Piston; Piston Ring - Honda Motorcycle Service Manual

PISTON
The piston moves at high speed in the cylinder, and is exposed
to the extreme temperature of combustion. Pistons are
therefore made of a specially forged light alloy type aluminum,
which is not only lightweight but also less susceptible t o ther-
mal expansion.
The piston itself is a high temperature part, being cooled only
by the release of heat t o the cylinder through the inhaled
airifuel mixture and the piston ring. The piston head is
therefore fabricated to have a somewhat smaller outer
diameter than the skirt, due to the high temperature exposure
and large thermal expansion. With a 2-stroke engine, the
cylinder is distorted and the clearance with the piston tends to
partially decrease, as there are t w o different parts with inequal
t h ~ r m a l exoansion in the cvlinder: a Dart cooled bv the airlfuel
PISTON MARK
PISTON RINGS
PISTON PIN
NEEDLE
BEARING
(2-STROKE
ONLY)
. . ~.
mixture around the piston, such as the scavenging port, and
a
part exposed to the extreme heat near the exhaust port. To
OFFSET
resolve this problem the piston head of the 2-stroke engine is
oval and designed to have appropriate clearance during
driving.
At the small end of the connecting rod of the 2-stroke engine,
a needle bearing is used. For the 4-stroke engine, however, a
PISTON RlNG
plain bearing is used at this point.
The reciprocating motion of the piston is converted into a rota-
tional motion of the crankshaft through the connecting rod. To
smooth the motion conversion, the pin hole of the piston is
slightly offset against the center shaft of the crankshaft.
If the piston is assembled in an incorrect direction, the piston
4-STROKE
The piston rings are inserted within the grooves in the piston
Plain TOP RING
Rings are made of a material with considerable wear
resistance, because the piston rings move at high speed with
Bevel Edge SECOND RlNG
the piston while being pressed against the cylinder wall by
their own tension.
The ring arrangement for the Cstroke engine is with t w o com-
CONNECTING ROD
( 7 p J
CRANKSHAFT
Piston rings for 4-stroke and 2-stroke engines have identifi-
cation marks near the end gap of the top and second rings.
These ring marks must face upwards on the piston when
assembling.
strikes the cylinder wall due to reversed offset, causing rapid
wear or seizure.
To assemble the offset correctly, the assembly must be done
pression rings sealing the combustion gas and a pair of oil rings
removing the oil from the cylinder wall.
Although the t w o compression rings are similar in appearance,
they are different in detail. Therefore, when removed, their in-
Date of Issue: March, 1995
0
HONDA MOTOR CO.,LTD
by following the marks indicating the piston head assembly
direction.
stallation position must be noted and marked before storing to
prevent incorrect reassembly. If identification is difficult, the
difference in shapes should be remembered; the top ring is
usually plain and the second ring has a beveled edge. Most of
the top rings are chrome plated on their sliding surface in order
to increase wear resistance. A few second rings are, however,
also plated.
COMPRESSION RINGS
PISTON RlNG GAP
\ 1 "
MARK