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MG Midget and Sprite Technical - Over cooled!

A couple of years ago, after a hot summer with which the standard radiator (twice recored) could not cope, I fitted an aluminium one. Made to order by Coolex. No problems with overheating after that, quite the reverse. All was fine until the cold weather arrived. After 55 years of driving Sprites I was used to a gale of warm air from the heater, but not now. The faster I go the colder it gets. I normally blank off the oil cooler in winter, but that makes no difference. I pulled out the thermostat and tested it. Opens at the correct 82ºC.

The cylinder head has no bypass, so I always drill a couple of 3/16" holes in it. I'm wondering if I should not do this. Just waiting for a new thermostat to arrive, but meanwhile does anyone have any thoughts?
Les Rose

Hotter ‘stat?

Moss list a 90ºC ‘stat for cold climates, although I thought they used to be 88ºC.
Dave O'Neill 2

When my 1966 Sprite's vertical flow rad, the original rad, finally became too blocked to clear, and I found the recore cost or a new replacement unacceptable, I converted to xflow.

Immediately I found it over cooled. So I run an 88c stat even in the summer. Even on the hottest days, sitting in traffic, it doesn't get close to 190f.

I thought there might be a faukt with the stat. But testing in a pan of hot water, it opens and closes as it should.

So like you Les, in really cold weather, it's too cool. On very cold days, I'm lucky to see 160f on the temp gauge, and the heater is 'warm', not hot.

Sometimes I partially block the airflow through the front of the rad, because running so cool isn't good for the engine.

I've considered removing the belt fan and replacing it with an electric controlled by a stat. I must give that a try.

Yep, that's a good idea. I'll remove the fan blades from the pump temporarily, and see what happens.




anamnesis

New 82 thermostat WITHOUT THE HOLES will fix it
Drilling the thermostat is different to having a bypass.
A bypass simply recirculates the coolant through the engine block at whatever the temperature of the coolant is at the time--Basically it's got nothing to do with temperature control apart from giving the coolant somewhere to go while the thermostat is shut-

Drilling the thermostat is a different animal, in doing this the coolant circulates through the cool radiator instead of the warm engine. It's more or less like holding the thermostat open all the time, it'll take longer to warm up and temp. won't be regulated properly- The engine 'might' warm up in town but as soon as there's airflow cooling the rad. the engine/heater temp will plummet.

If you're running without a bypass then all that's needed is a single 1/16" hole (no bigger) to allow trapped air bubbles to escape and just go easy on the revs until your thermostat gets up to temp and opens. Even just removing the little jiggle pin that's on some thermostats leaves too big a hole and corrupts thermostat operation.

IF the thermostat is working properly then it's impossible to have too large a radiator the thermostat is the only temperature controller and I believe 82-83 deg is the perfect temp stat for these older cars-

William Revit

Have you guys running without the heater hose who picks up cool air from behind the radiator.

Ive nevver fitted it, not on my 1500 engine and not on my 1.8K, so my heater blows warm enginebay-air thrue the heater radiator onto my feet.
Works very well for me
try it and see if it works well enough for you?
A de Best

Arie,

My Sprite ran withouth the hose for years until a competition scrutineer told me off! The missing air duct compromised the fire wall. He even told the owner of the Midget next to me that his factory hose wasn't satisfactory as it was plastic. If you don't do competitions it won't matter either way. I was also mildly concerned about the likelihood of drawing exhaust fumes into the car but never actually noticed any.

I now run with a length of flexible aluminium hose.

On the plus side, the scrutineer's happy and I get a decent flow of warm air to the screen (and knees) on rainy days so the screen doesn't mist up. On the minus side, the hose looks a bit incongruous.

I've not noticed any difference in the engine temp with an 82 degree thermostat. Ever since I fitted a new vertical flow radiator the temperature has stayed consistently low. The highest the gauge ever goes now is to the N (not sure what temp that is) whereas, with the older radiator it went higher.

C
C Mee

Got me wondering about my Stat now Willy.

I haven't drilled it, but pretty sure it has a 'factory' hole, and jiggle pin.

Pretty sure GTS106, 88c, is the winter stat for A series in the UK.



anamnesis

Thanks chaps. I should have said that it's still a vertical flow rad, and I have an electric fan with thermo switch. It hardly ever comes on.

