Hi Fabrizio: I have 2 Eclipses, and both do not have any dihedral. Are you installing the stab tube into the halves and just holding them together to check?? I would think that if the cores and stab tube holes are cut properly at CA then it would have to be straight when you slide in the stab tube. I would do as Steve mentioned and draw a centerline and then install the stab tube to see if it all lines up correctly. PS - you will really like the way your Eclipse flies!!