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 rear brake master cylinder
NOTE:
Before removing the rear brake master cylinder,
drain the brake fluid from the entire brake system.
1. Disconnect:
Rear brake light switch coupler
2. Loosen:
Rear brake light switch "1"
3. Remove:
Rear brake master cylinder bolts
Rear brake master cylinder
4. Remove:
...
Checking the front fork legs
The following procedure applies to both of the
front fork legs.
1. Check:
Inner tube
Outer tube
Bends/damage/scratches
Replace.
WARNINGDo not attempt to straighten a bent inner
tube as this may dangerously weaken it.
2. Measure:
Spring free length "a"
Out of sp ...
Main switch/steering lock
Main switch/steering lock
The main switch/steering lock controls
the ignition and lighting systems, and is
used to lock the steering. The various
positions are described below.
ON
All electrical circuits are supplied with
power, the meter lighting, taillight, license
plate light and ...