210
Two-Stroke Engines
Copyright Goodheart-Willcox Co., Inc.
exhaust side of the piston will have heavy scor-
ing, Figure 9-61. The piston expands because
of the heat. Fresh air-fuel charge enters at the
intake port, so the intake side of the piston
skirt remains relatively cool. However, because
the piston expands from the heat and there is
no lubrication in the fuel, the side of the piston
skirt that covers and uncovers the intake port
becomes scored. See Figure 9-62. Connecting
the score locations forms an “Y” pattern across
the piston head, Figure 9-63.
Lean seizure scoring on the piston comes
from the engine running at excessive speeds,
causing the engine to overheat and the piston
to seize. Lubrication is reduced because of the
lean air-fuel charge. The engine loses the cool-
ing effect from the normal fuel quantity. Lean-
ing the mixture increases the combustion
temperature. As a result, the cooling fins can-
not remove the excess heat. The piston heats up
quickly and expands. The first locations of scor-
ing on the side of the piston correspond to the
cylinder wall surfaces that are adjacent to the
exhaust port, Figure 9-64. Air-fuel charge is
still entering the intake port, but because it con-
tains less fuel, it has lower cooling capacity than
a normal air-fuel charge. As a result, scoring on
the intake side of the piston skirt occurs over
a larger area than it does in a raw-fuel failure,
Figure 9-65. Connecting the score locations
forms an “X” pattern across the piston head.
See Figure 9-66. As the engine continues to run
lean, severe scoring takes place at the exhaust
port, Figure 9-67. Eventually the engine seizes.
Running lean for long periods of time also
causes the connecting rod crankpin bearing to
fail, Figure 9-68. This results from the bear-
ing running at a much higher speed than nor-
mal. In addition, lubrication is reduced because
of a lean fuel mixture. The bearing prematurely
wears out.
Exhaust scoring
Intake scoring
Goodheart-Willcox Publisher
Figure 9-61. Without lubrication, the exhaust
side of the piston becomes heavily scored.
Goodheart-Willcox Publisher
Figure 9-62. An absence of lubrication causes
the intake side of the piston to score. The heavi-
est scoring is on the piston skirt adjacent to
where the skirt closes off the intake port.
Intake scoring
Exhaust scoring
Goodheart-Willcox Publisher
Figure 9-63. When viewed from the top of the
piston, the points of exhaust and intake scoring
can be connected by lines that form a “Y,” indicat-
ing a raw gas failure.