Ducati MONSTER 821 Workshop Manual page 606

Remove the chain.
Engage the first gear and loosen nut (3).
Remove the nut (3) and the lockwasher (4) on the sprocket.
Remove the front sprocket (5) from the gearbox secondary shaft.
Remove the spacer (6) with O-ring (7) and washer (8) from the gearbox secondary shaft.
Important
The O-ring must be renewed upon removal.
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