Yamaha YFM660RN Service Manual page 214

CYLINDER AND PISTON
NOTE:
Measure cylinder bore "C" in parallel to and at
right angles to the cylinder matching surface.
Then, find the average of the measurements.
Cylinder
bore "C"
(3.9372 ~ 3.9388 in)
If out of specification, rebore or replace the
cylinder, and replace the piston and piston
rings as a set.
2nd step:
Measure piston skirt diameter "P" with a
micrometer.
b 5.0 mm (0.20 in) from the piston bottom edge
Piston skirt diameter "P":
Standard
If out of specification, replace the piston and
piston rings as a set.
3rd step:
Find the piston-to-cylinder clearance with the
following formula.
Piston-to-cylinder clearance =
Cylinder bore "C" –
Piston skirt diameter "P"
Piston-to-cylinder clearance:
0.05 ~ 0.07 mm
(0.0020 ~ 0.0028 in)
<Limit>: 0.15 mm (0.0059 in)
If out of specification, rebore or replace the
cylinder, and replace the piston and piston
rings as a set.
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ENG
Standard Wear limit
100.005 ~
100.100 mm
100.045 mm
(3.9404 in)
X+Y
C =
2
99.945 ~ 99.995 mm
(3.9348 ~ 3.9368 in)