Andreas,
When you stated a prestress in the bolt, are you refering to a preloaded bolt? I ran the model with two planes of symmetry, and you can see the stress contours in the plates (von Mises and max prin). It sounded like, in your original post, that you're really interested in the stress in the plates. Putting contacts betweent the bolt shank and the bores won't do any good, because there is really no contact between these bodies.
