Monster 659 Owner's Manual page 105

Minimum tread depth
E
Measure tread depth (S) at the point where tread is most
worn down;
It should not be less than 2 mm, and in any case not less than
the legal limit.
Important
Visually inspect the tyres at regular intervals for
detecting cracks and cuts, especially on the side walls,
bulges or large spots that are indicative of internal damage.
Replace them if badly damaged.
Remove any stones or other foreign bodies caught in the
tread.
104
S
fig. 110