I agreed with fanka but there is biggger problem.
SP recognize your bolt and nut as a "ball" shape, not the way they looks.
collison area is the problem. search with the word on this board.
In fact, it is very hard to make a body with complexs and concave shape in SP.
For example, if you model a car in sketchup, not with sketch physics tools(box,cone..etc),SP recognize it as simple 'box' shape.
[image: VW31_c1.JPG]