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Non Pressurized Radiator/Expansion Tank Caps

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GreenB Avatar
GreenB james robinson
Staten Island, New York, USA   USA
The cap on on top of radiator must have a vacum valve . For the system to work as Steve described

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barry s Avatar
barry s Silver Member Barry Stoll
Alexandria, VA, USA   USA
1972 MG MGB GT
1974 MG MGB
1976 Triumph TR6
1980 MG MGB
Steve - You may be unaware that there are thousands of B owners/drivers that have suffered no need to periodically (seldom nor frequently) top up the radiator coolant. You've got to come up with a more compelling reason for adding a non-OE overflow tank. BTW if the overflow tank is open to the atmosphere, what enables the radiator to suck in fluid? The radiator overflows when the pressure in the radiator exceeds the pressure of the cap. As a radiator cools the pressure in the radiator lowers until it reaches ambient. As the overflow port is located above the cap seal, what suction is applied to the auxiliary tank/container?

Steve S Avatar
On The Road, Lost in So Cal, USA   USA
In reply to # 3661361 by barry s Steve - You may be unaware that there are thousands of B owners/drivers that have suffered no need to periodically (seldom nor frequently) top up the radiator coolant. You've got to come up with a more compelling reason for adding a non-OE overflow tank.
I can't tell if that was serious or not. confused smiley

I never said anyone needs to keep filling up their radiator. I only said the level will drop due to fluid expansion and overflow. A radiator without an expansion cannot run completely full, and that's fine. It will be full to the top when hot, and appear a bit low when cold. It's normal. If you continue to fill the radiator, all you will do is dump coolant on the ground every time the engine warms up. I'm not trying to convince anyone to install an expansion tank, I was merely answering the question. I run several cars without them and they work fine. Some don't even run a pressure cap, just atmospheric pressure.

In reply to # 3661361 by barry s BTW if the overflow tank is open to the atmosphere, what enables the radiator to suck in fluid?
If you look inside an expansion tank, you'll notice the outlet pipe starts near the bottom of the tank. To put it simply, it works like a straw.

In reply to # 3661361 by barry s The radiator overflows when the pressure in the radiator exceeds the pressure of the cap. As a radiator cools the pressure in the radiator lowers until it reaches ambient. As the overflow port is located above the cap seal, what suction is applied to the auxiliary tank/container?
Because the contracting fluid has to be replaced with something. The only place it can draw from is the overflow tube, connected to the expansion tank. Therefore fluid is drawn back in through the vacuum valve in the pressure cap. And if you don't like this cap arrangement, you can switch them (pressure cap on expansion tank). It works either way. Or if you happen to have a pressure cap without a vacuum valve as James mentions then you'd have to do it this way anyway.



Edited 1 time(s). Last edit at 2018-01-14 01:32 AM by Steve S.

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Donthuis Avatar
Donthuis Don van Riet
Rijswijk, ZH, Netherlands   NLD
Do not mix up both types of systems: this is only valid for the non-pressurised overflow tank, when using any pressurised expansion tank the cap on the radiator should NOT have a valve. eye rolling smiley

There are long threads for both systems on MGE. I hereby include a picture of my own overflow setup I used before converting to the OEM expansion tank setup. The expansion tank was already in place, but was not yet connected. So the original overflow hose still ends in an open bottle. For the expansion tank setup with the closed cap on the rad see my earlier post....

In reply to # 3661360 by GreenB The cap on on top of radiator must have a vacum valve . For the system to work as Steve described


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IMG_7687.JPG

Donthuis Avatar
Donthuis Don van Riet
Rijswijk, ZH, Netherlands   NLD
One can keep a higher coolant level in the CBB radiator without loosing fluid, because the overflow or expansion tank takes it up and let it move back in again after cooling. IF you go the expansion tank route raising the pressure level by a 13 or even 15 psi cap on the expansion tank also raises the boiling point of the coolant.

Of course both actions are not strictly tied to each other, but since BL also combined both modifications in the RBB line I went the whole expansion tank way as well, with the last 15psi cap being used MG

Personnaly I found the overflow system less attractive, but this is a matter of taste. Anyway, any system will work better with a 15psi cap if the radiator is in normal shape...

In reply to # 3661280 by barry s What is the benefit of an overflow tank on a CBB?

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