Yamaha WR426F(N) Owner's Service Manual page 406

VALVES AND VALVE SPRINGS
5. Measure:
NOTE:
When installing a new valve always replace
the guide.
If the valve is removed or replaced always
replace the oil seal.
6. Eliminate:
7. Inspect:
8. Measure:
Measurement steps:
4 - 36
Runout (valve stem)
Out of specification
Runout limit:
0.01 mm (0.0004 in)
Carbon deposits
(from the valve face and valve seat)
Valve seats
Pitting/wear Reface the valve seat.
Valve seat width a
Out of specification
seat.
Valve seat width:
Intake:
0.9 ~ 1.1 mm (0.0354 ~ 0.0433 in)
<Limit>: 1.6 mm (0.0630 in)
Exhaust:
0.9 ~ 1.1 mm (0.0354 ~ 0.0433 in)
<Limit>: 1.6 mm (0.0630 in)
Apply Mechanic's blueing dye (Dykem) b
to the valve face.
Install the valve into the cylinder head.
Press the valve through the valve guide
and onto the valve seat to make a clear
pattern.
Measure the valve seat width. Where the
valve seat and valve face made contact,
blueing will have been removed.
If the valve seat is too wide, too narrow, or
the seat is not centered, the valve seat
must be refaced.
ENG
Replace.
Reface the valve