suggestions. Actually the engine and its cooling system are in very good stock condition. The radiator and water pump are a year old and I've flushed and changed the coolant twice in the past year. The coolant is composed of 20% anti-freeze and 80% distilled water. The temperature gauge is only a few weeks old and seems to report about the same readings as the old gauge before it failed.

Based on my experience with similar vintage cars, I believe the stock cooling system is doing about as well as can be expected. I'm really looking for ideas, preferably proven, to modify the stock system beyond its normal capacity. I want to avoid cutting holes in the bodywork, but I'm not opposed to trying non-original parts.

Steve's suggestions of fan and shroud are items I've been considering. The aluminum radiator is tempting, but for that much money, I'd like to be sure it will do some good. Anyone got one of these to report on? -G.
Glenn G

Glenn, you might want to consider removing the rear bonnet seal if you have and/or bending a couple of sections along the scuttle channel where the bonnet sits to allow air to flow out the rear of the bonnet. I know a lot of V8 guys do this and also louvre the bonnet to aid colling in OZ. My roadster does get hot in our 100 plus summers but I don't drive in in stop start traffic. It also cools really quickly once you get going. I know when I was younger, watching the needle climb on a hot day is very nerve wracking just waiting for the steam to arrive! In those days I did away with the thermostat by punching the centre out for more coooling. In contrast to Pauls wise words I recall MG's and many other english cars stuggling in the heat we have here. In stop start traffic they always heated up a lot. The pent roof rover v8 in the range rover was a particularly bad car for boiling in traffic.
A J Ogilvie

Will the MKI shroud fit with the yellow plastic fan installed?

Or was the shroud designed for the 3 blade metal fan.
Steven 67GT

Did you check the ignition timing and fuel mixture? Running to lean with retarded ignition will raise the temperature, too.

Before doing radical things I'd check all the thing already mentioned, I'd start with :
- radiatorcap pressure, try a 13 or 15lb cap.
- put foam around the radiator to force the air through the radiator instead of around it.
- as long as it doesn't boil over: try to relax as if in a sauna, in the extra warmth created by the heater at full blast.

Willem vd Veer


Have you tried an electric fan?

Also, even though your radiator is new, the engine may be packed with crud. I pulled one apart for a rebuild, and when I pulled the core plugs, the water jacket was about half full of old hard crud. I'm sure normal flushing would not have removed it. It took me about an hour with probes, an air gun, and water hose to remove it all. One sign that it was packed was that the block drain would not let anything out.

C R Huff

I echo others here in that with regular servicing, maintenance and repair the car should have no problems

good to hear you use distilled instead of tap water but 80%(?)

you must have the correct pressure rad cap for your system

cooling is from good condition and flow of oil and coolant/anti-freeze/water

regular complete full changing of oil in the engine, filter and oil cooler (as much as possible) - without damaging yourself empty the oil as warm as you can for as long as you can

changing gearbox and rear axle oil every 3-5 years is also a good idea

the (water) cooling system should be considered as a whole -
heater valve,
rad cap,
water pump,
engine block,
heater matrix,
coolant mixture

all items need to be clean, unclogged and fully operational

check blade grommets to keep fan secure, rad pressure cap is in good condition, heater valve fully opens, the rad and surround seal is complete (to stop cool air missing the rad), you have the correct thermostat

cleaning and flushing the whole (water) coolant system - for a coolant change drain the whole system - engine block, rad and heater matrix if you can get any of these out to give them a good shake at the same time as flushing and back flushing then all the better - the sequence is use cleaner as per its instructions then flush, back flush and flush and continue this until water runs clear, refill with the correct coolant mixture but bear in mind in tap water isn’t always the best water to use

As new rubber cooling hoses can be poor quality now this is also a good chance to change them to silicone and rounded-edge clips

as Paul put the B was aim at your market so should work there, the secret is to stop high heat build up in the first place
N Atkins

I wouldn't bother with the radiator diaphragm. I installed one, several years ago, and it made absolutely no difference in the engine operating temperature at low or high speed. When I replaced my water pump and radiator, last Summer, I left it off. In theory, it should have made a difference, but the reality was somewhat contrary. RAY
rjm RAY

