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 lean angle sensor
1. Remove:
Lean angle sensor
2. Check:
Lean angle sensor output voltage
Out of specification Replace.
a. Connect the lean angle sensor to the wire
harness.
b. Connect the pocket tester (DC 20 V) to the
lean angle sensor coupler as shown.
Positive tester probe
yellow/green "1"
Neg ...
Checking the spark plug
1. Remove:
Right side cover
Refer to "GENERAL CHASSIS" on page 4-1.
2. Disconnect:
Spark plug cap
3. Remove:
Spark plug
CAUTION:Before removing the spark plug, blow away
any dirt accumulated in the spark plug well
with compressed air to prevent it from falling
...
Checking and adjusting the steering head
1. Stand the vehicle on a level surface
WARNINGSecurely support the vehicle so that there
is
no danger of it falling over.
NOTE:
Place the vehicle on a suitable stand so that the
front wheel is elevated.
2. Check:
Steering head
Grasp the bottom of the front fork legs an ...