Yamaha Wr 450 F Owners Manual

4 - 38
ENGVALVES AND VALVE SPRINGS
9. Lap:
Valve face
Valve seat
NOTE:
After refacing the valve seat or replacing the
valve and valve guide, the valve seat and
valve face should be lapped.
Lapping steps:
Apply a coarse lapping compound to the
valve face.
CAUTION:
Do not let the compound enter the gap
between the valve stem and the guide.
Apply molybdenum disulfide oil to the
valve stem.
Install the valve into the cylinder head.
Turn the valve until the valve face and
valve seat are evenly polished,...

4 - 39
ENGVALVES AND VALVE SPRINGS
Valve spring
1. Measure:
Valve spring free length a 
Out of specification → Replace.
2. Measure:
Compressed spring force a 
Out of specification → Replace.
bInstalled length
Free length (valve spring):
Intake:
37.03 mm (1.46 in)
: 35.17 mm (1.38 in)
Exhaust:
37.68 mm (1.48 in)
: 35.79 mm (1.41 in)
Compressed spring force:
Intake:
111.3 ~ 127.9 N at 27.87 mm
(11.3 ~ 13.0 kg at 27.87 mm, 
24.91 ~ 28.66 lb at 1.10 in)
Exhaust:
127.4 ~ 146.4 N at 27.38 mm
(13.0 ~ 14.9 kg...

4 - 40
ENGVALVES AND VALVE SPRINGS
Combination of cylinder head and valve 
lifter
1. Combination:
For this combination, match the paint
color on the cylinder head with that on the
valve lifter according to the chart below.
NOTE:
When you purchase a cylinder head, you can-
not designate its size. Choose the valve lifter
that matches the above chart.Combination
Cylinder head mark 
a (color)Valve lifter mark 
b (color)
Blue Blue
Yellow Yellow
Purple Black
ab
ASSEMBLY AND INSTALLATION
1. Apply:
Molybdenum...

 
4 - 41
ENG
 
VALVES AND VALVE SPRINGS 
3. Install: 
 
Valve cotters
NOTE:
 
While compressing the valve spring with a
valve spring compressor   
1  
 install the valve 
cotters.
4. To secure the valve cotters onto the
valve stem, lightly tap the valve tip with a
piece of wood.
CAUTION:
 
Hitting the valve tip with excessive force 
could damage the valve. 
Valve spring compressor:
YM-4019/90890-04019 
5. Install: 
 
Adjusting pad   
1  
  
 
Valve lifter   
2  
 
NOTE:
 
 
Apply the engine oil on...

 
4 - 42
ENG
 
CYLINDER AND PISTON
CYLINDER AND PISTON 
CYLINDER AND PISTON 
Extent of removal: 
1  
 Cylinder removal  
2  
 Piston removal
Extent of removal Order Part name Q’ty Remarks 
CYLINDER AND PISTON 
REMOVAL  
Preparation for removal Cylinder head Refer to “CYLINDER HEAD” section.
1 Bolt (cylinder) 1
2 Cylinder 1
3 Gasket 1
4 Dowel pin 2
5 Dowel pin/O-ring 1/1
6 Piston pin clip 2
Use special tool.
Refer to “REMOVAL POINTS”. 7 Piston pin 1
8 Piston 1
9 Piston ring set 1
2
1 

 
4 - 43
ENG
 
CYLINDER AND PISTON 
REMOVAL POINTS
Piston  
1. Remove: 
 
Piston pin clips   
1  
  
 
Piston pin   
2  
  
 
Piston   
3  
 
NOTE:
 
 
Put identification marks on each piston head
for reference during reinstallation. 
 
Before removing each piston pin, deburr the
clip groove and pin hole area. If the piston
pin groove is deburred and the piston pin is
still difficult to remove, use the piston pin 
puller   
4  
.
CAUTION:
 
Do not use a hammer to drive the piston 
pin out.
Piston...

 
4 - 44
ENG
 
CYLINDER AND PISTON 
Cylinder bore “C”95.00 ~ 95.01 mm
(3.7402 ~ 3.7406 in)
Taper limit “T” 0.05 mm (0.002 in)
Out of round “R” 0.05 mm (0.002 in)
“C” = Maximum D
“T” = (Maximum D 
1 
 or D 
2 
) 
– (Maximum D 
5 
 or D 
6 
)
“R” = (Maximum D 
1 
, D 
3 
 or D 
5 
) 
  
– (Minimum D 
2 
, D 
4 
 or D 
6 
) 
 
If out of specification, replace the cylinder,
and replace the piston and piston rings as
set. 
2nd step: 
 
Measure the piston skirt diameter “P” with
a micrometer. 
a 
8 mm (0.315...

 
4 - 45
ENG
 
CYLINDER AND PISTON 
Piston ring 
1. Measure: 
 
Ring side clearance
Use a feeler gauge  
1 
. 
Out of specification  
→ 
 Replace the pis-
ton and rings as a set.
NOTE:
 
Clean carbon from the piston ring grooves and 
rings before measuring the side clearance.
2. Position: 
 
Piston ring 
(in cylinder)
NOTE:
 
Insert a ring into the cylinder and push it
approximately 10 mm (0.39 in) into the cylin-
der. Push the ring with the piston crown so that
the ring will be at a right angle to the...

 
4 - 46
ENG
 
CYLINDER AND PISTON 
Piston pin 
1. Inspect: 
 
Piston pin
Blue discoloration/grooves  
→ 
 Replace,
then inspect the lubrication system.
2. Measure: 
 
Piston pin-to-piston clearance 
ASSEMBLY AND INSTALLATION
Piston 
1. Install: 
 
Piston rings
Onto the piston.
NOTE:
 
 
Be sure to install the piston rings so that the
manufacturer’s marks or numbers are
located on the upper side of the rings. 
 
Lubricate the piston and piston rings liberally 
with engine oil. 
Measurement steps:...

4 - 47
ENGCYLINDER AND PISTON
2. Position:
Top ring
2nd ring
Oil ring
Offset the piston ring end gaps as
shown.
aTop ring end
b2nd ring end
cOil ring end (upper)
dOil ring
eOil ring end (lower)
3. Install:
Piston 1 
Piston pin 2 
Piston pin clips 3 
NOTE:
Apply engine oil onto the piston pin and pis-
ton.
Be sure that the arrow mark a on the piston
points to the exhaust side of the engine.
Before installing the piston pin clip, cover the
crankcase with a clean rag to prevent the
piston pin clip...