It started with the cap on the plastic plug broke. And when I drilled it out the hole apparently got bigger. So as a last resort I treaded it out to 12mm and screwed a metal plug with plumbing tape. but it still leaking, just when the engine is really hot.

But the part with the plug seems just to be plastic and not a part of the radiator. But it most be, right?!

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X5 3.0D M57 (EUR), 2003-03-24, BLACK SAPPHIRE METALLIC / LEDER DAKOTA/HELLBEIGE 2, Sports package