Yamaha YFM660 01 User Manual

							5 - 31
ENGVALVES AND VALVE SPRINGS
3.Install:

Valve cotters
NOTE:
Install the valve cotters while compressing the
valve spring with the valve spring compressor
1
 and attachment 2
.
Valve spring compressor:
P/N. YM-04019, 90890-04019
Valve spring compressor 
attachment:
P/N. 90890-01243
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. 
						

							5 - 32
ENGCYLINDER AND PISTON
CYLINDER AND PISTON
Order Job name/Part name Q’ty Remarks
Removing the cylinder and piston
Remove the parts in the order below.
Water jacket inlet housing Refer to “WATER PUMP” in CHAPTER 6.
Cylinder head Refer to “CAMSHAFT AND CYLINDER 
HEAD”.
1 Cable guide 1
2 Cylinder/O-ring 1/1 Refer to “INSTALLING THE CYLINDER”.
3 Cylinder gasket 1
4 Dowel pin 2
5 Piston pin clip 2
Refer to “REMOVING/INSTALLING THE 
PISTON”. 6 Piston pin 1
7 Piston 1
8 Piston ring set 1
For installation, reverse the removal pro-
cedure. 
						

							5 - 33
ENGCYLINDER AND PISTON
REMOVING THE 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.
2.Remove:

Piston rings 
NOTE:
Spread the end gaps apart while at the same
time lifting the piston ring over the top of the
piston crown, as shown in the illustration.
Piston pin puller:
P/N. YU-01304, 90890-01304
CHECKING THE CYLINDER AND PISTON
1.Check:

Cylinder and piston walls
Vertical scratches →
 Rebore or replace the
cylinder and the piston.
2.Measure:

Piston-to-cylinder clearance 
***********************************************
Measurement steps:
1st step:

Measure the cylinder bore “C” with a cylinder
bore gauge 1
.
a50 mm (2.0 in) from the top of the cylinder 
						

							5 - 34
ENGCYLINDER AND PISTON
NOTE:
Measure cylinder bore “C” in parallel to and at
right angles to the cylinder matching surface.
Then, find the average of the measurements.

If out of specification, rebore or replace the
cylinder, and replace the piston and piston
rings as a set.
2nd step:

Measure piston skirt diameter “P” with a
micrometer.
b5.0 mm (0.20 in) from the piston bottom edge

If out of specification, replace the piston and
piston rings as a set.
3rd step:

Find the piston-to-cylinder clearance with the
following formula.

If out of specification, rebore or replace the
cylinder, and replace the piston and piston
rings as a set.
***********************************************
Standard Wear limit
Cylinder 
bore “C”100.005 ~ 
100.045 mm 
(3.9372 ~ 3.9388 in)100.100 mm 
(3.9404 in)
C =
Piston skirt diameter “P”:
Standard99.945 ~ 99.995 mm
(3.9348 ~ 3.9368 in)
Piston-to-cylinder clearance = 
Cylinder bore “C” –
Piston skirt diameter “P”
Piston-to-cylinder clearance:
0.05 ~ 0.07 mm
(0.0020 ~ 0.0028 in)
: 0.15 mm (0.0059 in)
X+Y
2 
						

							5 - 35
ENGCYLINDER AND PISTON
CHECKING THE PISTON RING
1.Measure:

Ring side clearance
Use a feeler gauge. 
Out of specification →
 Replace the piston
and rings as a set.
NOTE:
Clean carbon from the piston ring grooves and
rings before measuring the side clearance.
Side clearance
Standard Limit
Top 
ring0.04 ~ 0.08 mm
(0.0016 ~ 0.0031 in)0.13 mm
(0.0051 in)
2nd 
ring0.03 ~ 0.07 mm
(0.0012 ~ 0.0028 in)0.13 mm
(0.0051 in)
2.Position:

Piston ring 
(in cylinder)
NOTE:
Insert a ring into the cylinder and push it
approximately 50 mm (2.0 in) into the cylinder.
Push the ring with the piston crown so that the
ring will be at a right angle to the cylinder bore.
a50 mm (2.0 in)
3.Measure:

Ring end gap 
Out of specification →
 Replace.
NOTE:
You cannot measure the end gap on the
expander spacer of the oil control ring. If the oil
control ring rails show excessive gap, replace
all three rings.
End gap
Standard Limit
Top 
ring0.30 ~ 0.45 mm
(0.0118 ~ 0.0177 in)0.70 mm
(0.0276 in)
2nd 
ring0.30 ~ 0.45 mm
(0.0118 ~ 0.0177 in)0.80 mm
(0.0315 in)
Oil
ring0.2 ~ 0.7 mm
(0.0079 ~ 0.0276 in)— 
						

							5 - 36
ENGCYLINDER AND PISTON
CHECKING THE PISTON PIN INSPECTION
1.Check:

Piston pin
Blue discoloration/grooves →
 Replace, then
inspect the lubrication system.
2.Measure:

Piston pin-to-piston clearance
***********************************************
Measurement steps:

Measure the piston pin outside diameter a
.
If out of specification, replace the piston pin.

