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There wasn't a year where they came from the factory as aluminum. The newer 4.8L motor in the 2008 MY had a slightly different design and the coolant pipes weren't an issue. All the 03-06 MY 4.5L motors had plastic coolant pipes.
Porsche really never went to aluminum coolant pipes. They simply replaced every failed plastic one with the metal parts. In other words, you cannot buy replacement plastic pipes.