quote:Maybe because the coolant overflow tank can rust some on the inside? I would think that water would be recirculating in the system from the overflow, but apparently not due to the different water color I saw.
Water & overflow tanks from mid-'72-up were nickel-plated inside and out, then spray-painted flat black- maybe for slightly more heat radiation. If you remove the paint, you'll have a nice shiny stock tank. But plating is never a continuous sheet. Instead, its a stack of flat crystals, and its very possible for water to wormhole its way under the plating just as on an old bumper. If coolant isn't changed yearly, the corrosion inhibitors get used up and rust starts on the underlying mild steel. I've seen stock tanks with NO bottom and one with a soldered-on coffee can bottom! Also, the out-pipe is not at the bottom of the tank, so even though there is (or should be!) circulation, stuff still collects in the bottom.