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 Midget and Sprite Technical - Searching for a coolant thermal switch

Good afternoon Gentlemen
I need a thermal switch with a 3/8" UNF thread or a maximum diameter of approx. 12-14mm. It must not have a lenght - thread and probe - of more than approx. 20mm. A switch with two electrical connetors is preferable, but not a must.
Temparature range would be between approx. 83 and 87 degrees Celcius, right?
Does anyone recall a thermal switch to look for?
Jan Kruber

You might find what you want here
Dave O'Neill 2


3/8"UNF is a small thread for a thermo switch - is it correct?

Most are bigger at 3/8" BSP or NPT, 5/8" UNF, 14mm or 16mm+.

There are some Intermotor 14mm switches in the attached picture.

Richard Wale

More time to edit PLEASE!

Picture below:

Richard Wale

Great, I could use one of the 14mm switches, thank you

Good work, you managed it at last :-)
I ask for a 3/8 UNF because in the switch housing there's a 3/8 UNF thread. The switch that came with the kit is a 3/8 pipe thread and an even larger 1/2" pipe thread adapter, but as mentioned, the only fitting hole in the housing is a 3/8" thread, weird.

Which temperature range would you guy's think I should aim for?

Good work Richard, you did it at last :-)
Jan Kruber


When you say 3/8" UNF is that correct as a 3/8" pipe thread refers to a nominal 3/8" bore pipe and the OD of 3/8" pipe is about 0.66" (16.66mm). 1/8" BSP is slightly larger than 3/8" OD.
David Billington

The thread in the housing IS for sure a 3/8" UNF / 24 TPI thread, checked it with a 3/8" UNF bolt
Jan Kruber

Be carefull there
A 3/8"UNF will screw into a 1/8BSP---sort of
If it's a straight BSP(28tpi) thread it will screw into a short thread, like the length of a nut but be loose in the thread --If it's a tapered BSP a 3/8 UNF will screw in for about 3 or 4 threads then bind up
William Revit

Could you drill the housing out and tap to 3/8 BSP and use the switch you have already
William Revit

As you can see, there is a single switch body fitting hole, with the 3/8" UNF thread (with a bolt inserted to visualise it)
On the picture you can see the 3/8" pipe thread switch body. If I enlarge the fitting hole and cut a 3/8" pipe thread, I imagine the switch body to go too far in the liquid passage. Fitting the switch body in the 1/2" adaptor would move the switch body back, but there is not enough material in the housing for the 1/2" pipe thread.
So I will buy a 14mm" thermal swith from the "Carbuilders", bore out and cut a 14mm thread for it. The 14mm switch body looks shorter and I dont think it will go too deep in the housing.

Jan Kruber

and one more

Jan Kruber

I don't know about others but I can't see that you've given enough information to decided what temperature switch you might consider.

Where is this fitting going, top hose, bottom hose, elsewhere - back up/second or only fan - size and location - water stat rating (if fitted), which engine (1275?) and state of tune - oil cooler with or with a stat.

Or perhaps I'm making it over complicated and 83-87c is fine regardless.

Nigel Atkins

It will be placed in the top hose between the elbow thermostat housing and the radiator.
Only for the fan, a 9" Revotec blower.
Standard tune.
No oil cooler fitted.

Jan Kruber

Yep, got the picture now, the 14mm switch sounds like the answer- even if it is a little long and pokes into the flow a tiddle it would be ok
On the temp. selection, I'd imagine the switch they supplied in your kit would have a temp. written on it ,just go the same
If it hasn't got a temp. on it you might have to test it in hot water and see what temps it comes on and goes off at
Usually the off temp is 3-4 or5 deg lower than the on to stop the fans continuously cycling
William Revit

just for info - the Revotec midget kit (which this is not) normally comes with a temperature adjustable switch with a setting range of 70c-120c, and a 5c difference in on and off. Unfortunately switch range of 3/4 of a turn is unmarked so it
Nigel Atkins

I have the Revotec Midget fan kit with (now uprated) 9" blowing fan and the adjustable electronic fan controller described to Willy fitted in the top hose, 82c water stat, engine fan removed, mildly uprated engine, uprated alternator, no oil cooler.

My fan switch is set about halfway through its sweep so as a pure guess might be at 95c (switch on) (or perhaps 90-100c? or above or below this) but whatever the temperature at that point the setting seems about right to get my gauge needle back to its usual range of reading.

Nigel Atkins

The thermal switch that came with the kit has the range 175 - 185 degree Fahrenheit, equals 80 - 85 Celsius.
The housing with the switch will be inserted in the top hose.
Would you think this is the right temperature range, considering the location?
It is certainly a quite lower range than yours, Nigel.

Jan Kruber

I just soldered a boss in the radiator after I had tried one in the top hose which came on too early.

mark heyworth

The one I bought from Carbuildersolutions is 88/79C. I also bought the boss for the radiator, although I haven't tried it yet, so I don't know if that's the correct range.
Dave O'Neill 2

Years ago I had a discussion about thermal switch location with a auto race electrician, I mentioned I had fitted mine in the input side of the radiator, he said standard practice was to put them in the output side so the fan was only turned on when additional cooling was required.
David Billington

Always a matter of debate whether the switch should go on top or bottom hose, also covered in previous threads here - or indeed different locations, input/output, warmer/cooler.

You can in theory/fact adjust the setting on a variable range switch to suit/compensate.

As my electric fan is the only cooling fan on my car I think I prefer it nearer the start of possible heat build up - but of course if I was really nervous I'd install an override switch too so that I could cover emergency/bark for myself.

But as always it's each to their own (tastes and requirements) and there are many variances in the same model of car let alone different makes and models.
Nigel Atkins

I don't know about the hose adapter you have, it doesn't look like a Revotec one by the images on their website. I can't remember if it was you or someone else that said their adapter was 28mm but it surprised me as the ones I've had have been 25mm (for presumably 1" hoses).

Do bear in mind the (wide range of) figures I put up for mine were guesses (never checked) for fan switch on.

If you have an 82c 'water' stat then (allowing a degree or two either way) that is the temperature the 'water' stat starts to open and may not be fully open until 89c-95c.

Given this and the close proximity of the switch in the top hose, allowing for heat build up and/or dissipation between the two, I'd have though but don't know that a switch on? or off? of the fan at 80c/85c or 83c/87c might be a bit too soon/low.

What do others think?

Nigel Atkins


You can buy the hose adapters with a choice of threads, from a range Imperial sizes and types and a range of Metric sizes that would suit your switch:

Also look at:

M Wood

Where did you put the adapter in the radiator, in the top or in the bottom? And in which temperature range is your switch?
Jan Kruber

An adjustable temperature range (thread fitting) switch would cover anything within its temperature range.

Just as an example only, no doubt others makes are available and the Revotec ones will be available from other suppliers at lower prices -

(negative Earth EFC 3/8" BSP external thread also listed)

Electronic Fan Controller Threaded Fitting -
Nigel Atkins

ETA: for the same cost you could sell on your existing hose adapter and switch and buy a an adjustable temperature switch hose fitting kit that you could put in the top or bottom hose (or buy and make up your own kit perhaps) -

Electronic Fan Controller Hose Fitting -
Nigel Atkins

This thread was discussed between 11/12/2018 and 14/12/2018

MG Midget and Sprite Technical index

This thread is from the archive. The Live MG Midget and Sprite Technical BBS is active now.