e92 335i 2008 water leaking from the headgasket,

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hello, i have e92 335i 2008, recently i had a water leaking from the headgasket,a week after replacing the headgasket, everything was fine until i opened the oil filler to see theres water in it, now im thinking that the headgasket is not installed properly or the head itself have crack or something, the garage told me that the head was already checked and its in perfect condition and the headgasket is new and cannot be misinstalled. he is telling me that it could be the oil housing is leaking from inside , not the oil filter housing gasket or the headgasket. can anyone explain what could it be ? Thanks