Measure the piston inside diameter b
.

Calculate the piston pin-to-piston clearance
with the following formula.

If out of specification, replace the piston.
***********************************************
INSTALLING THE 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.
Outside diameter (piston pin):
21.991 ~ 22.000 mm
(0.8658 ~ 0.8661 in)
Piston pin bore inside diameter:
22.004 ~ 22.015 mm
(0.8663 ~ 0.8667 in)
Piston pin-to-piston clearance =
Bore size (piston pin) b –
Outside diameter (piston pin) a 
Piston pin-to-piston clearance:
0.004 ~ 0.024 mm
(0.00016 ~ 0.00094 in)
: 0.07 mm (0.003 in) 
						

							5 - 37
ENGCYLINDER AND PISTON
2.Position:

Top ring

2nd ring

Oil ring
Offset the piston ring end gaps as shown.
aTop ring end
bOil ring end (lower)
cOil ring end (upper)
d2nd ring end
3.Install:

Piston 1
 

Piston pin 2
 

Piston pin clips 3
 
NOTE:

Apply engine oil onto the piston pin, piston
ring and piston.

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 from falling into the crankcase.
4.Lubricate:

Piston

Piston rings

Cylinder
NOTE:
Apply a liberal coating of engine oil.
New
INSTALLING THE CYLINDER
1.Install:

Cylinder

O-ring

Bolts (M10)

Bolts (M6)
NOTE:
Install the cylinder with one hand while com-
pressing the piston rings with the other hand.
CAUTION:

Be careful not to damage the timing chain
damper during installation.

Pass the timing chain through the timing
chain cavity.
New
T R..42 Nm (4.2 m · kg, 30 ft · lb)
T R..10 Nm (1.0 m · kg, 7.2 ft · lb) 
						

							5 - 38
ENGAC MAGNETO
AC MAGNETO
Order Job name/Part name Q’ty Remarks
Removing the AC magneto
Remove the parts in the order below.
Engine oil Drain.
Shift pedal link Disconnect.
1 Oil pipe 1 1 Disconnect.
2 AC magneto coupler 2
3 Starter idle gear cover/gasket 1/1
4 Dowel pin 1
5 Starter idle gear 1 1
6 Starter idle gear 1 shaft 1
7 Bearing 1
8 AC magneto cover/gasket 1/1 Refer to “REMOVING/INSTALLING THE 
AC MAGNETO ROTOR”. 
						

							5 - 39
ENGAC MAGNETO
Order Job name/Part name Q’ty Remarks
9 Dowel pin/O-ring 3/2
10 Lead holder 1
11 Pickup coil 1
12 Stator assembly 1
13 Starter idle gear 2 1
14 Starter idle gear 2 shaft 1
15 Bearing 1
16 AC magneto rotor 1 Refer to “REMOVING/INSTALLING THE 
AC MAGNETO ROTOR”.
17 Starter wheel gear 1
18 Woodruff key 1
19 Bearing/washer 1/1
For installation, reverse the removal pro-
cedure. 
						

							5 - 40
ENGAC MAGNETO
REMOVING THE AC MAGNETO ROTOR
1.Remove:

AC magneto cover
NOTE:
Loosen each bolt 1/4 of a turn at a time, in
stages and in a crisscross pattern. After all of
the bolts are fully loosened, remove them.
2.Remove:

AC magneto rotor nut 1
 

Washer
NOTE:

While holding the AC magneto rotor 2
 with
the sheave holder 3
, loosen the generator
rotor nut.

Do not allow the sheave holder to touch the
projection 4
 on the rotor.
Sheave holder:
P/N. YS-01880, 90890-01701
3.Remove:

AC magneto rotor 1
 
NOTE:
Use the flywheel puller 2
.
Flywheel puller:
P/N. YU-33270, 90890-01362
CHECKING THE COIL
1.Check:

Stator assembly

Pickup coil
Damage →
 Replace.