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16
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3.
E N G I N E - - - - - - - - - - - - - - - -
3-3
CYLINDER, PISTON AND PISTON RINGS
A.
Construction
The
cast iron
cylinder
has a
lubrication
oil channel
to the
cylinder
head
cover,
push
rod
holes
and
lubrication
oil return
passage.
Each
gasket
has an 0-ring to effectively pre-
vent oil leaks.
The cylinder is subject
to
wear due to
the
movement of the piston sliding
up
and down
against the
cylinder
wall.
Wear
is greatly
accelerated
when
dust
in
the air or dirt or mettalic
particles
in
the oil get into
the
cylinder.
Therefore the proper
servicing
of
the
air cleaner
and
oil filter
is
necessary.
The piston receives
the
explosive energy of the
gas/
air mixture and
transmits
rotating
motion to the crankshaft through the connecting rod and crank pin.
As
it
moves up and down
in
the cylinder at high
speeds,
special
core must be
taken in
selection of the material of the
piston.
In the Hondo
90,
the piston
is
mode
of
cost
aluminum
alloy
and is 1 mm elliptical in
shape.
As the piston
is
heated
to
high
temperatures,
considerable thermal deformation occurs
and
expansion
toward
the
ends
of the piston pin will lessen the clearance between the
cylinder and
piston.
Therefore
the
clearance
is
most important.
If the
clearance
is too
large, a
strong
shock
will be
causeci
when the
piston reverses
its
motion
at the
top
and
bottom of the
stroke,
causing
the
piston
to slap
against the cylinder and allowing oil up
into
the combustion chamber from the crankcase.
If the
clearance is
too small, the
formation of
a lubricating oil film on the cylinder
wall is
prevented,
causing
piston
seizure
or at least
lowering
output through increased
fr iction
loss.
Along
with
the
cylinder
and
piston,
the
piston
rings
have great bearing on pressure
in
the combustion chamber
and
prevention of crankcase
oil
from
getting
into
the
combustion
chamber.
The
piston
rings
are made
of
special cast
iron.
The
top
and
second rings serve to
maintain compression, while
the oil
ring
scrapes oil
from
the cylinder wall.
All
the
rings also
transfer heat from the piston to the cyl inder.
Wear of the top ring i5 especially
rapid
as
it is subjected to
high
temperatures
from
the exploding gas mi xture.
Therefore, it
is coated
with hard chrome
to improve its resistance
to
wear.
The
second
ring
increases the
area
of
cylinder
in
contact
and
decreases
the
pressure per unit
so that
it expands to
fit
closely
against
the cylinder wall rapidly.
The
oil ring has a concave
section shape
to
scrape
oil
off
of the cylinder
wall
effectively.
The clearance between
the ends of the
rings must
be
correct.
Too small
clearance
will allow the ring
ends
to
contact
when heated
and bulge against the cylinder
wall,
causing
increased
friction
loss
or
seizure.
Too
large
clearance
will
allow combustion
pressure
to
escape
into
the
crankcase or
oil
to
come
up
into
the
combustion
chamber.
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