Use the micrometer to measure at several points around the ring.
Piston Ring Thickness Standard:
Top [A] 0.77 
0.79 mm (0.0303
0.0311 in.)
Second [B] 0.78 
0.79 mm
(0.0307 
0.0311 in.)
Service Limit:
Top 0.70 mm (0.0276 in.)
Second 0.71 mm (0.0280 in.)
If any of the measurements is less than the service limit on either of the rings, replace all the rings.
NOTE
When using new rings in a used piston, check for uneven groove wear. The rings should fit perfectly parallel to the groove sides. If not, replace the piston.
Piston Ring Groove Width Inspection
Piston Ring End Gap InspectionRear Wheel Removal
Raise the rear wheel off the ground with the stand [A].
Remove:
Cotter Pin [A]
Rear Axle Nut [B]
Washer [C]
Rear Axle [D] (from Right Side)
Remove the rear wheel rotation sensor from the caliper
bracket (see Rear Wheel Rotation Sensor Removal in the
Brakes chapter).
...
Drive Chain Wear Inspection
Remove the chain cover (see Drive Chain Removal/Installation
in the Final Drive chapter).
Rotate the rear wheel to inspect the drive chain for damaged
rollers, and loose pins and links
If there is any irregularity, replace the drive chain.
Lubricate the drive chain if it appears dry.
...
Special Tools and Sealant
Compression Gauge, 20 kgf/cm²:
57001-221
Valve Spring Compressor Assembly:
57001-241
Valve Spring Compressor Adapter,
25:
57001-1019
Valve Seat Cutter, 45° - 27.5:
57001-1114
Valve Seat Cutter, 45° - 32:
57001-1115
Valve Seat Cutter, 32° - 28:
57001-1119
Valve Seat Cutter ...