Only the outer sphere matters.
Exactly speaking, SP takes the outher 'box-shaped boundary' of a group to calculate the mass of it.
So the shape dosen't matter.
If u click a group, you can see a box composed of blue lines. SP take total volume of the box and multiply it's density to calculate mass of the group.
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.
If you got SP3x, you can change it in UI window.
SP3x download link.
http://code.google.com/p/sketchyphysics2/downloads/list?can=1&q=&sort=-uploaded&colspec=Filename%20Summary%20Uploaded%20Size%20DownloadCount
I attached an example.
Hellow I am enjoying SP3x resently and I need some help.
When I thouch "frozen thruster" with my "magic hand" during simulation, it unfreeze and move freely.
But when "frozen thruster" touched by other object, it moves during contact and it freeze again when it lose contact (with other object).
I attache file with the situation I wrote about.
Any solution with this problem? I expected "noautofreeze" option could be helpful but it dosen't. Actually I dont know the effect of the option and what is the "autofeezing".
I am sorry for my poor English.