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...

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...

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...

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...

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,...

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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...

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”. 

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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...