bolt pre-stress and unions in Comsol

Hi,
I'm trying to model a simple axysymmetric assembly that is joined with a bolt as you can see in the attached file.
But I'm worried about the pre-stress I'm introducing in the model. In spite of not saying anything to Comsol, if the different geometries are in contact with each other, the program understands that there is an union between them. But, how does it work?
I really don't understand what Comsol is doing in this cases. I know that there are pair that we can define, but it seems that Comsol ties different geometry without doing anything.
I think that this is important just because I'm not able to control these unions, I cannot assign any stiffness value to the union or anything similar. And it directly affects to the stiffness of the assembly.
The pre-stress value that I should enter is in relation with this too, because if Comsol is considering a perfect union, it' s introducing an stiffness to may model, and I'd be over-stressing my model.
I don't know if it is clear what I'm talking about...
Thanks a lot,