Another tank

[has2tig]

I just got done building this tank for a friends rx-7. Its a air water separator for the cooling system. Made it out of 3/16 5053 aluminum all tig welded together. The bottom has a custom machined fitting to restrict flow.

[Fishy Jim]

If you're getting "air" in the cooling system it most likely means you have a cracked intermediate housing and the engine needs rebuilding. The S4 and S5 13b's are notorious for this failure.

Rebuilding the rotaries is actually rather simple. The engine doesn't weigh much more than 200lbs so you can lift it out with a friend and a piece of 2x4 spread between you if you don't have an engine hoist.

[has2tig]

Its a FD and as far as i know all came with a air water seperator.

[Fishy Jim]

Ah, this seems to be another of the basterdization of automotive terms that brought you the "sway bars" (they're ANTI-sway bars).

What you built was an "expansion tank." Something that's very common on a lot of cars these days.

[has2tig]

lol jim call it what you want i just saying thats what mazda called it. I would think it is not a expansion tank due to the fact that the coolent is always running through it and it is always filled and pressurized. But hey i dont know much about this it just what i was told i could be wrong.

[Fishy Jim]

The function is identical. As the coolant heats up and expands it fills the cavity of that tank. My 87 325i had one.

To call it a separator would imply that air is entering the system somewhere, but the reality is that it's located at the highest point in the system, so as air is purged (after maintenance) it cannot re-enter.

[has2tig]

i might be stupid but i dont think it works quite the same. here it is right after the radiator and then goes directly into the engine. If this were to explode all of the coolant would gush out. And yes it is notorious for exploding due to the high heat, the presure and the original being plastic. A expansion tank is just a tank that sits off the side and collects coolant as it expands and to my knoledge are not inline with the coolant system.

[has2tig]

I just looked at how it is set up and it is kindof a eapansion tank but its also not lol. But jim you are correct it is a expansion tanks also. My bad

[Fishy Jim]

There's no need for coolant to pass through it in more than one location. The bubbles will rise on their own.

I suppose its entirely possible that the rotor housings around the exhaust ports could get hot enough to boil the most robust coolant mixtures. If that was the case (not an ideal condition), then it would in fact be a separator, as well as an expansion tank.


I'm curious where the restricted port takes off from? The unrestricted one goes directly into the radiator, right?

[has2tig]

i believe the unrestricted one comes from the rad and the restricted one is the outlet. If it was unrestricted then suposidly the air doesnt have time to rise and i just talked to the owner and it is not a overflow tank. Actually if you look at the top there on the cap there is an nipple for the overflow tank to connect. The purpose of the tank (acording to the owner who restores them) is to get the air bubbles out that is casued by the cavitation of the water pump.