Original heater cores were produced using an obsolete Òexpanded tubeÓ design. They performed poorly as there was little contact between the thin fins and the water tubes. This core is made with Òtube and finÓ technology, where several layers of folded aluminum fin are sandwiched between wide, flat tubes and then topped with header tanks. The entire assembly is oven brazed so that a much greater surface area of fins is welded to the tubes, resulting in far higher efficiency. Oven brazing involves dipping the loosely assembled core in a brazing liquid and then passing the core through a special ammonia-infused oven, which causes the brazing fluid to flow and melt into all the cracks and crevices and solidify.