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Two-Stroke Engines
Copyright Goodheart-Willcox Co., Inc.
The basic terms related to the piston are the:
• Ring land.
• Piston crown.
• Piston skirt.
Piston
The piston in a two-stroke engine receives pres-
sure from combustion and changes it into linear
motion. The piston transmits this linear motion
to the crankshaft by way of the connecting rod.
The crown of the piston is the top surface
of the piston. Combustion pressure acting on
the crown creates a force that pushes the pis-
ton down in the cylinder. The piston is typically
composed of an aluminum alloy that can toler-
ate the heat of the combustion process.
Ring(s)
Rings on the piston keep combustion pressure
above the piston and out of the crankcase. A pis-
ton ring is a metal ring that encircles the piston
close to the crown. Its purpose is to create a seal
between the piston and the cylinder wall to pre-
vent combustion gases from entering the crank-
case. Depending on the engine, the piston could
have one or two rings. The ring is installed in
a groove cut around the piston. The ring has
an inside diameter that is slightly smaller than
the diameter of the piston. This keeps the ring
seated in the ring groove. See Figure 5-12. The
raised area between ring grooves is called a ring
land.
The ring is split to allow it to be installed
around the piston and to allow it to expand to
stay in constant contact with the cylinder wall.
The rings are not manufactured as a perfect
circle that would easily slide into the cylinder
because the ring would not apply an outward
force on the cylinder wall. Instead, they are
manufactured as a perfect circle that has been
permanently expanded or sprung out. The rings
are compressed before the cylinder is installed
onto the piston. Once the rings are installed,
they try to regain their original shape, which
causes them to push against the cylinder wall
with constant pressure. This is called the inher-
ent force of the ring.
The gap between the ring ends when the ring
is placed squarely in the cylinder is called the
ring end gap. Each ring end has a semicircular
notch, which must be aligned with an alignment
pin in the ring groove when the ring is installed.
The alignment pin keeps the ring from rotat-
ing, or “oscillating,” around the piston while the
engine is running.
If there were no alignment pin in the ring
groove, the ring could possibly rotate around
the piston while the engine is running and the
ring ends could be exposed to the exhaust port.
The inherent force of the ring would allow the
ring ends to expand while in the exhaust port
and break off, causing serious mechanical dam-
age to the cylinder and piston. The alignment
pin keeps this from happening.
Piston Pin
The piston pin is a round, hardened, precision
ground and polished metal rod that connects the
piston to the connecting rod. See Figure 5-13.
The piston pin is held in place in the piston pin
bore, a precision drilled hole through the pis-
ton, by a retaining clip, a circular wound wire
of spring steel. A groove is machined just inside
the edge of the piston bore to hold the retaining
clip in place after the piston pin is installed.
Some piston pins need only one retaining clip
to hold the pin in place. The opposite end of the
piston pin is retained by the shoulder of a coun-
terbore. A bore is a hole drilled through a part
from one side of the part to the other. A coun-
terbore is created by drilling the hole larger
but stopping the drilling before going through
to the other side of the part. A shoulder is cre-
ated where the second drilling stopped. See
Figure 5-14. When the piston pin is installed
in the counterbore, the pin stops at the shoul-
der. The retaining clip groove is machined on
Ring groove
Ring land
Alignment
notches
Alignment pin Ring
Goodheart-Willcox Publisher
Figure 5-12. The piston ring fits into a machined
groove around the piston called the ring land.