Ducati MONSTER 821 Workshop Manual page 491

If the difference between the minimum and maximum values measured is higher than 0.40 mm, replace
the phonic wheel.
Once verified that such difference is lower than or equal to 0.40 mm, perform the shimming procedure of
the speed sensor (1) or (2) using suitable calibrated shims (B), after tightening the front (3) or rear (4)
retaining screw of the relevant ABS sensor to a torque of 7 Nm ± 10%.
The shims used are of 0.2 mm or 0.5 mm.
Use them as required to reach the correct shimming.
Warning
Shim pack shall never exceed 3 mm.
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