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How does a header tank cooling system work?

Posted: 06 Jul 2010, 18:55
by jfryjfry
I'm building my own little hot rod and have it set up with the old fashioned radiator-with-pressure-cap-on-neck-with-overflow-tube-into-recovery-tank.

But the talk about a header tank got me wondering if there might be a better system for my car. I am having a hard time finding online how a system like that works and wanted some thoughts on how it works and if it is better/necessary.

I read something about the cap being the highest part of the system, and the top of my radiator is probably 4" below the top of the head.


Re: How does a header tank cooling system work?

Posted: 06 Jul 2010, 20:09
by Midlana1
The short answer is you'll probably be just fine as-is if you don't have any problems with overheating or purging the system of air. All the problems start when the radiator is lower than the engine, but is very engine-dependant. It's made worse when the radiator hoses drop down near the floor when going from a mid-engine setup to the front.

Unless your engine already has a history of trapped air, there's no need for the added complexity.

Re: How does a header tank cooling system work?

Posted: 07 Jul 2010, 08:17
by jfryjfry
Cool. That was my plan. But how does a header tank system work? What are the components and what do they do?

Re: How does a header tank cooling system work?

Posted: 07 Jul 2010, 14:34
by Jagmandave
The original name for the device is "expansion tank", and that explains exactly how it works. As water heats it expands - there is only so much room in the system so the extra water has to go somewhere. Back in the day, they left room in the top of the radiator for some air to absorb the extra space, if you filled the radiator too full the extra water pushed out the overflow tube onto the ground. After a couple times of this you're low on water.....

Someone had the bright idea to have a tank to catch the water, and used a different type of cap on the radiator itself - the expansion tank caught the extra water and when the system cooled it drew it back into the radiator - that way you were never low on coolant. Simple....

Re: How does a header tank cooling system work?

Posted: 07 Jul 2010, 16:17
by jmcglynn
Actually a "header tank" is different than an expansion or overflow tank.

A header tank is part of the pressurized system, and is typically used to collect coolant from all the various bleed lines (cylinder head swirl pot, radiator, etc.) The pressure cap is on the header tank and you fill the system there instead of on the radiator.

An expansion tank or overflow tank is outside of the pressurized system, it catches coolant that is pushed past the radiator cap (past the 15-22 lb spring-loaded pressure relief valve) in a pressurized cooling system, a siphon feed line allows the displaced coolant to return to the pressurized system when it cools down. Pretty much every vehicle has an overflow/expansion tank, but header tanks are only used to solve a problem where excess air can't be bled out of the system because the fill cap for the system is lower than other parts of the system.

Re: How does a header tank cooling system work?

Posted: 07 Mar 2011, 10:12
by Lotusrk
Having worked as a foreign car mechanic for several decades, I've come across numerous makes/models that trap air bubbles and won't give them up easily. The easiest sollution is to jack the end of the car about 3+ feet off the ground which moves the radiator cap higher than the engine. Another sollution that helps is to use a pressure tester to force coolant past the thermostat (where the air bubbles like to hide). Sometimes the same principals apply to hydraulic systems. I could not get my Ranger clutch hydraulics to bleed, even with a pressure bleeder, until I ran the front wheels up on a 4' pile of dirt. Then I pumped the pedal furiously about 100 times, and the problem was solved. On the subject of overflow tanks: ethylene glycol is poisonous to all animals (including us) and expensive to replace. Once it spills on the ground, it doesn't just go away, it ends up in the water table where we pump it out of the ground and drink it. The antidote for ethylene glycol poisoning is alcohol; lots of it. This dialates the kidney pores and allows it to pass. Did your dog drink antifreeze? Get him drunk. The moral is; put an overflow tank on it and only keep it 1/3 to 1/2 full so there is room for expansion.

Re: How does a header tank cooling system work?

Posted: 29 Aug 2013, 06:56
by TRX
There is a discussion (with drawings) of various header tank systems in one of Caroll Smith's books, but I don't remember which one offhand.

There are various different arrangements that all fall into the general category of "header tank." Some older vehicles, like some late '50s-early '60s Ford 352s and 390s, had the tank as a high spot to fill the cooling system. That sort of system (or a fill cap on the thermostat housing) came back later when hoodlines went down and radiators got shorter, and might be lower than the highest spot inside the engine.

I have some old Cosworth installation instructions that show an elaborate baffled swirl tank arrangement, like you might find on a dry sump oiling system. Since the instructions also talked about the proper type of stop-leak to use, I figure the swirl pot was to remove combustion gasses leaked through the head gaskets into the cooling system.

Some newer cars have swirl-like arrangements in "circulating" header tanks, where coolant flows continually through. This helps remove bubbles from boiled coolant, and it theoretically acts as a self-bleeding system to get rid of trapped air.

Trapped air in the cylinder head(s) means that where air is, coolant isn't. On cars like the C4 Corvettes, the engines were mounted level instead of at the usual 5 degree or so nose-up angle. The tilt of a normal engine helped move trapped air out and to the radiator; in the C4, it stayed trapped in the heads, so GM added small air bleed tubes to the backs of the heads to bleed the air out.

Many newer vehicles have elaborate air bleed procedures; some of them have one or more small bleed screws, which must be loosened until coolant comes out, then tightened shut.

Air is often trapped in the water pump housing; look inside a used pump and you can often see "ring around the collar" where the housing is stained or corroded where the water level is when the engine is shut off. I've tapped several pump housings 1/8 NPT and added 1/8 AN bleed lines back to the top of the radiator to avoid this; the pump isn't nearly as efficient pumping foam as it it is pumping solid water. In a couple of cars, it made a noticeable difference in coolant temperatures.

I used 4AN because I had some brass JIS(?) fittings on hand; they're the farm-equipment version of AN, but they seem to be unobtainium nowadays. Speedway had preformed hoses and the AN fittings for the water pump for cheap. I just drilled the radiator tanks and soldered the brass fittings in. However, simple brass horse barbs and small hose will work. Or, if you have plastic tanks, you can use one of the inline hose adapters meant for installing temperature sensors; Pegasus sells several varieties of those. You can bleed to the header tank instead, of course.

1/8" oil pressure sender line would work, except there always seems to be some bit of scunge floating around the coolant system just waiting to stop up the tiny lines. I never had any luck with them.

Re: How does a header tank cooling system work?

Posted: 30 Jan 2014, 13:08
by bgkast
Here is a good (and long) read on the subject:

Re: How does a header tank cooling system work?

Posted: 05 Jul 2017, 18:26
by Leggman
One of my favorite toys is a mazda rx7 that I swapped in a ford 302 v8. The radiator cap at the radiator is not the highest part of the system so I installed a aftermarket neck with another cap.
This I put at the highest part of the system and I use it to purge out the air