New 82º thermostat just arrived. No jiggle pin so I will drill it 1/16"

Interesting story about the air duct. My plastic one is about 40 years old and still fine.
Les Rose

I wonder how efficient the heater box itself is in transfering heat to the air passing through it.
My suspicion with Arie's arrangement is that better heat in the cockpit is only partially due to it using warmer under bonnet heat. I think that the ram effect when air is taken through the duct produces a greater volume of air when at speed, than the little heater rad can cope with.
GuyW

Guy, I think the heater is fine as the matrix has been regularly flushed out and flows water well. The faster I go the colder the engine runs, when it should be the other way round. There is lots of heat at idle with the heater fan running. That of course is when the rad fan turns on.
Les Rose

"I wonder how efficient the heater box itself is in transfering heat to the air passing through it."
Good point.
Irrespective of how warm the engine was, the heater only ever blew tepid air at best. Taking it apart revealed the foam insulation had mostly dissolved and the air was taking the path of least resistance i.e. above and around the matrix.
Replacing the foam forced the air through the matrix and it's now too warm at times, especially with the (heater) fan on.
Sorry not a lot of help for you Les but the transformation was remarkable.
Jeremy MkIII

Les, I wasn't sure from your original
post how you were determining that it was running cold. You seemed to suggest the clue was that the cabin heater wasn't as warm as it used to be. But maybe you had measured the coolant temperature as well? Those infra red 'gun' thermometers are useful for that and cheap enough.

But if you are saying this purely based on the heater performance then Jeremy's explanation is worth considering, even if you know that the heater core is good. The faster you go the more cold air gets forced through the heater and if it is even partially leaking past the core it will come through as colder air.
GuyW

one interesting thing i have noticed with thermostats, here anyway. Years ago when thermostats failed the majority of failures were thermostats failing to open causing overheating. But nowadays it seems the opposite is the norm as they seem to fail open or slow to open causing cold running. Fuel injected cars suffer from it badly as their fuel mixture is regulated against engine temp.the result being slow warm up so running richer for longer. people doing short running and not getting up to temp get woefull fuel economy. And then when their car does get up to temp and they venture out on the highway, the temp plummets and the poor old computer sees cold and not up to closed loop running specs so thinks the engine is cold and pokes more fuel in again--I'd say a good 5% of cars presented for servicing suffer from this but it gets overlooked time and time again because the customers don't read their gauges properly---if at all and are happy with their temp gauge sitting well down.and the service people just do their thing and a quick road test without even looking for a fault like this.
William Revit

Hi Willy. Something you said earlier made me think what you also just said.

I run an 88c stat in summer, and don't overheat. Still too cold in winter.

I tested the stat and it fully opens and fully closes.

BUT, there is quite a delay in closing.

So if on average, it is fully open for longer than it should be, that would keep the engine cooler than it should be too.

New stat, so must be faulty?



anamnesis

sounds a bit like it could be on it's way out. I've had them where you pull one out of a hot engine and sit it on the bench and instead of closing up straight away they'll just sit there for ages and when they eventually shut, they're not properly closed off and you can see plenty of light around the door causing slow warmups and cool highway running in use.
William Revit

Thanks Willy.

That might be the answer to mine and other's too cool problems then. Crap stat, even if new.

That's a spring job lined up for me then. Stat out, boil it, and time how long it takes to close up.

I wonder if there's a spec' sheet for stat opening and closing times.

anamnesis

The thermostat often gets overlooked. In olden days it would often get changed at service intervals or for summer & winter. I run both of my Sprites on 74 deg Stats but must admit to not changing them in 22 years.
Alan Anstead

I used to swap mine over twice a year but winters arent as cold in recent years and I am older and less enthusiastic. Also mot so easy if you have switched to a 4life type of coolant.

Oftentimes radiator blinds were once used as well. Useful when your 'classic' was your everyday car and winter mornings you set off in the dark at 5.30 to clear the ice and frost for your 50 mile commute for a 7am start.
GuyW

Guy
Even earlier for me as my shift in London started a 0645hrs. I left home at an ungodly hour!
My Frogeye was my only car back then so commuted to & fro in it from 72 to well into the 80s.
It was fitted with a Shorrocks Supercharger. I used to lag its long inlet with asbestos rope against freezing. If snow had fallen the build up between the tyre worn areas on the road, central to the car, would build up in front of the car and eventually bringing it to a halt necessitating me getting out and clearing it with a WW2 trenching tool. The front bumper acted in reverse of a snow plough!
Alan Anstead

Guy...how many of these still exist?.. l press it into use as and when needed
Dave

David Cox

David, I have one, only ever used it on the odd occasion mostly for the reasons made by Guy.
Philip Sellen

To clarify, while the lack of heat to the cabin is obvious, this is correlated with the dashboard gauge showing only about 65ºC at 60 mph. At idle it stabilises at about 95ºC when the rad fan comes on, and the heater blows hot.

