MGB Register Forum

I have managed to snap the Capillary Thermostat probe located in my top hose.
I plan to replace with a like for like but unsure if I should also replace the top hose? the present hose is 30 years old and I feel a replacement hose will be more forgiving in sealing around the probe area where it comes out of the hose.
Has anybody had the experience of fitting a replacement & was are the main concerns. On Installation I was planning to seal the hose with a high temp sealant.

Is this the sensor for the electric fan? I'm guessing you have a late RB car?
I think any hose which is that old should be replaced when an opportunity like this arises. Make sure to get the best quality replacement you can find - cheap ones are unlikely to last another 30 years!

Ian F

As Ian says, get the best replacement you can, both switch and seal.
The one I bought hasn't moved since it was installed in 2019.
You can turn the hose clamp on the top hose so that the barrel of the clamp sits directly behind the connector for the switch, holding it in place.

As for hoses, spend a bit more on them as the quality of reproduction rubber parts can be hit and miss. I bought two track rod ends from a well known MG parts supplier and the rubbers perished before the car even got onto it's wheels. They lasted about three months.
I was surprised to find that my local motor factor was able to get new branded items and they have been grand. I paid a little more.
Worth checking local motor factors for other parts like hoses.

Personally, I wouldn't put silicone sealant anywhere near the cooling system, especially given how prone the heater tap on the block is to clogging up. With the right gaskets and hose clamps the system should be grand without it.

When you mention a capillary thermostat probe fitted in the top hose this says your car is using an aftermarket thermocouple probe based temperature switch to control the electric fan. This isnt a standard part, its a generic aftermarket switch someone has fitted to your car.

Where an electric fan was factory fitted this is controlled by a dedicated 'otter' switch which is a press fit into the radiator top tank, fairly close to the exit to the top hose.

If your radiator has a provision for a proper otter switch it would be worth fitting one. However if you have the earlier type radiator without the facility to fit an otter switch then the most practical course of action would be to replace the damaged aftermarket probe based unit. I think they are only about £ 12 on Ebay, They have their place and are generally reliable. I fitted one on my daughter's 15 year old Nissan Micra recently as a simple work around when that car's unnecessarily complex electronic fan control circuit failed.

Thanks for clarifying that Chris - I'm not up to speed with all the additional bits fitted to the later MGBs.

Ian F

Thanks for the response guys. I'm driving a 1970 model and it doesn't allow a otter switch to be installed. I'll pick a replacement hose from Moss, and the Capillary is available on line from numerous outlets. I'll seal the hose with gasket sealant.

You should be ok without using sealant if you are fitting a new hose. None was needed on my daughter's car. The hose sealed as soon as i started tightening the jubilee clip.
Avoid sharp bends on the wire to the probe and keep electrical wiring neatly cable tied away from the moving parts of the fan and the job is done.

Thanks for the response guys. I'm driving a 1970 model and it doesn't allow a otter switch to be installed. I'll pick a replacement hose from Moss, and the Capillary is available on line from numerous outlets. I'll seal the hose with gasket sealant.

You would be wasting your time with gasket sealant for this unless you are attempting to reuse an old hose where the rubber has hardened and started to go brittle. Any recent, serviceable hose will seal itself around the very thin wire. These things are fitted as a workaround or to control aftermarket fans all the time, there no need to overthink this one. It is as simple as it sounds.

Nice one ill fit a new hose & skip the sealant then see if it does the trick.the sealant was a precautionary measure.