Crossflow Cooling System -Overflow Tank or Expansion Tank

[San Augustine]

Probably used the wrong terms here, the car has an overflow tank and I want to convert it to an expansion tank. 

[MV8]

Then all you need is a tee in the heater circuit (or the one you ordered to tee a radiator hose) and an expansion tank.

 

Examples:

https://www.ebay.com/itm/256717068259?_trkparms=amclksrc%3DITM%26aid%3D1110006%26algo%3DHOMESPLICE.SIM%26ao%3D1%26asc%3D20240415110138%26meid%3Db75df59a2efe47d8be0a0a2eabdf5a5d%26pid%3D101875%26rk%3D2%26rkt%3D4%26sd%3D365014549820%26itm%3D256717068259%26pmt%3D1%26noa%3D0%26pg%3D2332490%26algv%3DSimplAMLv11PairwiseFitmentPromotionV2WithCassiniQueryEmbRecallKatReco&_trksid=p2332490.c101875.m1851&itmprp=cksum%3A256717068259b75df59a2efe47d8be0a0a2eabdf5a5d|enc%3AAQAKAAABQCggmqi%2BjGBvS8WShKTeLW5J2g6qXnzgH61T5t8ujPfO4ZJHWSDajmSreRV%2BXNto1elxKNVVM64kNmwVgiMN2szhK0tlR1011FXMzSAB1PUgkXdlAtp1SJ2P1Ia246YdDc6Lz8RqY5J3Udbo2ifiNz9Ty%2F0PHH1s8EKnHfOcCBzmEgRxCIN2Zn9HgmCS1k6AZeBNTMKlaZRZ2WNDksfpX44iTQnZvnI4Dt722Qdc8VyykIRbEgO5GisZJbzb%2B4%2B7btodHPi%2FWbaXOQkCse4E2DR5GNj%2FM8xvWUmB%2FJ%2BYhG2ozgYXZwg9jwK34x2WrP0aKibkYvLCbiZ37z%2FbLlNt3Na19qY6Y%2FHye68UsDQ9KEquaE22WvyPZmoo9fQfGWT7%2B6E4V9CTXmflBAIL0Q%2BeNeJ8GMtxahjs4rOKTrX8go4k|ampid%3APL_CLK|clp%3A2332490&itmmeta=01KBJQWRCSVA0EGAMN5S2HW764

 

A tee (assumes 1/2 inch hoses and fitting for expansion tank): https://www.ebay.com/itm/267284759799?_skw=heater+hose+tee&epid=230386134&itmmeta=01KBJR1N5F8BM5KNMCHP85WEX1&hash=item3e3b69c4f7:g:rcQAAOSwwt5oRyTA&itmprp=enc%3AAQAKAAAA0FkggFvd1GGDu0w3yXCmi1cwvKWLr3m5skZUhd7IzrdGPI5hLO6qnV81qhoM62naBqKkKn8KMIXKQ8JyDjr2v68JiSRbCJIhGEEvJ8IZY6OlP3hMT4r6RLSDdFuNO9MDivKi8zmuwWUcSrQqr0uNWRswARDTgDL8Gaxge%2BwLP8%2FRDMXxDCUeseGXbKziP6wcNhAZRBrsY4d5GytIEVmXFq1fNW9UtaMtPEX%2FlKzREgSb2LuZY0i%2FpMax%2FMh2eMXaP359dWD6ia6pK4Ur66mF5zI%3D|tkp%3ABk9SR4jThtjcZg

Edited December 3, 2025 by MV8

[San Augustine]

Right, but you see I am a rather visual learner, that is why I need pictures.  

[MV8]

ok. I'll post a pic shortly.

[MV8]

You can tee this hose anywhere between the engine and the heater. If the expansion tank you select has a small port next to the filling cap, then it can be capped with fuel hose and clamps or tee'd into the overflow tank up front. The other heater hose could be used also even if the heater valve were left closed.

Edited December 3, 2025 by MV8

[NSXguy]

for your visual interest… 

Sharing some photos from when I did a closed system. I have since removed as I wanted to declutter the engine bay and simplify. Either system (open or closed) works well if properly installed. 
 

as MV8 mentioned, Tee off the heater hose circuit or if no heater then just connect to the water pump neck  (reference the photo with the arrow ) and route the line (larger ID) to the bottom of the expansion tank. 
 

Smaller ID hose will go from nipple on existing thermostat housing to small nipple on expansion tank
 

you’ll want to cap off the existing thermostat housing with a non pressure rad cap and then use a ~13lb rated cap at the expansion tank. 

some expansion tanks have a purge/air bleed nipple as well.  

