Yamaha YZF-R125 Service Manual: 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
Replace
the piston pin.
3. Measure:
- Piston pin bore diameter "b"
Out of specification
Replace
the piston.
4. Calculate:
Piston-pin-to-piston-pin-bore clearance
Out of specification 
Replace the
piston pin
and piston as a set.
- Piston-pin-to-piston-pin-bore clearance =
Piston pin bore diameter "b" -
Piston pin outside diameter "a"
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 ...
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 ...
Other materials:
Checking the front wheel
1. Check:
Front wheel axle
Roll the wheel axle on a flat surface.
Bends → Replace.
WARNINGDo not attempt to straighten a bent wheel
axle.
2. Check:
Tire
Front wheel
Damage/wear → Replace.
Refer to "CHECKING THE TIRES" on page
3-23 and "CHECKING TH ...
Checking the cylinder head
1. Eliminate:
Combustion chamber carbon deposits
(with a rounded scraper)
NOTE:
Do not use a sharp instrument to avoid damaging
or scratching:
Spark plug bore threads
Valve seats
2. Check:
Cylinder head
Damage/scratches Replace.
Cylinder head water jacket
Min ...
Installing the primary drive gear and balancer gears
1. Install:
Balancer driven gear "1"
Lock washer
Balancer drive gear "2"
Primary drive gear
Washer "3"
Balancer driven gear nut
Primary drive gear nut
NOTE:
Align the punch mark "a" in the balancer drive
gear "2" with the punch mark "b" in the balancer
driven gear "1".
...