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:
Checking and replacing the rear wheel sprocket
1. Check:
Rear wheel sprocket
More than 1/4 tooth "a" wear
Replace the
rear wheel sprocket.
Bent teeth Replace the rear
wheel sprocket.
b. Correct
1. Drive chain roller
2. Rear wheel sprocket
2. Replace:
Rear wheel sprocket
a. Remove the self-locking nuts and the ...
Assembling the front fork legs
The following procedure applies to both of the
front fork legs.
WARNING
Make sure the oil levels in both front fork
legs are equal.
Uneven oil levels can result in poor handling
and a loss of stability.
NOTE:
When assembling the front fork leg, be sure to
rep ...
Installing the cylinder head
1. Install:
Cylinder head
NOTE:
Pass the timing chain through the timing chain
cavity.
2. Tighten:
Cylinder head bolts "1"
Cylinder head bolts "2"
NOTE:
Lubricate the cylinder head bolts and washers
with engine oil.
Tighten the cylinder head bolts in the proper
tighten ...