 

[San Augustine]

Is it possible to have an interruption if I go off the heater inlet when the water valve is shut off since there is no continuous flow?

[MV8]

No interruption but why not use the other hose?

[San Augustine]

Sorry, I am an idiot. Got my hoses mixed up. Didn’t realize the hose you marked went to the h2O pump.

[MV8]

What is the issue with the cooling system as-is? The filler should stay as full as it will hold under the cap and the level is maintained by the overflow. Minimum cold engine level in the overflow covers the hose end inside the overflow.

 

If the filler neck is not kept full, then the cooling fan switch (unplugged in your picture) does not pickup the engine temp and the cooling fan doesn't come on when it should. I believe one wire goes to a chassis ground and the other to the fan motor to provide ground.

[San Augustine]

As you know I bought the car on BAT last week. I knew that I would need to check over the car before doing any driving, one of which was the cooling system.

As to your question- the fan wires were plugged into the housing. But what got my attention was that the cap on the catch can was askew and the can was empty and dirty. The hose itself was dry and needed replacement. 
 

When I removed the radiator cap no coolant was visible. I then removed the housing to inspect the thermostat and found it cruddy but functional in the boiling water test. The coolant under the housing was clear and tested to be almost 100% h2O.

 

So after these findings I knew that at a minimum I was replacing the coolant, the old thermostat, and radiator cap. And after reading this topic I thought that maybe it would be prudent to upgrade the cooling system.
 

 

Edited December 4, 2025 by San Augustine

[NSXguy]

2 hours ago, San Augustine said:

As you know I bought the car on BAT last week. I knew that I would need to check over the car before doing any driving, one of which was the cooling system.

As to your question- the fan wires were plugged into the housing. But what got my attention was that the cap on the catch can was askew and the can was empty and dirty. The hose itself was dry and needed replacement. 
 

When I removed the radiator cap no coolant was visible. I then removed the housing to inspect the thermostat and found it cruddy but functional in the boiling water test. The coolant under the housing was clear and tested to be almost 100% h2O.

 

So after these findings I knew that at a minimum I was replacing the coolant, the old thermostat, and radiator cap. And after reading this topic I thought that maybe it would be prudent to upgrade the cooling system.
 

 

there’s really no need to “upgrade” the cooling system especially for a mostly road driven 7; unless you just want to, of course. It works well in current configuration even in a track situation- I can attest . For your cooling system baseline I would say:

-replace the thermostat

-new gasket on the housing

-new cap (I believe 13lbs)

-new 50/50 mix ford green coolant,

-the overflow hose to the overflow tank

- if overflow tank holds, no need to replace unless for aesthetics

-probably won’t hurt to flush the rad (and heater heatexchanger) while everything is off

-water pump is fairly inexpensive; would throw a new one in as well (while you’re in there)

-make sure you put enough coolant and bleed system thoroughly

 

should provide lots of trouble free miles and smiles, at least for the coolant side.

 

I have a more comprehensive checklist for a “new to owner” analog car baseline process. 

 

 

 

 

 

Edited December 4, 2025 by NSXguy

[MV8]

The previous owner(s) may have used water without antifreeze for superior cooling but added a rust preventative. If not, the radiator core may be partially plugged with rust particles. I'd use a garden hose to flush everything into a basin to pour into a white bucket to see the condition. If no significant "muddy" water (just momentary flashes of solids pouring out), then just flush every hose each direction until water is clear, then fill with a 50% premixed coolant.

 

If the level under the cap keeps going down without leaks, I'd suspect the head gasket is leaking internally to the exhaust.

If the overflow level never goes down, I suspect the radiator cap outer sealing ring is torn or missing or head gasket.

 

 

 

[San Augustine]

Good to know about detecting head gasket leaks. I have already sourced a radiator cap, thermostat, and water pump. 

[MV8]

https://www.summitracing.com/parts/sum-tc001?seid=srese1&ppckw=pmax-oils-fluids-sealer&gclid=EAIaIQobChMI9-3SlsKmkQMVpZtaBR1KHiIMEAQYBCABEgJqfvD_BwE

 

https://www.evapo-rust.com/thermocure-cooling-system-radiator-rust-remover/

 

Video is just an example of using it and not concurrence with the kids assumptions.

Edited December 5, 2025 by MV8

[IamScotticus]

I'm back...

Just saw this and I think its about perfect, for metal.

