Yamaha YZF-R125 Service Manual: Checking the cylinder and piston
1. Check:
- Piston wall
- Cylinder wall
Vertical scratches
Replace the
cylinder, and replace the piston and piston rings as a set.
2. Measure:
- Piston-to-cylinder clearance
a. Measure cylinder bore "C" with the cylinder bore gauge.
NOTE:
Measure cylinder bore "C" by taking side-to-side and front-to-back measurements of the cylinder.
Then, find the average of the measurements.
b. If out of specification, replace the cylinder, and replace the piston and piston rings as a set.
c. Measure piston skirt diameter D "a" with the micrometer.
b. 5.0 mm (0.20 in) from the bottom edge of the
piston 
d. If out of specification, replace the piston and piston rings as a set.
e. Calculate the piston-to-cylinder clearance
with the following formula. 
f. If out of specification, replace the cylinder, and replace the piston and piston rings as a set.
Removing the piston
1. Remove:
Piston pin clips "1"
Piston pin "2"
Piston "3"
CAUTION:Do not use a hammer to drive the piston
pin
out.
NOTE:
Before removing the piston pin c ...
Checking the piston rings
1. Measure:
Piston ring side clearance
Out of specification Replace the
piston
and piston rings as a set.
NOTE:
Before measuring the piston ring side clearance,
eliminate any carbon deposits ...
Other materials:
Checking the clutch plates
The following procedure applies to all of the
clutch plates.
1. Check:
Clutch plate
Damage Replace the clutch
plates as a
set.
2. Measure:
Clutch plate warpage
(with a surface plate and thickness gauge "1")
Out of specification Replace
the clutch
plates as a set.
...
Installing the thermostat
1. Install:
Thermostat
NOTE:
Install the thermostat with its breather hole "a"
facing up.
2. Install:
Copper washer
Coolant temperature sensor
CAUTION:Use extreme care when handling the
coolant
temperature sensor. Replace any part that
was dropped or subjected to ...
Installing the water pump
1. Install:
Water pump assembly "1"
O-rings "2"
NOTE:
Align the projection "a" on the impeller shaft
with the slit "b" on the camshaft sprocket bolt.
Lubricate the O-rings with a thin coat of lithiumsoap-
based grease.
2. Fill:
Cooling system
(with the specified amou ...