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 brake fluid level
Front brake
Front brake
Minimum level mark
Rear brake
Rear brake
Minimum level mark
Insufficient brake fluid may allow air to
enter the brake system, possibly causing
it to become ineffective.
Before riding, check that the brake fluid
is above the minimum level mark an ...
Installing the cylinder head
1. Install:
Cylinder head
NOTE:
Pass the timing chain through the timing chain
cavity.
2. Tighten:
Cylinder head bolts "1"
Cylinder head bolts "2"
NOTE:
Lubricate the cylinder head bolts and washers
with engine oil.
Tighten the cylinder head bolts in the proper
tighten ...
Checking the fuel pressure
1. Check:
Pressure regulator operation
a. Remove the fuel hose connector cover "1"
and disconnect the fuel hose "2" from the fuel
pump.
CAUTION:Although the fuel has been removed from
the fuel tank, be careful when removing the
fuel hose, since there may be fuel remaining
i ...