Ducati Diavel Workshop Manual page 709

Cylinder head assemblies: valves - rocker arms
Locate the spacer (17) on the shaft.
Locate the spring (18) on the spacer (17) as shown, fitting one end of the spring in its seat in the cylinder head.
Locate the closing rocker arm (16) into its seat, making sure that the through hole is in line with the closing rocker arm
shaft and drive the shaft home.