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•
8-4 TOP END MAINTENANCE
1.
Since
the
formation
o
f carbon
is
a product of
two-s
troke
operation,
deca
rb oni
za
tion
at
regular
inter-
va
ls
is
essen
tial.
Removing
the
cy
linder
head
and
scrapi
ng
the
carbon
from
p iston crown
and cylinder
l1ead shou ld be done at
every tune-up or every
40
ho urs of
operation.
During
every other
decarboniza
tion, the cylinder
sh
ou
ld
be
removed and
the
exhaust
port decarbonized.
2.
When the
cyli
nder and piston
are
removed
,
there are
several
checks
which
shou
Id be performed.
First
of
all,
the cy
l
inder
bore
shou ld
be
checked fo
r wear or
scoring
each time
it is
removed.
Th
e
''out
o
f round''
and ''ta
per'' can be
checked
with
an
inside
micro-
meter. The
out
of round of the
cylinder bore
circu
lar
direction
90°
at a
time.
The differen
ce
in measure-
ments
shou
Id
not exceed
0.002'' (0.05
mm).
Honing
or
reboring should be
d
one
if
this tolerance
is
exceeded.
Taper
of
t
he
cy
li
nder
bore
is
measured
by
moving
the micrometer
from
t
h
e
top to
the
bottom of
th
e
cylinder
being
carefu
I
to miss
t he
port openings.
The
d
i
fferen ce in these measu reme nts
(between
top
and
bottom)
should
not exceed
0.002''
(0.05
mm).
If
upon
inspect
ion
of
the
cyl
i
nder bore, scoring
of
the
cy
I
inder
wal
I
is
observed
hone o
r
rebore unti
I
these
scores
d
isa
ppear.
Before assem bly,
always
clean the
ca
rb
on
from the exhaust
por
t
.
Also wash
out
the
cy I inder with solvent or
soap
and water to
insure
that
al I
hone materia
I
has
been
removed.
Fai
lure
to
do this
may
resu
lt in
premature w
earing o
r
sco
ring of
the
cy
Ii nder and
p iston.
Fig
.
8-31
•
ENGINE MAINTENANCE -
Top End Maintenance
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•
I
I
-
..
..
-
A
.,___ 9
I
..
-
--
'
I
..
- - -
-·
--
-
--
Fig
.
8-32
3
.
The
piston-to-cylinder
wall
clearance
sl1ould
be
checked
before reassembly.
Th
is is
done by measuring the
largest
outside
diameter
of
the piston and the smallest inside
diameter
of the
cylinder
bore
.
The largest diameter of the
piston
is
checked 0.4'' (
10
mm) above the
bottom
of
the piston
sk
irt
and
at
right angles
to the
piston pin
hole. New
piston to
cy
linder
wal
I
clearance
is
0.0018
'""J
0.002''
(0.045
'""J
0.050 mm).
.
.
...
..
.
Fig
.
8-33
,
4.
The
piston
pin-piston fit
can
be
checked
by
inserting
the piston pin into
the piston and
checking
the amount
of
resistance
encountered.
The piston pin should fit
into
the
piston
w ith
a snung,
thumb press fit.
Also
check
the needle bearings and
needle bearing cage
for excessive
wear,
or overheating .
I '
•
•
Fig. 8-34
-69-
•
.
•
•
•
f
~
'
J
•
•
.
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