Yamaha YZF-R125 Service Manual: 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 from the piston
ring grooves and piston rings. 
2. Install:
- Piston ring (into the cylinder)
NOTE:
Level the piston ring into the cylinder with the piston crown.
a. 40 mm (1.57 in)
3. Measure:
- Piston ring end gap
Out of specification
Replace
the piston ring.
NOTE:
The oil ring expander spacer end gap cannot be
measured. If the oil ring rail gap is excessive, replace
all three piston rings. 
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. Mea ...
Checking the piston pin
1. Check:
Piston pin
Blue discoloration/grooves
Replace the
piston pin and then check the lubrication system.
2. Measure:
Piston pin outside diameter "a"
Out of specification ...
Other materials:
Multi-function display
1. Multi-function display
2. "RESET/SELECT" button
The multi-function display is equipped with the
following:
a speedometer (which shows the riding speed)
an odometer (which shows the total distance
traveled)
two tripmeters (which show the distance traveled
since they were last ...
Checking the spark plug
The spark plug is an important engine
component, which is easy to check.
Since heat and deposits will cause any
spark plug to slowly erode, the spark
plug should be removed and checked
in accordance with the periodic maintenance
and lubrication chart. In addition,
the condition of the ...
Aftermarket Parts, Accessories, and Modifications
While you may find aftermarket products
similar in design and quality to
genuine Yamaha accessories, recognize
that some aftermarket accessories
or modifications are not suitable because
of potential safety hazards to you
or others. Installing aftermarket products
or having other modif ...