Cub Cadet MTD P90 Series - Manual (Page 115)

Failure Analysis
109
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 lubri-
cation system is probably not the cause.
As an example: a piston skirt shows metal transfer 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 probably related to cylinder temperature.
The hall mark of a lubrication failure is the presence of
discoloration and/or metal transfer on all friction surfaces
within the engine. See Figure 11.7.
An important thing to note is that just because there are
signs of insufficient lubrication, that does not mean that
was the cause of the failure. It may only be a symptom of
the real cause of the failure.
Larger size abrasive particles can render the lubricants
ineffective, leading to an engine failure. An overheated
engine can cause the oil to break down leading to a failure.
In an engine overspeed, the oil is pushed away from the
bearing surface leading to a failure.
In all three of the above cases, the signs of insufficient
lubrication are symptoms not the cause. There will also be
signs of heat or discoloration around the parts affected by
the lack of lubrication. See Figure 11.8.
Figure 11.7
Metal to metal
transfer
Discoloration
Figure 11.8
Discoloration