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took a little investigating and a some help from a previous thread, but I finally wrapped my mind around the radiator overflow issue. Here's a good article to read regarding overflow tank vs expansion tank:
http://www.custompistols.com/cars/articles/overflow.htm

With a typical radiator (filler neck and pressure cap on radiator) the overflow port is above the bottom seal of the radiator cap, so when the cooling system pressure overcomes the maximum pressure rating of the cap, coolant passes by the seal and flows out the overflow port. Not a good idea to let it just spill on the ground, so you put a hose on the port that leads to an overflow tank or "catch can". This is all well and good, but what happens to the coolant after it gets to the catch can? It just sits there? I don't want to just catch coolant every time the engine warms up and then have to refill the radiator when it cools off... Just doesn't make sense! I knew there had to be a way for the coolant to get from the tank BACK INTO the radiator.

Went back to the trusty '69 Chevy Service Manual and VIOLA! On page 6K-1 it describes the inner workings of the radiator cap. It is indeed not a one-way valve like I had thought, there's both a pressure relief valve and a vaccuum relief valve. Thus, as the system cools and pressure drops below atmospheric, the vaccuum relief valve opens and fluid is sucked IN through the overflow port. If the attached hose leads to the bottom of a coolant resovoir it pulls the liquid back into the system. If you don't have a hose or resevoir it sucks in air and you have to top off your coolant every time you drive :down:

Now comes the question of what to replace my old crappy plastic overflow tank with... I was intrigued by this link:
http://www.valvoline.com/carcare/articleviewer.asp?pg=pht20040301cc

But if there are other suggestions I'd love to hear them!

Sorry for the long post that I'm sure was common sense to most of you guys. It's just a relief (no pun intended) to get this all straightened out so I can quit putting water in my radiator!
 

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Then let me confuse you even more. My old time radiator guy "gabby", no longer with us told me he loved when they added coolant recovery tanks. He always told me to fill the radiaator up tp 3/4" below the filler neck to allow for expansion. He said when ccolant recovery came out, he had more business do to overfilling, added pressures involved with purging, vacuum cycling., burping fluid instead of air.

So now i ask you this. When you fill the system, it heats,expands, and burps out the excess. When it cools off, it contracts. Why do you then top it off? When you warm up the engine, it is going to expand right back to where it was when it burped.:laughing: Topping it off only gradually thins out your coolant.:crazy:

Just my 2 cents.
 

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When you warm up the engine, it is going to expand right back to where it was when it burped.:laughing:

Just my 2 cents.
Actually, yes. If you look at most GM overflows (not ours, I think) there is a "hot" and "cold" level on the ovreflow. All things being equal, it'll heat up, overpressure and flow out. When you turn it off, it'll cool down, dcontract, draw vacuum and suck the fluid back in. There should not be any addition required. If you do need to add water for whatever reason, better to add it to the rad but even if you add it to the overflow, it'll mix soon enough. Not too much icing to worry about in our climate.
 

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The primary reason for the recovery tank was environmental, so you don't puke out antifreeze and ruin our streams and rivers and kill the little kittykats too.:laughing:

It does have an additional advantage of purging the system of any air after a few cycles and then sucking back coolant only, thus allowing the newer lower hoods to have shorter rads with the same efficiency.

There is absolutely nothing wrong with the later c3 plastic overflow tanks. Fits perfectly out of the way under the fender, Unless you prefer an shiny alum one:D

there are 2 types of rad caps for the recovery tank use.
The best


Without the spring
 

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010752: That was also my understanding as far as the purpose of the overflow tank. Note also that if you drag race and your radiator pisses out on the starting line or anywhere near it, they will kick you off the track.

Dep
 
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