i posted this here because there was only 5 posts in heating and cooling and it didnt look to promising . besides more people read here . . okay on to the question at hand. the lil water neck on the side of the radiator water fill spout that you would hook your over flow tank to. is it necessary to have one . if the radiator didnt come with a spout for the over flow tank do i need to have one or can i just run a completly sealed system on thwe street.? does it even matter ? i mean if you are using a alum can and clamping the hose at both ends its a sealed pressure anyways right. meaning no pressure or water seepage. ? correct. and if the water gets pulled back into the radiator from the over flow can it would still be there without a can . here i am goin in circles again . lol do i need the thing or not ,.? thank,s guys

Certainly, you COULD do it.

If you do, make certain you leave enough room for expansion so you don't blow off your hose, cap, or create a hole or crack in your radiator when it gets hot.

A pressurized cap still might also work as a safety valve for you.

I'm certainly not a cooling expert but it doesn't sound like the best plan to me.
Consider that even with a 30+ lb cap you will relieve coolant to the "overflow tank" when you shut off the engine after leaning on it or after extended driving.
I have never measured the pressure in a cooling system but I would not be surprised to see a pressure rise to say 75 to 100#s /sq in without relief & probably much higher if you over heat the engine & I don't think most radiators are designed to run at this pressure.
If it were simple to run a closed system I think the automakers would have done it a long time ago.
If you want good info try posting your question in the forum here for Jack Wilson. He is pretty sharp. ;)
http://www.stewartcomponents.com/Tech_Tips.htm

-------EDIT---------
As for leaving air in the system for expansion, I don't agree with this at all.
The reason for the puke tank is to keep air out of the system.
Air is a horrible coolant medium in an engine :D

You definetly need one. Once the pressure builds it will find someway to releave itself,intakegasket,waterneck,or maybe push a freeze plug out.Especially in the Sacramento valley heat.

Sure, just blow holes in my theory - or the radiator, whatever.... :rolleyes:

I guess I wasn't that clear - I was trying to imply that you shouldn't do that but it still might work. The way they used to run an overflow tube was without any catch can, and the radiator would always dump antifreeze until there was enough air in the radiator to keep that from happening. Probably not the brightest thing to try on a fully closed system.

okay all of it makes sense to me . so using a alum over flow bottle that is close once you clamp on a line isnt the same .?

the overflow needs some sort of a vent to the outside, unless you run an expansion tank like the late models do where the pressure cap is on the overflow tank.
wait- i guess even then the cap is what vents to the outside.
the cap not only lets excess pressure out when it gets hot, it also lets pressure back in when the system cools down and the coolant contracts.
in a nutshell- do it like oem and don't worry about it. they spent millions of $$$$ figuring this out, so why try to re-invent it?
of course, there is always the option of running that Evans coolant that uses no water and runs at atmospheric pressure. you can even run clear hoses with that stuff.

hmm for years i have always ran a be cool alum can and clamped the hose at both ends and never had a problem. to hence a sealed system

Depending on the actual mix and its properties, coolant expands roughly 6% of the total system volume - ambient vs. operating temperature.

hmm for years i have always ran a be cool alum can and clamped the hose at both ends and never had a problem. to hence a sealed system You had a closed system but there was air in the overflow tank wich can be compressed. This would act the same as an expansion tank in a closed boiler heating system.

okay i got the idea now thanks .