On a 91 330E, and probably other M103's, there is an O-ring that should be installed.

The Fan bearing bracket mounts to the upper timing cover. As you face the fan bearing bracket, the bottom right bolt (I believe it's the one that the bottom of the belt tensioner shock mounts to), goes through an enlarged hole in the timing cover. There may be a taper/chamfer at the opening on the t-cover and/or a step. This is where the O-ring goes (the bolt goes through it). At the MB parts counter this is called the "timing cover o-ring". I found that the O-ring didn't fit tight around the bolt at all and neither did it stick out past the t-cover surface far enough to make any kind of a seal. I read somewhere that you are supposed to use the black MB (or equivalent) sealant on the mating surfaces of the bracket and t-cover at that hole AND under the washer of the bolt on the outside as well. Failure to do this could result in an oil leak whose source is very hard to spot.