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:
Removing the crankshaft
1. Remove:
Crankshaft "1"
NOTE:
Remove the crankshaft with the crankcase
separating tool "2".
Make sure the crankcase separating tool is
centered over the crankshaft.
CAUTION:
To protect the end of the crankshaft, place
an appropriate sized socket between the
c ...
Loading
Adding accessories or cargo to your
motorcycle can adversely affect stability
and handling if the weight distribution
of the motorcycle is changed. To avoid
the possibility of an accident, use extreme
caution when adding cargo or
accessories to your motorcycle. Use
extra care when ridi ...
Installing the piston and cylinder
1. Install:
Top ring "1"
2nd ring "2"
Oil ring expander "3"
Lower oil ring rail "4"
Upper oil ring rail "5"
NOTE:
Be sure to install the piston rings so that the
manufacturer marks or numbers face up.
2. Install:
Piston "1"
Piston pin "2"
Piston pin clips "3"
NOTE:
...