Honda 2009 TRX90X Owner's Manual page 146

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Drive Chain
5. With the upper or lower section of drive chain tight between the
sprockets measure the distance between a span of 21 pins from pin
center to pin center. If the distance exceeds the service limit, the chain
is worn out and should be replaced.
Service limit:
10.6 in (268 mm)
Measure a span of 21 pins
(cont'd)
Servicing Your Honda 137