Yamaha YZF-R125 Service Manual: Checking the valves and valve guides
The following procedure applies to all of the valves and valve guides.
1. Measure:
- Valve-stem-to-valve-guide clearance
Out of specification
Replace the
valve guide.
Valve-stem-to-valve-guide clearance =
Valve guide inside diameter "a" -
Valve stem diameter "b" 
2. Replace:
- Valve guide
NOTE:
To ease valve guide removal and installation, and to maintain the correct fit, heat the cylinder head to 100 C (212 F) in an oven.
a. Remove the valve guide with the valve guide remover "1".
b. Install the new valve guide with the valve
guide installer "2" and valve guide remover
"1". 
a. Valve guide position
c. After installing the valve guide, bore the valve guide with the valve guide reamer "3" to obtain the proper valve-stem-to-valve-guide clearance.
NOTE:
After replacing the valve guide, reface the valve
seat. 
3. Eliminate:
- Carbon deposits (from the valve face and valve seat)
4. Check:
- Valve face
Pitting/wear
Grind the valve
face. - Valve stem end
Mushroom shape or diameter larger than the
body of the valve stem
Replace
the valve.
5. Measure:
- Valve margin thickness D "a"
Out of specification Replace
the valve.
6. Measure:
- Valve stem runout
Out of specification Replace
the valve.
NOTE:
- When installing a new valve, always replace the valve guide.
- If the valve is removed or replaced, always replace
the valve stem seal.
Removing the valves
The following procedure applies to all of the
valves and related components.
NOTE:
Before removing the internal parts of the cylinder
head (e.g., valves, valve springs, valve seats),
make sure th ...
Checking the valve seats
The following procedure applies to all of the
valves and valve seats.
1. Eliminate:
Carbon deposits
(from the valve face and valve seat)
2. Check:
Valve seat
Pitting/wear Replace the ...
Other materials:
Removing the primary drive gear and balancer gears
1. Loosen:
Primary drive gear nut "1"
NOTE:
Place the aluminum plate "a" between the balancer
drive gear "2" and the balancer driven
gear "3", and then loosen the primary drive gear
nut.
2. Straighten the lock washer tab.
3. Loosen:
Balancer driven gear nut "1"
NOTE:
Place ...
General chassis
Installing the air filter case
1. Install:
Air filter case joint clamp
NOTE:
Align the projection "a" on the air filter case with
the slot "b" in the air filter case joint clamp.
...
Installing the front fork legs
The following procedure applies to both of the
front fork legs.
1. Install:
Front fork leg
Temporarily tighten the upper and lower
bracket pinch bolts.
NOTE:
Make sure the inner tube end position "a" is 24.5
mm (0.96 in) from the top of the upper bracket.
2. Tighten:
Lower br ...