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MG Midget and Sprite Technical - Thermostat: drill or not to drill

Here's a good old one!

I have a 1275 in a Frog. Vertical flow rad. It takes a long time to warm up and currently has the rad 50% covered. The other day the gauge sat at 160 deg (70deg C).

Ordered a new thermostat but during the strip down found the old thermostat is an 88 deg c stat and works at that too. Because the head has no bypass I drilled one 3mm hole in the stat.

Is that hole enough to bypass the stat with enough water to keep the temp down? Should I try the new stat without any holes or is there a risk of causing head damage?

And yes the gauge is ok.
Graeme Williams

I'd say that hole isn't big enough to bypass the stat with enough water to keep the temp down. Took me a time to find it but if you haven't seen this already see from 1:26 - 2:14 - https://www.youtube.com/watch?v=wWFHXu2q8tA

Even if your stat already has a hole (sometimes with bent pin) I don't think another 3mm hole would make that much difference.

I can't remember now but doesn't the vertical rad hold more coolant.

Perhaps the older gauges were more accurate but I've had three of the later type in my midget (2 brand new) and all under-read.

Have you got an oil cooler without an oil stat?

Nigel Atkins

3mm hole is big enough to slow the warmup considerably--i'd try 1mm, you need just enough to let any trapped air through so the stat doesn't get caught in a bubble

Running without a bypass is an issue but lots do it without a problem, better off with one to keep the coolant circulating through the block but if you havn't got one you havn't got one----------
William Revit

Yes, definitely don't go too big to start with, I put 2 3mm holes in once and had to weld one up as it wouldn't get warm. Never had a problem running without a by pass. In fact I don't think I've ever had an A Series with one!
John Payne

Just to clarify, the 3mm hole may slow warm up but there will still be warm up.

An 88c stat would be in tolerance to start opening 86–89.5c (187–193f) and fully open 94.5c–100.5c (202–213f) but I've never found that even though the gauge temperature reading bulb and stat are situated close to each other in the 1275 head that they correlate too closely on the gauges I've had in my 1275 Midget. (I've used both 82c and 88c stats).
Nigel Atkins

William: the stat comes with a "bleed hole" anyway.
If I don't add extra holes I am concerned the engine will be restricted on water flow and locally overheat. On the other hand, the stat is sitting in water in the head and that should heat up with the engine and trigger the stat. I hope!
Graeme Williams

Graeme,
if the stat has the hole then that covers the air issue JT said about, John and Willy say lack of bypass needn't be a problem. Check the Archives rather than rely on my memory but I think it's said that modern made stats tend to fail open rather than closed(?)

In case you missed it before, have you got an oil cooler without an oil stat?

Nigel Atkins

No oil cooler Nigel.

Although technically I can't fault it, I can't agree with your comment about "there will still be warm up" because at the moment there isn't. 88 deg C t'stat, working ok but water temp in top of rad is barely 70 deg C. After 20 miles!

And the gauge is correct!
Graeme Williams

There must be some warming otherwise the needle on your gauge wouldn't move.

Something appears to be wrong, with an 88c stat and the rad half covered.

Are you sure the stat and gauge are working OK and that accurate. Did you try Guy's suggestion elsewhere of feeling the top hose for when the stat starts to open?

If you used the same 1275 and vertical rad with the temp bulb in the same position last winter have there been any changes to the car and what temperature readings did you get on the same gauge last winter?
Nigel Atkins

Where is the temperature sensor? In the top of the radiator (as it should be for a std frogeye) or in the head (as it should be for a 1275?).

If it's the former then wouldn't you expect a lower temperature reading together with an apparent slower warm-up as the sensor is on the cold side of the thermostat?

My frogeye has the sensor in the radiator. The temperature takes quite a while to register and dosen't climb much above 160 or so in the winter months.
Simon Wood

Simon, it's in the top of the vertical flow rad. Graeme has an 88c stat and his rad half covered which I'd have thought would push the temperature up a bit, no bypass though, also I don't know if you get more cooling with a Frogeye bonnet over a later 1275 style body wings and bonnet.

