Yamaha YZ450F Owner's Service Manual page 160

The front fork with a built-in piston
rod has a very sophisticated inter-
nal construction and is particularly
sensitive to foreign material.
Use enough care not to allow any
foreign material to come in when
the oil is replaced or when the
front fork is disassembled and re-
assembled.
CHECKING THE BASE VALVE
1. Inspect:
• Base valve "1"
Wear/damage → Replace.
Contamination → Clean.
• O-ring "2"
Wear/damage → Replace.
• Piston metal "3"
Wear/damage → Replace.
• Spring "4"
Damage/fatigue → Replace base
valve.
• Air bleed screw "5"
Wear/damage → Replace.
CHECKING THE COLLAR
1. Inspect:
• Piston metal "1"
Wear/damage → Replace.
CHECKING THE FORK SPRING
1. Measure:
• Fork spring free length "a"
Out of specification → Replace.
Fork spring free length:
470 mm (18.5 in)
<Limit>: 465 mm (18.3
in)
CHECKING THE INNER TUBE
1. Inspect:
• Inner tube surface "a"
Score marks → Repair or replace.
Use #1,000 grit wet sandpaper.
Damaged oil lock piece → Re-
place.
• Inner tube bends
Out of specification → Replace.
Use the dial gauge "1".
Inner tube bending limit:
0.2 mm (0.008 in)
The bending value is shown by one
half of the dial gauge reading.
Do not attempt to straighten a bent
inner tube as this may dangerous-
ly weaken the tube.
CHECKING THE OUTER TUBE
1. Inspect:
• Outer tube "1"
Score marks/wear/damage → Re-
place.
5-19
FRONT FORK
CHECKING THE ADJUSTER
1. Inspect:
• Adjuster "1"
• O-ring "2"
Wear/damage → Replace.
ASSEMBLING THE FRONT FORK
1. Wash the all parts in a clean sol-
vent.
2. Stretch the damper assembly ful-
ly.
3. Fill:
• Front fork oil "1"
To damper assembly.
Recommended oil:
Suspension oil "S1"
Oil capacity:
3
197 cm (6.93 Imp oz,
6.66 US oz)
• Be sure to use recommended
fork oil. If other oils are used,
they may have an excessively ad-
verse effect on the front fork per-
formance.
• Never allow foreign materials to
enter the front fork.
4. After filling, pump the damper as-
sembly "1" slowly up and down
(about 200 mm (7.9 in) stroke)
several times to bleed the damper
assembly of air.
Be careful not to excessive full stroke.
A stroke of 200 mm (7.9 in) or more
will cause air to enter. In this case, re-
peat the steps 2 to 4.