Radiator shrouds and improved fans for MGBs are a significant aftermarket industry so overheating of the 1800 engine is a common problem. Thankfully, they're usually tough enough to handle it without significant engine damage. One thing I've always noticed is that they are quite motion sensitive. When the gauge temperature is rising from sitting, it goes down quickly when the car begins moving and air flow is applied. Thus, it's readily apparent that a more focused air flow has advantages and Steve's suggestions are worth considering. If you're in Fort Worth, there is a very good british car shop in your area. I've been by there for rare parts a couple of times when I was in the area on business. The shop often has Austin Healeys or Jaguar E-Types in there for work and a good place to stop by and see some hard to find cars. I think it's "British Auto Specialists". Being located in your area, they should have lots of experience addressing your problem.
Rick Penland

>>Radiator shrouds and improved fans for MGBs are a significant aftermarket industry so overheating of the 1800 engine is a common problem<< often caused by poor servicing, maintenance and repair of the vehicle

solving the cause of the problem is best, improving on the original system is good but only after getting the best out of what you've already got
N Atkins

"Radiator shrouds and improved fans for MGBs are a significant aftermarket industry"

Not so sure about that, AFAIK there is one for the early rear fill rad and that is it. Can't be much different from the later centre fill, you may say, but I have read at least one account where someone tried and gave up. It may help the fan to pull more air through the rad when stationary instead of taking a short-cut due to the gap between fan and rad, but at the expense of reducing flow when under way.
PaulH Solihull


I had exactly the same overheating trouble that you have on my ´63 MGB, and it was cured with a plastic radiator shroud from Moss. It was quite a job to make it fit in the car, had to cut away a lot of it at the LH side and split it at the bottom. But it was worth it!

Before I decided to go for the shroud, and not the electric fan that many of my MG friends use, I read this article on overheating Sunbeam Tigers:

At the same time I installed the shroud I also changed the thermostat, so of course that may have played a part too. But I doubt it :-)


You need a high efficiency radiator core- 120 tubes, 3 rows deep. I run these cores in my A and B with only normal mark II MGB fans and 10 lbs radiator caps- no shrouds or any other items. In southern California no overheating ever, I have even covered my oil cooler. (Of course, all other mechanical items noted such as clean system, no leaks etc. are first to check). There is no need to have overheating when you have enough radiator. I use a 180 deg. thermostat.
M. H. Dabney

The Moss shroud worked well on our car and allowed me to keep the completly original metal fan. The shroud was easy to fit. I did have to trim it a bit and it overlaps the bottom of the rad but it still works fine. Being black plastic its surprisingly unobtrusive for its size.
Please ignore the temporary fixing for the Boyer Bransden ignition amplifier, this was a quick functional test.

Stan Best

I must be the only one who has had no luck with the radiator shroud offered by Moss. 1/4 of it hangs below the radiator, which allows air to be pulled into the fan from under the car. Since I'm running a supercharger, which has the belt tensioning setup located on the bottom of the alternator, it made it almost impossible to adjust the belt tension once the shroud was in place. Even without the shroud, adjusting the tensioner is a PITA. RAY
rjm RAY

Again, thanks for the ideas. Tore, the story about testing cooling system components on the Sunbeam was a very thorough piece and very welcome among the wealth of anecdotes. It seems airflow is the most important factor. Based on that and other suggestions as well as my own experience (40 years in Texas and 30 years in old sports cars), I've decided to modify the existing radiator with a more efficient core and add a fan shroud. When it's done I will report back.

Ray, I feel your pain, buddy, because the idler pulley for the fan belt on my car will probably be inaccessible after the shroud is in place. May need to fabricate a tool to deal with this. -G.
Glenn G

Glenn, I carry a 1/2" crowsfoot wrench as well as a deep 3/8" drive socket and ratchet so I can adjust the tension of my serpentine belt while on the road and away from my regular tools. RAY
rjm RAY

This thread was discussed between 02/06/2011 and 09/06/2011

MG MGB Technical index

This thread is from the archives. Join the live MG MGB Technical BBS now