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MG MGB Technical - Thermostat for my 77 Roadster

Well, a week ago I got my 77 roadster, which I bought as a dead car in mid July, on the road. It is quite a pleasure to drive one that hasn’t been “rode hard and put up wet”.

I need to replace the thermostat because it takes too long to warm up, and on cool evenings it never makes it up to the “N” on the gauge. Currently the ambient temps here are running between 30 and 50 F.

From searching the archives, I see that many people recommend a 195 F thermostat, while my Haynes indicates a maximum of 190 F (88 C).

I am inclined to go with the 190 because the electric fan switch is supposed to kick on at 194 F (90 C). So, it seems to me that if I use the 195 F stat, the thermostat and the fans (dual fans) will be fighting each other.

Your thoughts please?

Thanks, Charley
C R Huff

All of my thermostat failures have been open. This means the car in cold weather won't warm up right.

I'd go with either a 180 or 190 given your fans. I use a 195 year round on my 65 that has a manually switched electric fan that I use on hot days in slow traffic. The heater works much better with a 195 than a 180.
Robert McCoy


A nice easy test for the thermostat: drop into a can of water, heat the water with a thermometer to check out the temperature figure as weel as the thermostats behaviour.

Charlie - I believe that it depends on what brand of thermostat you get, some are labeled 190°F and others are labeled 195°F. I don't think there is much difference in the final outcome.

“rode hard and put up wet”. - I thought that I was the only one around who still used that phrase (I learned it from my grand father). One doesn't hear it much anymore, but it does paint a picture. Cheers - Dave

David DuBois

Thanks guys,

I'll check at the auto parts store to see what they have available, and will shoot for the 190 if I can find it, or 195 if I can't find the 190.

Yeah, David, I've used that expression for a long time, but don't remember where I first heard it. It does a good job of describing many of the cars I have owned.

C R Huff

Well, after calling all around with no luck, I decided to walk into and Autozone since it seemed from internet searches that the balanced thermostats were supposed to be there.

They weren't finding it when I called because they weren't looking in the performance parts section. We found under the Mr. Gasket name hanging on the rack instead of in the back of the store. There were only three choices, 160, 180, and 195 F. I got the 195 F. I hope it doesn't start a fight with my fan thermostat.

C R Huff

I don't understand what you mean by "fight with my fan thermostat"?
The fan switch is going to close when the temperature of the coolant going into the radiator is around 195 degrees and open when the temperature drops to about 180 degrees.
The thermostat in the head is going to open at around 195 degrees and close at around 180 degrees.

The fans aid in cooling the coolant in the radiator.
The thermostat allows coolant to circulate through the radiator to be cooled.


I don’t know exactly what combination of temperatures will cause the fight, but if you want to get the concept, mentally push the temps to an extreme. Say the fan switch were set at 175 and the thermostat were at 200. The result is that the fan keeps trying to make the coolant cooler,the thermostat keeps trying to make it warmer, and the fan runs too much.

In 1984 I bought two new Mazda diesel pick up trucks that both came out of the factory with a bad match between the thermostat and the thermatic fan. The result was that the temp gauge continually cycled up and down about a quarter inch on the gauge. I solved the problem by having the dealer install a different thermostat, and then the temp remained rock steady.

In the case of the MG, the new thermostat is 195 F, and according to the Haynes manual, the fan switch is set at 194 F. Also, according to the Haynes, the thermostat for the late model MG was either 180 F normal or 190 F cold climate. So, I don’t have the split between the two as I should have, and in fact have the thermostat higher than the fan switch.

I will see what happens. It should be better than running too cold. If it doesn't work out, I can probably order a 190 F.

C R Huff

Well, that was a disappointment.

I put my fancy new $12 195 F balanced t-stat into my 77 B, drove it 13 miles home, and the gauge never moved out of the blue cold block. To add insult to injury, when I opened the hood to fondle the radiator and hoses to check the accuracy of the gauge, I found that my t-stat housing was seeping through the new gasket.

Oh well, tomorrow is another day.

C R Huff

Charley, are the fans running?? Seems to me with your cool temps this time of year they should not run at all [except for a short time after parking]. And you should try disconnecting if they are running to see the effect on the heat.
I have a 77 that has one electric fan and in summer, 75F, it only runs after parking for a short time.
Barc Cunningham


Mine has been converted to the later style twin fans, but they are not running prematurely.

The thermostat I took out was a 180 F. With that one, it would run in the center of the gauge in the warmer part of the day, but would only run about 1/3 of the way up in the cool evening.

With the new 195 F t-stat, the needle barely made it off the peg. I think it’s defective. I guess I’ll try a plain cheap style 190 F stat and see what happens.

C R Huff

As far as testing the stat goes, just feel the top of the rad as the engine is warming up. You shouldn't feel much warmth at all, until the stat opens then the rad will suddenly get very hot. If the rad warms up gradually with the rest of the engine then the stat is stuck open. At temps between 30F and 50F I *would* expect to see the gauge get up towards N.

As far as the fan stat and the thermostat fighting each other goes this is more of an issue with cars that have the fan switch in the head (like the V8, which is actually in the inlet manifold but it is still before the stat) and not the radiator header tank. Put too high a stat in (or too low a fan switch) and the fan switch will come on and run the fans, but because the stat is still closed there will be no heat in the radiator for the fans to cool, so the fans will run continuously. With the fan switch in the header tank as per 4-cylinder cars this can't happen.
Paul Hunt

I replaced the fancy balanced thermostat with an ordinary Stant 195 F thermostat, and now the temp gauge runs in the center even on cool evenings.

Also, Paul, it passes the test you describe. I didn't try that as it was warming up with the first stat, but after I had driven 13 miles I could comfortably hold my hands on the radiator or hoses and the gauge barely got off the low peg.

I haven't yet figured out how much the fans are running. Since I just got this car on the road, I don't know if I can hear them while I'm driving.

C R Huff

I drove it home this evening with ambient temps at about 60 F, and the gauge reached the center in about two and a half miles. Looks like all is well now regarding that problem.

Thanks to all for the help.

C R Huff

This thread was discussed between 28/10/2008 and 02/11/2008

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