Ducati Diavel Workshop Manual page 690

Cylinder head assemblies: camshafts
When correctly installed, the sealing ring should be flush with the bevel of the cylinder head bore.
Cylinder head cover
Apply sealant (Three Bond 1215 liquid gasket) at the four points of the cylinder head cover shown in red in the photo.
Fit the gaskets (4) and (9) on the cylinder head cover, as shown in the figure.