I have a 72 with a GM crate motor and an aftermarket aluminum radiator in my car...nice setup, but the radiator has a cap on it for overflow which is fine but I also have the original overflow tank used with radiators that don't come with caps? I purchased the car like this and haven't paid much attention to this except when I started sitting in traffic it would overheat? So now I have to fix it.....

So what do you think I should do? Are these two tanks working against each other? Should I get an aftermarket overflow tank?

I dont even need the heater hoses hooked up, i will disconnect them soon.



Thanks,
jim

 

i think you would only need one pressure cap on either of the two units

i think i would use a pressure cap onthe radiator and have a vented cap on the overflow unit

 

So do they sell non pressure caps for my overflow tank?

 

just drill an 1/8 hole in the center

 

You can't do that!...lol. You'll lose all your coolant that way. The heater hoses are pressurized and plumbed to the recovery tank. Either get the correct radiator (no cap), or remove the tank and re-route the heater hoses and use a plastic vented tank.

 

why not??? the first radiator cap is the pressure relief and the second radiator cap is redundant? i guess the question is where does water flow into the overflow and is it below the radiator cap?


edit maybe my suggestion wasn't clear i'm talking about venting the overflow cap

 

I did get a Dewitts radiator off of e-bay 941B but it doesn't fit...it's too short by about an inch. Tom says the radiator brackets must have been altered but I didn'e see anything that suggested it was.

I guess I will have to get a aftermarket overflow container and use whats there. It does cool off nicely after you start moving.

 

i wouldn't get rid of the original overflow they are cool looking....i still don't see why you can't vent the second cap.....

 

Like I previously said, the tank would still need to contain the pressure, as the heater hoses tee into the tank.

 

Ok ...I got it...scrap that idea or block off the hoses

 

You could make that radiator work with the factory expansion tank. Been there done that...I got a super deal on a aftermarket rad. but it lacked the fitting to the tank and has a cap...You'll need to have 3/8" nipple installed in the side tank to run to the expansion tank. Then run the highest pressure cap you can find on the radiator(effectively sealing it off) then run a 10-13 # cap on the expansion tank, keep your heater hoses where they are, and everything will be right with the world.

 

I had no overflow tank when I bought my Vette and was always loosing coolant after a run. I installed an aluminum tank as you have pictured but converted it to only an overflow unit.

I don't have it connected as you do. I have the hose from the radiator going to the bottom of the tank via reducers/connectors. I have the other bottom outlet of the tank blocked off with plumbing fittings. No heater hoses here.

I have the tank overflow outlet as the upper one near the cap and blocked off the other cap outlet.

Here's an old pic:

 

Radiators are confusing and the 68-72 era is the most confusing. That's because GM used (2) two different radiators for small blocks. Aluminum w/ surge tank and Copper/Brass without.

A common misconception is that there was some special engineering logic behind this and there wasn't. GM was in the mode of cutting costs and c/b radiators were much easier (and cheaper) to build.

It's not really that had to figure out which radiator is correct (originally installed by GM) for the car. A lot of catalogs call the aluminum radiator the "manual, No Air" radiator but that's not the whole story. The LT1 or L46 could be a manual w/o air and it's the 26" copper/brass unit.
Simple formula is the (no auto, no ac, no rpo) =aluminum with tank
If the car got any one of these, or any combo, then= copper/brass

Now, for your post. Looking at the picture on top I see a couple things. The aluminum radiator you have in the car is not right and someone did modify the core support, upper brackets, or both. The stock aluminum radiator you bought on ebay should have bolted right in. I suggest you buy a new core support and possibly an upper bracket and install 941b. Hook it up to the surge tank the right way and you'll only have one cap. Done!

I also noticed that the fan is sitting way too far inside the shroud and that could be the cooling problem you had in the first place. The fan should be positioned about half way in the shroud.

I see the double cap thing all the time and I just shake my head and wonder The only thing that could be worse is when the tank is connected from the radiator cap overflow. That's the tube just under the cap. This connection is dead, no pressure, ever. It should go directly to the ground and nothing should ever come out of it. There are about ten different combinations of hooking up a double cap, depending on the heater hose connections, equalizer hose, and overflow connection. None of which I like.

 

This might be the only GOOD way to install the double cap system but I still don't see the reason for it. This radiator has end tanks on it and they have room for the fluid to expand and contract. So just fill the system about 2" from the top and forget it, that's what GM designed for the 26" c/b. If you really want that extra 2" of coolant, then fill the radiator up to the top, install a non-pressured plastic catch can and run the cap overflow into it. When the coolant expands out it will go into the catch can, when it cools down it will suck it back in.

Nice job on the plumbing though

 

I got this new rad for next to nothing...that's the good news. The bad news is it's for an earlier big block Vette, and I'm bulding a blower motored small block Vette. I've used the Moroso expansion tanks(which are very similar to the GM pieces) in the past and they do a great job of "burping" the system, particularly when the rad is below the motor, so....why not keep the stock tank and plumb around it. Besides I polished it, and I'm gunna' use it dammit!

 

Like You said, the cap overflow is not sealed - it is open to the atmosphere. Any overflow may dribble out of it, but it will never get drawn back in, right? In fact, if you hook the cap overflow up to the expansion tank, the entire radiator system will be operating in a non-pressurized manner which defeats the purpose of pressurized caps. Your coolant will boil at a lower temperature.

 

The expansion tank has to be used as such and not as a coolant recovery system...my setup functions just as the original...minus the heater.

 

I know this sounds strange and I have trouble believing it but I run a pressure gage on my cooling system and I found when I had two 7 pound caps in series like in the picture you ADD the two caps together. A pair of 7 pound caps gave me 14 or 15 pounds of pressure. Without a gage you don't relize that a pair of 16 pound caps are actually making the system operate at 32 pounds.
The caps do add up if in series.

 

I don't think so.....when the pressure reaches 7 lb., the cap will vent...Betcha....

 

So your telling me when I run a 29# cap on my rad and a 13# cap on my expansion tank my system will hold 42# of pressure....I don't think so. The cap on the expansion tank doesn't know if the rad has a cap or is sealed.....