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 diode
1. Check:
Diode
Out of specification Replace.
NOTE:
The pocket tester or the analog pocket tester
readings are shown in the following table.
a. Disconnect the diode from the wire harness.
b. Connect the pocket tester ( × 1)
to the diode
terminals as shown.
c. Check the diod ...
Rider seat
To remove the rider seat
Insert the key into the seat lock,
and then turn it clockwise.
Seat lock
Open.
Pull the rider seat off.
To install the rider seat
1. Insert the projection on the front of
the rider seat into the seat holder
as shown.
Projection
...
Checking the switches
1. Clutch switch
2. Main switch
3. Front brake light switch
4. Sidestand switch
5. Rear brake light switch
6. Neutral switch
7. Turn signal switch
8. Horn switch
9. Dimmer switch
10.Pass switch
11.Engine stop switch
12.Start switch
Check each switch for continuity with the pocket tes ...