Ducati MONSTER 821 Workshop Manual page 1085

Lubricate the race (P) of the driven gear (4) with engine oil.
Fit the driven gear (4) on the starter clutch (13), ensuring it is properly seated.
Note
To help insertion, turn the driven gear in the direction indicated by the "green" arrow.
Check that the driven gear can rotate freely in the direction of the green arrow but not in the direction of
the red arrow.
If either of these two conditions is not met, this means that the starter clutch has not been fitted correctly.
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