C1 or C2 Corvette expansion tank and AC 307, 362 (13lbs) or RC-26 (15lb) radiator cap.

 

These are reproduced by DeWitts 

 

C1, side exit:

https://www.dewitts.com/collections/1961-62-surge-tanks

 

C2 end exit with bracket:

https://www.dewitts.com/collections/1963-67-surge-tanks

 

These caps work :

Chevrolet AC 307 & 362 (13lbs) or RC-26 (15lb) radiator cap.

 

The tanks start at around $300 (yea, ouch!)  the guys going for a production date code stamp pay more.

 

Of course there are Ebay Chinese copies made, and some of them look pretty good.  They will pop up when you search for this.

 

The C2 version with its end facing exit tube has a matching metal bracket that may give better mounting options in a 7.

This example on BAT, the engineer has fabricated his own.

Here are pictures from BAT:

 

https://bringatrailer.com/listing/lotus-super-seven/

 

In short, although I prefer translucent plastic, If I had found this tank sooner, I would have stopped here.  The caps for it are the correct pressure ranges for Crossflow, its mountable in a multitude of ways, its available in two exit port directions and looks the business. I don't see how it wouldn't be allowed on a race track.

Corvette...go figure.

 

What do I NOT like about it?  Wait till you see the prices of those AC pressure caps 

 

One of the web sellers of these is Top Flight.  I like their presentation of the products, no affiliation.

C1:

https://www.topflightautomotive.com/products/dewitts-radiator-1961-1962-chevrolet-corvette-coolant-expansion-tank-w-o-date-code/

C2:

https://www.topflightautomotive.com/products/dewitts-radiator-1963-1967-chevrolet-corvette-coolant-expansion-tank-w-o-date-code/

 

[IamScotticus]

Time to add more here.

 

This is for new guys who may not know the difference between the two types of coolant tanks.  This thread uses overflow, recovery, and pressure tank terms frequently without really explaining what is what.

 

Recovery or Overflow tank: an open air vessel that holds coolant that is purged from under a radiator cap when the engine is hot and during pressure surges like hard accelerations.  The cap and hose are usually located at the top of a radiator or on the head as part of a thermostat housing or swirl pot. The vessel holds the coolant until it gets pulled back in when the engine cools.  There should be enough extra coolant kept in a recovery tank to ensure the hose doesn't pull in air.

 

Expansion tank:  a vessel that holds extra coolant in circulation under pressure.  The hot coolant flows into it and pulls back when the engine cools.  There is not a radiator cap, the radiator is typically sealed.  The cap on the expansion tank regulates the pressure, but unlike the radiator cap, coolant does not flow through it, this cap stays above an air gap.   Typically,  a "closed" system expansion tank should be half full of coolant at all times, expanding with the temperature but not needing an overflow tank.  Most road vehicles since the 90's will have this type of system with an input hose at the top of the vessel coming from the thermostat housing hot side, an output hose at the bottom going to the lower radiator hose or the cool side and an overflow hose at the filler neck that drains to the ground, if needed.  For sporting applications, the overflow should be captured in a recovery tank to prevent spills and is usually a safety regulations requirement.

 

Location, location, location.

For open radiator cap systems, the height doesn't matter as air bubbles are purged out and vacuum pulls coolant back in.

 

For closed expansion tank systems,  the tank should be located where the tank cap can be the highest point for efficient air bubble egress. Above the highest coolant level in the engine or radiator.

 

Regardless of which system you use, and why, realize that a failure of the cooling system can destroy your engine.  Pressure caps are wear items and should not be expected to last forever.  In a 7 where the coolant recovery tank is often located in front under the nose, it's not very visible.  And even if you do see coolant in it, are you sure the cap is working?

 

Choose the system that you can watch.

I prefer a closed system because the level in the tank is the level in your whole system at all times and temperatures.   If there is a loss happening, it will be more visible from an expansion tank mounted on the scuttle than a recovery tank under the nose.

 

Can I use my current thermostat housing from Caterham for a closed sys?

Yes, if you can fit a cap that is not an overflow pressure cap.  The outlet barb should go to the top inlet barb on the expansion tank (not the overflow barb). On those pressure caps, only the lower seal holds pressure. The top seal has no guarantee of it.  It's better to use a housing without a cap so you don't have a failure point. Also be aware of those housings elbows that have the vertical nipple , those nipples are glued in and have been known to come off. 

Some Formula Ford tracks don't allow them. The nipple can be replaced with a threadded brass barb.

Edited 14 minutes ago by IamScotticus