Glad you've managed to extract it.

Every time I drain the rad, I now replace the o-ring on that screw. After the second time, I drained the rad mine started dripping and wouldn't seal. Maybe someone tried to tighten it to stop a drip and sheared the head off.

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12 E70 3.5i xDrive
03 E46 330i ZHP
01 E53 3.0i
98 E36 323is
12 Audi A4 Quattro
79 Triumph Spitfire
73 MGB