MG Restoration

Cooling System Overflow Tanks and Expansion Tanks

Les Bengtson

The topic of using an “overflow” tank seems to be one, which has generated much discussion, especially when we head into the summer months. Thus, it might be a good idea to discuss what these tanks do and how to use them.

First, the term “overflow tank” seems to be used in two different meanings, ones that are very dissimilar. There are two different types of tanks, which are used when referring to overflow tanks. The true overflow tank and the expansion tank, which was used on the MGC and late model MGB automobiles.

The true overflow tank is an add on item used with a conventional cooling system. The conventional cooling system uses a radiator, which has an attached filler neck to which a pressure cap is attached. The system is filled though the filler neck on the radiator and should be filled to about ½” to 1” below the bottom of the filler neck. If the system is filled any higher, it will be over full and the excess coolant will be vented from the system. This happens when the heat builds up within the coolant, it expands, increasing the system pressure above the pressure holding capability of the pressure cap (listed on the cap in pounds per square inch or PSI) causing the excess coolant to be vented out through the overflow vent which is exposed when the cap sealing plate is forced upwards. The excess coolant is then vented to either the atmosphere or into an overflow tank, which functions as a catchment basin. The overflow tank is under atmospheric pressure while the cooling system is under atmospheric pressure plus the pressure generated by the heat expansion of the cooling medium. As the cooling system begins to cool down, the internal system pressure drops until it allows the pressure cap spring to overcome the system pressure and the cap then forms a seal again on the bottom of the filler neck. This takes place when the internal system pressure drops to lower than the rated pressure of the radiator pressure cap. Thus, the cooling system pressure will always be above the atmospheric pressure of the overflow tank until the pressure cap again seals and the entire system is completely cool. At that time, both the cooling system and the overflow tank will be at atmospheric pressure. As can be easily seen, the flow of excess cooling can be in one direction only—out of the cooling system and into the overflow tank. The coolant in the overflow tank cannot flow back into the cooling system because a liquid cannot flow from a lower pressure to a higher pressure. Thus, an overflow tank serves strictly as a catchment basin, not as some for of extra coolant reservoir.

The expansion tank was used on the MGC and the later MGBs having a sealed radiator system. A sealed radiator system is one which has no filler neck and is filled from some remote source, most frequently some form of opening on the thermostat housing. To handle the coolant expansion, a remotely located expansion tank is fitted, along with a pressure cap to vent any excess pressure build up. Thus, the radiator and engine are full of coolant and the expansion tank is about one half full of coolant. The empty portion of the tank functions in the same manner that the empty space in a conventional radiator’s header tank functions—it allows the air inside to be compressed to allow the expanded coolant to have a place to be. With this form of system, as the system pressure is increased, excess coolant is forced out the radiator vent and into the expansion tank. As system pressure drops, cooling is drawn from the expansion tank and into the radiator and engine.

With this understanding, we can see that the addition of an overflow tank serves only one purpose—to catch overflow and keep it from being deposited on the ground. The fitment of an expansion tank, to a conventional radiator, will accomplish little. The MGB expansion tank is equipped with a 15 psi pressure cap while the average conventional radiator uses a 7 psi radiator cap. Thus, when the 7 psi pressure cap unseals, the excess coolant will flow into the expansion tank, which will not, due to its 15 psi pressure cap, relieve the system pressure, but will allow the system pressure to build up to 15 psi before system pressure is relieved. Thus, it puts an additional strain on the radiator, one the radiator was not designed to handle. Under these conditions, coolant will flow back into the radiator and engine as long as the total system pressure is less than the pressure rating of the pressure cap. At that point, the coolant flow will stop, making the expansion tank a closed system. This system will have no way to vent is pressure, about that of the radiator pressure cap rating, until the expansion tank pressure cap is removed. This can cause some problems with the expansion tank.

The expansion tanks are made out of formed copper sheeting which has been soldered together to form the expansion tank. Copper has a property called “work hardening” which means that as it expands and contracts, the copper gets harder. When it gets harder, it becomes brittle. Thus, it is no longer capable of expanding and contracting without small cracks forming in the surface. If these cracks are above the coolant level, you will only see small lines form in the painted surface. If the cracks are below the coolant level, you will see lines of coolant being forced to the outside of the tank. At this point, the cracks can be sealed by cleaning the area and flowing soft solder into the cracks to seal them. But, this is only a temporary measure and the cracks will begin to enlarge and the solder seal will be breached. At this point, the tank would have to be replaced, using a good used tank. New tanks are no longer available.

How to install an overflow tank

An overflow tank is installed by finding a place to bolt or screw it to the inner fender of the car. The radiator overflow vent hose is then routed to the overflow tank and inserted into it. You will still need to keep an eye on the coolant level in the radiator and keep it topped up to ½” to 1” below the bottom of the filler neck.

How to install an expansion tank.

The expansion tank is installed exactly as the overflow tank is, but should be located about the level of the radiator header tank. The radiator pressure cap is replaced with a blanking cap as used on the MGC and the expansion tank should have the proper pressure cap for your system used on it. The overflow line from the radiator filler neck is routed to the expansion tank. Hose clamps need to be used on this line, as it now becomes a pressure carrying line rather than an overflow line. The overflow line is the one on the filler neck of the expansion tank and it has a hose attached to it that vents to the atmosphere just as the conventional radiator overflow hose does. The radiator is filled to the bottom of the filler neck and the expansion tank is filled about one half full. The use of this system will add about one pint of coolant to the system, an insignificant amount.


The use of an overflow tank is done for environmental reasons. It prevents a slightly overfilled radiator system from venting onto the ground. The use of an expansion tank, with a conventional radiator, adds a very small amount of additional coolant to the system. If this small amount of additional coolant is sufficient to correct an overheating engine, there is some other form of cooling system problem, which should be addressed.

This monograph may be reproduced only for non-commercial use without other permission of the author. Reproduction for commercial use only by written permission.

Copyright © 2003 by Les Bengtson

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