the only way i can think of to keep it gap free (if you work in polygons) is to attach part a and part b together. if for some reason your model requires that part a and b are seperate, i would insert a edgeloop on part b as hamidz said, and then move the verts of edge in the same spot as the verts on part a. after that you select all faces of part b except the ones that connect to part a - and you smooth. it will add a smooth part b that still lines up perfect with part a no matter how close you get. you do end up with 5 sided faces though around the edge - which you can either leave as is, clean up manually, or apply another smooth to both part a and b - and maya will get rid of the 5 sides faces and create all quads again. that what you wanted? other ways would be to go nurbs fill the gap in, or go subd and go higher lvl on your rivet. if you go subd maya does internally basically the same as i mentioned above, only automatically w/o you ever needing to clean up afterwards.