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MG MGA - Closed Coolant System

Which tank gets the pressure cap? The radiator tank or the remote tank? I have two very well respected mechanics here local with opposite views. Currently, I have the radiator header tank with a blanking cap and the remote resevior with a 7# cap.

It would seem to me I have it set up correctly. As the liquid expands, it goes over to the resevior which is pressurized. As the liquid contracts it can be drawn back over to the unpressurized radiator tank.

Just to confused the issue, this is for a '59 Twin Cam but the system is a modification from original and both tanks are remotely located but for this communication, the Twin Cam header tank is the "radiator" tank.

Thanks for your help.

Paul
Paul Hanley

Paul - If you have a pressurized expansion tank (remote from the radiator header tank), it gets the pressure cap. If you are using an unpressurized coolant recover tank, the pressure cap goes on the radiator header tank. Both systems do the same thing in different ways. They both keep the radiator coolant level up to the top of the header tank, thus, no air in the cooling system. Cheers - Dave
David DuBois

Greetings Dave,

Here's what I've got: I have a small--maybe 1 quart in size--remote mounted, metal tank. In its current configuration, it has a normal 7# radiator cap. There's a rubber washer in the top of the cap, a spring loaded devise that reaches down into the neck of the tank and the end of the spring devise also is a rubber gasket to seal the bottom of the neck. In all, a normal readiator cap. When the car is warm, its under pressure for sure.

Also in the current configuration, the radiator header tank has what appears to be a normal radiator car except there's no guts to it. Simply a top cap with a rubber washer on the underside. It only seals the top of the radiator tank's neck. The hose connecting the two devises is just below the top of the filler neck on the radiator tank. When the engine is warm, the radiator tank is clearly under pressure also.

What the one mechanic was telling me today, was if I switched the two caps, I could add coolant to the remote tank if needed without having to let the car cool off completly.

Any of this make sense to you? Am I using the correct caps where they belong? Is there any advantage one way vs. another and is it possible to set the system up incorrectly?

Thanks,

Paul



Paul Hanley

Paul - From your discription, I would say that you have the system set up correctly. Modern cars don't have a cap on the radiator header tank at all, but older vehicles use the set up that you have. As for having to wait until the engine cools before adding coolant, I don't know why you couldn't do that with your present setup. Just like the radiator cap on a vehicle without an expansion tank, it is possible to add coolant when the engine is hot. The first notch of the cap, when removing it, releases the pressure of the inner seal, while keeping the outer seal engaged. This allows the pressure and any steam to be released through the overflow tube prior to releasing the outer seal when the cap is turned the rest of the way. I am not sure why one would want to add coolant when the engine is hot, but it could be done with your system as it can be done with a normal radiator.

The way the mechanic discribed to you, with the double pressure seal on the radiator header tank and the single seal on the expansion tank, turns the expansion tank into a unpressurized coolant recovery tank. Either method works well, with the non pressurized recovery tank doesn't have the problem of the metal (usually brass) work hardening and cracking from repeated expansion and contraction each time the car is driven (this is a common problem with the expansion tanks used on the later MGBs. I have a coolant recovery tank on the MGB and even on the TD, which doesn't have pressurized cooling system.

In case you don't know how a coolant recovery seystem works (a lot of people view it as nothing more than a catch tank to keep the coolant from going onto the ground), here is a discription. When the coolant heats up in the engine/radiator, it expands and pushes past the inner seal on the cap and out the overflow tube between the two seals of the cap. The overflow tube is connected to the bottom of the recovery tank, which is maintained at atmospheric pressure. After the engine is shut off and the cooling system cools, the coolant contracts and forms a vacuum in the system. This vacuum then draws the coolant in the recovery tank back into the cooling system through a small check valve in the lower seal of the radiator cap. Years ago, before the advent of coolant recovery systems, or the wide spread use of expansion tanks and thus, no check valves in the radiator caps, it was not uncommon to see cars with collapsed radiator hoses from the vacuum that developed when the coolant cooled to ambient temperature.

This is probably far more than you really wanted to know, but you asked :) Dave
David DuBois

Paul

David has said it all. A number of us run MGB or similar expansion tanks. This is my set up:

http://freespace.virgin.net/stephen.gyles/radiator_expansion_tank.htm

Steve
Steve Gyles

Steve - Should the expansion tank not be situated higher than the header tank?
joe

Joe

No. It does not make the slightest difference. The internal pipe goes to the bottom of the tank so the 'expanded' water gets sucked back regardless of its position relative to the top of the radiator.

Works absolutely fine, believe me.

Steve
Steve Gyles

I just need a space to install one. The spot in Mr Gyles' photo is in my car occupied by a large pulley and two belts for the SC......

Any idea where to get the mounting brackets for the brass tank?
dominic clancy

Thanks Dave. Just what I was looking for. Steve--your description very helpful too. Dominic, as far as space, mine is not vertical like the MGB but rather horizontal--perhaps more square--from an early Jag, I was told but not certain. It occupies the space where the washer fluid bottle wold have gone had it been fitted. A neat and tidy system that except for the trained eye, looks completely original and appropriate. And a very short distance from the rear of the header tank, across the rear shelf and into the remote tank.

Thanks all.
Paul Hanley

Dominic

I got the tank and strap separately but from the same MG scrap yard: http://www.mgbreakers.com/

They list second hand expansion tanks for £10 and when I spoke with them they were able to find me the bracket.

Best of Luck

Steve
Steve Gyles

Most modern cars use an unpressurized plastic tank. The radiator cap is slightly different than the old style in that it has a rubber seal around the upper seal instead of the brass one on original cap. The reason for the unpressurized system is it works and is much less expensive to produce. The reservoir can be mounted any where you have space. Daewoo and VW use presurized plastic tanks with screw on pressure caps what the advantage is I don't know.
R J Brown

Paul, I have a plastic expansion tank mounted in a bracket up behind the front valance panel below the radiator, connected to the raqdiator overflow tube, it works fine and is invisable..
John H

The conventional wisdom used to be (note past tense) that the tank for a unpressurized coolant recovery system should be mounted at approximately the same height of the radiator header tank. That is how I have mounted mine, but I am not absolutely convinced that that position is mandatory. The only reason that I can imagine that it would be necessary to locat the tank there is to prevent syphoning of coolant out of the system in case of an air leak somewhere else in the system. In the case of the sealed and pressurized expansion tank, the tank height would not make any difference at all on the operation. Cheeers - Dave
David DuBois

This thread was discussed on 08/03/2006

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