Ok so make a bolt in a 3D program and you will find when you print it.... the plastic shrinks. Your once perfectly measured design is now too small to fit the nut or bolt you designed it for!!!
Uggg File time! Right
Well now if you ever need to do a boolean in blender just gauge the nut and bolt hole and open the blender file that I attached. Grab the bolt head and start designing!
The blender file is in 2.63 Btw.