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MG MGB Technical - 77 MGB temp sensor popped out

I'm considering buying a 77 mgb roadster and during the test drive the temperature sensor near the top of the radiator popped out and spewed coolant. While this was very embarrasing for the seller it wasn't a deal-breaker for me but I am concerned as to why it popped out and is there anything that can be done to keep it from happenning again. Any help would be appreciated.
RL Lucus

It might just need a new seal, the later cars were different and they added a clip to hold it in, I have changed a few cars to a screw in switch. You have to solder a bung in the existing hole with the correct threads then you are good to go, a rad shop could do it for you if you can't.
Bob Fisher

Not familiar with the later cars but that experience would encourage me to be VERY thorough in looking at the car. There may be some overdue work on other components - a LOT of other components. No deal breaker necessarily, but certainly a price negotiation driver!
Roger T

As Bob says this isn't uncommon, either with old seals or new non-OE, which is why the later cars had a spring-clip as well. In itself it's not a cause for doom and gloom, but doesn't preclude the possibility of wear on other components of course, which should always be looked for, and doesn't necessarily warn against purchase like perhaps body problems might.
Paul Hunt

I wouldn't worry about that. As far as MG problems go that's minor!

On those later cars the switch is just held in by the rubber seal and good luck. Even with a reconditioned radiator and a new seal I could never get mine to stay in. I think even with the clip from memory which is just a piece of thin spring steel?

In the end I machined up a steel plug with a screw thread in the back. I made a decent steel bracket that clips over the top of the radiator and has a screw into the back of the plug. I'd already replaced the later style dual non-shrouded fans with one big shrouded fan which came with an adjustable sensor. That has a bulb on a capillary tube which goes into the top radiator hose. The sensor body is mounted nest to the radiator on my custom made fan mounting brackets.

You can see it in a picture half way down here.

Looking at that picture I should probably tie wrap that long capillary tube to the radiator top pipe so it doesn't vibrate too much and work harden and crack.


Simon Jansen

I wouldn't worry about the implications of this too much.
This once happened to a friend on his late model B on the path into a busy car show. This was a show for all cars: antique, classic and customized. He felt no end of embarrasment as it blew like a tea kettle in front of all those people! Bystanders were moving well out of the way!


Mr. Lucus is considering buying my car. I had heard of these sensors popping out, but it had never happened to me, even though I've had the car since 2003. I had another still in the package, so I replaced it immediately (with seal), and we went back out for the rest of the test drive. (I later fashioned a clamp and secured the sensor in place, just in case.)

5 miles down the road, it started chugging, and we pulled over to look. Ralph noticed a problem with the throttle linkage, and I later discovered the cotter pin had disappeared.

Yes, as Ralph noted, this was very embarrassing, especially since I told him the car has purred along for almost 2 years with no problems (I SWEAR it's true). I wouldn't blame him, and I told him so, if he decided he didn't want the car.

The car had been parked since the end of April...I don't drive it much in the heat of a Texas summer. It's wonderful the other 9 months of the year, though.
Fred Doyen


Any chance that some of the chugging problem was caused by the coolant bath?

C R Huff

That's the first thing I thought about, Charley, but it was the throttle linkage.
Fred Doyen

The fact you could easily figure out the problem and know how to rectify it shows just how simple and easy to maintain these little cars are.
Simon Jansen

Thanks to all of you for the helpful comments. As Fred stated he put in a new sensor and seal and fabricated a clip to hold it in place and that seems to be working just fine. I have since bought the car from Fred (my second MG by the way - I had a '76 midget that I bought new in '76 and would probably still be driving it if I hadn't totalled it in 1991). I'm looking forward to driving my MGB those other 9 months that Fred mentioned. Thanks again.
RL Lucus

I solved this problem by using a some thin steel wire to hold the lips of the spring clip firmly in place relative to each other and therefore to the top lip of the sensor. This stoped the sensor moving in relation to the clip and the whole assembly to remain "fixed" in in relation to the header tank. It was this movement that allowed the clip top lip to slip off the sensor , the sensor to skew and pop out and the boiling water to just miss my ear as I was peering into the Dizzy.
This simple mod has been in place for 4 years and is a visual check item.

I also soldered 2 wires , (slightly bigger in cross sectional area than the existing wires) onto the leads from the sensor ( which is basically a swith) , lead these back to a new "FAN" switch on the dash. There is a spare place on the switch panel on the dash so we dont have a mis matched switch panel. Now we can swith the fan on and off as required.
R. E Bester

Good job it was 'peering' and not something else or you would have got an even biger shock ...
Paul Hunt

This thread was discussed between 26/07/2009 and 04/08/2009

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