Bummer! The socket head on these bolts is pretty deep. If you are really, really lucky you will be able to clean out the socket well in the head of the bolt to get the hex driver in deeper. Also, buy some valve lapping compound and dip the hex driver in it before inserting into the bolt. The abrasive will help get a better bite. Many people suggest loosining the bolt with a surge instead of steadily increasing pressure on the wrench.
1984 300D Euro