My 1275 with an 88c stat showed on the gauge the equivalent of 60-70c in winter and might have gone lower, no engine fan, but the gauge I don't believe was accurate.
Nigel Atkins

If the weather is ok tomorrow I'll play around with a few things and see what occurs.

And Nigel both gauge and stat have been checked in the last couple of days. Both spot on.
Graeme Williams

Graeme
If your new stat already has a bleed hole(some don't)then i'd try it without a drilled hole
Some new t/stats look as though they don't have a bleed hole--the older type had a hole and a jiggle pin with a nylon ball but 'some' of the later ones without the pin just have a little nick out of the side of the door which does the same job as a tiny hole--
A bypass hose is there to circlate the coolant through the engine only, while the stat is closed normally---but lots and lots of A series are running around without them so i wouldn't get too concerned about not having one, once it's warmed up and the stat starts opening up it'll circulate ok anyway

On the 1100s (1275)and later clubman engines here they didn't have them anyway
William Revit

Thanks William. That was going to be my first step. I have a new 82 deg stat with beed hole so I was going to bung that in without any additional drillings.

I'll report back.
Graeme Williams

Graeme,
good news about the stat and gauge being spot on. I've very recently installed a brand new dual gauge and the water temp part read even lower than the previous two that read low. In theory by the manufacturing tolerances one 88c stat might be very close to opening at the same range as one 82c stat but in real life I don't think this is very likely and the 82c stats I've used have noticeable varied from the 88c stats I've tried.

It'll be interesting to see what you find.

You could also check your spark plugs before changing the stat and with the run using the 82c to see if you might be running rich by running too cool. I've read that within a range the cooler end will give more power and warmer end more economy but of course there are many factors to influence this temperature range.

Good luck.
Nigel Atkins

Put in a 82C (180F) stat without holes (other than the air purge).
During warm up on the drive I hit 175F on the gauge but on the road it sat a little below that. An improvement from the 88C stat which had a small drilled hole.
It took a long time to warm up, but always has done. but for all that I know that it has a tendency to overheat in heavy traffic.

Thank you for your advice, everyone.
Graeme Williams

As my head doesn't have a bypass, I've gone for a drilled thermostat. The engine warms up nicely and I tends to run at 90 degrees unless I'm in heavy traffic in hot weather, It's a fairly warm 1330 engine with a downflow rad.
See image


G Lazarus

I had one hole of that size! Don't know why my engine takes so long to warm up. It's 1330 with a lot of headwork, breathed on cam and balancing so it will have a bit more power than standard.
In my mindset I'm working on the idea that a stuck thermostat will boil. Mine doesn't so I must be getting enough water through and if it were heading that way, albeit relatively slowly, the stat would open and save the the day anyway.
That's the theory!
Graeme Williams

175f=80c so that t/stat appears to work fine
Maybe it was just the fact that your old 88 stat was failing towards cold and opening too early
Maybe a 1mm-2mm hole in the new stat to circulate some coolant while the stat is shut might be an idea
It could be getting warm at the rear and not reaching the thermostat area early enough
You can always solder it up if it creates an overslow warmup, but a tiny hole should be ok
OR
Does your frog have a heater or any other external hoses fitted
Really the t/stat is the end of the line till it opens and warmup should be reasonably quick
How long/far does it take to warm up
One persons perception on too long could be different to others
William Revit

Graham,
1330, more power means more heat to control so more surprising that you have lower temp readings unless you drive like a maiden aunt.

Some thoughts. -

Have you checked at what temperature on your gauge when the stat appears to open(?).

Any issues with coolant level in system(?).

Engine driven cooling fan would slow initial warm up but would help warming when the car is static and at low idle revs, do you have one fitted(?).

Is your heating/cooling hoses/piping plumbed in different to usual, is a heater fitted(?).

Have you tried checking the temperature (readings) at various points like top and bottom of rad, at temp bulb on rad, top and bottom hoses, front and rear of cylinder head(?).

What is your exhaust manifold like(?).

Do you have an electric cooling fan(s) fitted, if so where does the thermoswitch take its reading from(?).
Nigel Atkins

William: not the stat was spot on - opens at 88 within a degree or so.

All plumbing standard Nigel
Graeme Williams

This thread was discussed between 08/01/2020 and 11/01/2020

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