After a thorough search of the DUCATI spare parts catalogs, we have located for all models 2001 to now the correct assignment. So far we have assumed that there is only one thread size for all models, M22x1.5. Well, it turned out that there are also many models with the thread size M12x1.5 since about 2008.
http://www.stahlbus.com/info/images/Downloads/Oelablass-Gewinde-Motorrad.pdf