Electron diffraction patterns produced by free Ar-N clusters (10(3) < (N) over bar < 10(5)) display the characteristic features of the crystallographic face centered cubic structure. However, the existence of structural defects is observed upto the largest clusters, mainly through the broadening and the displacement of crystallographic lines. An analysis of these changes is tempted in order to determine the nature (deformation or twin) and the number of stacking faults. Incoherences are found in the quantitative results, which could mean that the standard crystallographic analysis fails in the description of defects, particularly twinning, in clusters. Finally, a comparison with another similar experiment is exposed, showing the influence of the gas expansion conditions upon the cluster structure.