by Lorne Goldman

Overflow Tank

The subject of using an “overflow” tank is one that has spawned much forum discussion and even more confusion. The term “overflow” tank commonly has two different meanings on forums. Meanings that are anything but the same. 

The true overflow tank is simply an item added on after manufacture. It is used with a conventional cooling system which consists of a radiator, fit with a filler neck and sealed with the recommended pressure cap. The system is filled through the filler neck. 

When the coolant heats, it expands, increasing the system pressure above the pressure holding capability of the pressure cap (listed on the cap in PSI). The seal on the cap is forced upward and this allows the coolant to be vented out through the overflow vent which becomes exposed. The excess coolant vents either into the atmosphere or the ground. However, one can fit an overflow tank which catches the overflow. The overflow tank is under atmospheric pressure. The cooling system is under atmospheric pressure plus any extra pressure created by the heat expansion of the coolant. 

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 seals the rad again when the coolant system pressure drops below the rated pressure of the pressure cap. Accordingly the cooling system pressure will always be either above the atmospheric pressure of the overflow tank until the pressure cap again seals and they are both at atmospheric pressure. So the flow of excess cooling can be in one direction and that is out into the overflow tank. The coolant which went into the overflow tank cannot flow back into the cooling system because nothing can flow from a lower pressure environment to a higher pressure environment. 

Ergo, an overflow tank serves only to prevent coolant flow onto the ground or car where it can cause great mischief. If using an overflow system on an older car, please look at Gerry's article below. The level of the coollant must be checked before any usage. 

An overflow tank is installed by finding a place to bolt or screw it. Try the valences (Inner wings) or the bulkhead. The radiator overflow vent hose is connected to the overflow tank. 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. If any system is filled higher, it will be overfull and the excess coolant will be vented from the system.

Expansion Tank

An expansion tank (aka a recovery tank) is fit to sealed radiator. It either has no filler neck or it is sealed with a blanking cap which does not react to or produce pressure. It simply seals the rad aside from the vent and hose to the expansion tank. The remotely located expansion tank is fitted, and sealed with a pressure cap to vent any excess pressure build up. The radiator and engine are full of coolant and the expansion tank is full from about 1/3 to 1/2l of coolant. The empty portion of the tank serves the same purpose as the empty space we used to leave i the old conventional rads header tank. The empty space allows the air inside to be compressed to allow expanded coolant a space to occupy. 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 back from the expansion tank and back into the radiator and engine. When filling a rad with this type of system, fill to the very top!

If fitting an expansion tank to an older Morgan, install it exactly as the overflow tank above, but locate it about the level of the rad top section. The radiator pressure cap is replaced with a blanking cap (looks like a rad cap but not spring or stopper on it) and the expansion tank should use the proper pressure cap for your system.. The overflow line from the radiator filler neck is routed to the expansion tank. Hose clamps now need to be used on this line, as you have made it into a pressure carrying line rather than an overflow line. The expansion tnk overflow line is the one on the filler neck of the expansion tank. It has a hose attached to it that vents to the atmosphere just as the conventional radiator overflow hose does BUT this should not be a regular event as, once the system finds its balance, the excess coolant with return to the rad and the engine..unlike before. The radiator is filled to the very top and the expansion tank is filled about 1/3 to 1/2 full. The use of this system will add about one to one liter to the system. However, this will depend on the size of the expansion tank and hose.


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 creates an entirely different system and effect. 
 In older Morgans, without either expansion or overflow tanks, the excess fluid produced by the hot temperatures was released onto the ground. Frequent checking of the coolant level and replacement is needed to avoid overheating on these cars. Many owners have retro fitted either overflow or expansion of tanks.

Remember, the biggest panic is at the start. People don't fill their cooling systems when their engines are hot. They fill them when their engines are the top. They start'em up, heat to running temp (a jump of 80C or so) and make puddles for a few days. It is unsettling and embarrassing but harmless (except to the environment). With an expansion system, after some driving, the collant will find a happy balance and level and expel any excess. If it doesn't, you have a problem that must be cured. 

Cooling a Big Plus 8
by Lorne Goldman on eMog

 After many years of trying I have found a happy recipe for all-weather cooling.

1. A bigger more efficient rad. I am not enthralled with those with a deeper core or many more fins. My experience of that is that they make it harder for a good air flow to pass through and too much gets deflected. The newest Mulberry rads are ideal. Longer and thinner. That means a bigger rad without prejudicing airflow and leaving plenty of room for a ...

2. Big fan. Something that pulls a LOT of air. I use a SPAL. So does the Factory. They pull an immense amount of air for their diameter, a must in a Morgan. Take care in the installation, the right SPAL can pull as much as 20+ amps and will require an MUCHLY UPGRADED 30 amp inline fuse, a relay and an alternate power connection (as the Morgan wiring is not rated that high, the wires can burn. Install separate wiring for the power and take the current from the alternator to the power side of the relay).    

WATCHPOINTS: When they first switched to SPAL, the MMC simply changed the fan fuse (at the fuse box) to 30 amps. However, the wiring to the fan was not upgraded to support this much amperage and the wire would melt. This, of course, made the fan non- functioning and that caused some serious sadness to owners (and their engine). They did upgrade the wiring but they kept using regulars 30 amp relays. With these, sooner or late, the relay will fail and the fan will stop functioning or function continually. The best solution is to use the SPAL wiring kit, the one they insist on using if you have any of their performance fans. The part number for the entire kit (which includes items you do not need) is # FRH-HO-K. Call them up and get a price on the relay and its wiring alone. 

3. Big expansion tank, 1.5 liters. A bit more fluid in the system and easier to work with when the engine is hot.  I have a Mulberry on one car and a Griffin on the other. I do not shirk on size. They are both large (1 to 1.5 liters) The Mulberry is canted at the bottom so that it sits vertically on the Morgan bulkhead and that forces more air into it. Admittedly the RDR (RON Davis Racing) rads cool better than the Griffin, but they are diffiuclt to fit because of their depth in the case of my bigger Plus 8.. 

4. I use a 15-16 pound cap.

This combo gets us through absolutely anything without a problem, at least in North America and Europe.