206
Two-Stroke Engines
Copyright Goodheart-Willcox Co., Inc.
has smeared. Excessive heat on the piston sur-
face can come from a lean running engine, raw
gas used as a fuel, or overheating of the cylinder.
Roll the piston down below the exhaust
port to see the intake side of the cylinder. In
Figure 9-52, the cylinder looks smooth and
shiny. Very fine abrasives will cause this surface
to look smooth but dull. Large abrasives will
cause visible scratches running up and down
the cylinder wall. A raw gas failure will cause
the cylinder wall and piston to heat up from a
lack of lubrication. Piston surface material will
begin to melt and adhere to the cylinder wall
in the form of score marks. An example of this
adhesion smearing is shown in Figure 9-53.
Any evidence of abrasive ingestion during this
visual analysis of the inside of the engine means
that, in all likelihood, the rings are worn and
will need to be replaced. A worn piston skirt
indicates the piston, and maybe the cylinder,
will need to be replaced. Scoring of the piston
means that the piston and cylinder will need to
be replaced.
No visible
machine marks
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Figure 9-50. The surface of the piston side looks
smooth. The original machine marks have been
worn off by abrasive ingestion.
Smeared aluminum
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Figure 9-51. Excessive heat from the cylinder
overheating, a lean running engine, or using raw
gas as a fuel in a two-stroke engine can cause the
surface temperature of a piston to reach melting
point and begin smearing.
Intake side
of cylinder
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Figure 9-52. An undamaged cylinder will be
smooth and shiny. A dull, scratchy, or scored
cylinder indicates an engine that was not running
correctly.
Scoring
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Figure 9-53. An overheated piston surface
reaches its melting point and transfers aluminum
to the cylinder wall, causing the scoring. Using
raw gasoline (no two-stroke oil premixed) results
in a lack of lubrication on the intake side of the
cylinder.