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:
Checking the shift drum assembly
1. Check:
Shift drum groove
Damage/scratches/wear Replace
the shift
drum assembly.
Shift drum segment "1"
Damage/wear Replace the
shift drum assembly.
Shift drum bearing "2"
Damage/pitting Replace the
shift drum assembly.
...
Engine break-in
There is never a more important period
in the life of your engine than the period
between 0 and 1000 km (600 mi). For
this reason, you should read the following
material carefully.
Since the engine is brand new, do not
put an excessive load on it for the first
1000 km (600 mi). The vari ...
Removing the rear brake caliper
NOTE:
Before disassembling the brake caliper, drain
the brake fluid from the entire brake system.
1. Remove:
Union bolt "1"
Copper washers "2"
Brake hose "3"
NOTE:
Put the end of the brake hose into a container
and pump out the brake fluid carefully. ...