FAILURE ANALYSIS
83
8. Because the oil absorbs the abrasive particles,
the engine components that are immersed in oil
will show definite signs of abrasive ingestion
especially around the connecting rod and main
bearing journals.
See Figure 11.5.
NOTE: Abrasives that are trapped in the oil will
cause the lower portion of the combustion cham
-
ber to wearing more than the upper portion.
NOTE: Wear of only one bearing surface on a
new engine could be a sign of a manufacturing
defect.
NOTE: Abrasive particles can also be imbedded
into materials that are softer than the abrasive.
This will cause the affected part to act like a
piece of sand paper or a grinding wheel.
See Figure 11.6.
Figure 11.5
Figure 11.6
Imbedded abrasives
Insufficient lubrication
The bearing surfaces in an engine are not smooth.
As a result of the machining processes to make the
engine parts, there are little peaks and valleys that are
only visible on a microscopic scale. These peaks are
called asperities. As the engine breaks in, the asperi
-
ties break off leaving plateaus that become the bearing
surface. The valleys become reservoirs for the lubri
-
cant.
When an engine is properly lubricated, all of the
moving parts glide on a thin film of oil. If that film breaks
down or carries enough grit to bridge the film, damage
will occur.
1. When the parts are at rest, they push the lubri-
cant or oil away resting on the bearing surfaces.
As the parts rotate, they climb over the oil, pull
-
ing the oil between the bearing and the part,
riding on a film of oil.
The asperities are the first thing to make contact
between two moving engine parts with an insuffi
-
cient oil film between them. This creates friction
and causes a transfer of metal between the
parts. The heat and friction further breaks down
the oil film, accelerating the process.
2. Insufficient lubrication failures include:
• Low oil level
• Wrong oil for the application
• Contaminated oil
• Degraded oil (heat, age, acids)
3. Metal transfer is the primary indicator that the
film of oil between two engine parts has been
violated.
If the damage is localized, a general failure of
the lubrication system is probably not the cause.
As an example: a piston skirt shows metal trans-
fer to the cylinder wall. The connecting rod and
wristpin show some signs of excessive heat. The
main bearings and camshaft are not damaged.
This would indicate that the problem was proba
-
bly related to cylinder temperature.
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