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It is really important that this bolt which hold the gear pulley for the timing belt in place is really tight. In some AAZ engines this bolt came loose due to some small manufacturing flaws on the crankshaft surface. If the bolt is loose engine timing changes and it can lead to timing belt and engine failure.
Always replace this bolt when the pulley is taken off. TDI engines may use a different bolt
also check the gear pulley: 028 105 263 D