I haven't tried the new thermostat yet, because I want to change the housing which is corroded. I have a new one, but the thermo switch doesn't fit. The housing and switch are for a Marina 1.3, and I can't understand why at some point the factory changed the thread in the housing. I need either to get a switch that fits, or make an adapter. But I can't identify the thread in the housing.
Les Rose

On my 1275 the fan thermostat fits into an adapter in the top rad hose. I think I got it from an early ford fiesta but I think Burton Power do a similar fitting. This then retains the standard thermostat housing.
GuyW

Les,

What's the thread on your current thermo switch? Many I've encountered use a metric thread with 1.5mm pitch.
David Billington

Dave, the current switch has a pitch that looks like 1.6mm. The new housing is nearer 2mm. In fact a 3/4" plumbing adapter screws into it.

Les Rose

Les,

What's the OD of the thermo switch thread?
David Billington

21.5mm
Les Rose

I'm using a modern fan switch, which is M22 x 1.5
Dave O'Neill 2

That was going to be my guess as to what the thread was M22 x 1.5mm and a common size for fan switches. A 3/4" pipe thread shouldn't fit as too large, a 1/2" pipe thread is slightly smaller at around 21mm so might screw in.
David Billington

The problem though is that the modern M20 one won't fit as the thread in the housing is coarser. Is it an imperial size? The 3/4" pipe goes in but a bit loosely.
Les Rose

What I need is to bore out the housing and to fit an insert with an M20 thread. I can do all that except that I don't have an M20 tap.
Les Rose

1/2" NPT ones are available, 14 TPI, one of the few that have a common pitch with BSP. A 3/4" pipe thread is a bit over 26mm OD. I have a M20x1.5mm tap as it's a metric conduit thread size, but it's about 40 mile from you, maybe something can be arranged.
David Billington

Thanks for the offer Dave. I can buy a tap for about a tenner so not sure the trip is worth the fuel. I'll have a go at making the insert and then decide how I'm going to tap it.
Les Rose

I have a 3 row custom radiator. This is the winter solution.

Flip Brhl

👍
anamnesis

I am getting a pair of taps, taper and plug, all the way from China for £9 post paid.
Les Rose

Bonkers isn't it Les?
I buy stuff on Temu. Minimum order £15 which includes air freight and arrives, usually in just over a week. If any items are a pos, you ask for and get a refund or a credit.
Some of the stuff is surprisingly decent (e.g. inlet valve for a WC for £2) and a lot appears on eBay or Amazon at a significant mark up.
Jeremy MkIII

I was going to say you should buy locally to support UK industries. But chances are your local supplier of much of this stuff is buying in their stock from China in the first place!
GuyW

Flip, my guess is you dont have a fan infront of your radiator? LOL!!
A de Best

I have completed the modification of the thermostat housing. This required:

Offcut of aluminium tube (£4, eBay)

24mm blacksmith's drill bit (£7, eBay)

M22 x 1.5mm taper tap (£7, online, not eBay). Yes, the switch is actually M22 not M20. All items were free postage.

I made a jig to mount the housing on the lathe carriage, and used the 24mm drill bit in the chuck to bore it out. I turned a tiny bit off the tube so that it would slide tightly into the housing, leaving a slight flange at the bottom so it couldn't be pulled out when screwing in the switch. I then glued the tube in with Araldite Metal.

The bore was a bit small for the M22 tap, and I didn't have the right tool to ream it out (and was too mean to buy one), so I honed it with emery and the cordless drill until it looked and felt about right.

I used the lathe again to get the tap started, as the home made jig kept the housing square to it. Once a couple of threads were cut, the tap was held in the vice and the housing used as its own tool to cut the remainder (I didn't have a tap brace big enough).

All back together now, with new thermostat. A rather satisfying job. Someone will say that leaving the tube insert with only a 1mm wall is a bit thin, but there was no need to tap it all the way through as the switch is only threaded about 7mm. I was constrained by the materials and tools I could obtain.
Les Rose

This thread was discussed between 21/01/2026 and 21/02/2026

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