7.2.2 Instructions and comments on endoscopic and visual examination of cylinder
liners
Terms used for endoscopic examination
Use the terms listed below to describe the condition of the cylinder-liner surface in the endoscopic ex-
amination report.
Findings
Measure
Light scoring
Minor dirt scores can occur during the assembly of a new engine (honing prod-
ucts, particles, broken-off burrs). Removed cylinders clearly exhibit such scoring
on the running surface under endoscope magnification. Cannot be felt with the
fingernail.
Findings not critical.
Single scores
Clearly visible scores caused by hard particles. They usually start in the TDC area
and cross through the honing pattern in the direction of stroke.
Findings not critical.
Scored area
These areas consist of scores of different lengths and depths next to one anoth-
er. In most cases, they are found at the 6-o'clock and 12-o'clock positions (inlet/
exhaust) along the transverse engine axis.
Findings not critical.
Smoothened area
Smoothened areas are changes to the running surface but almost the whole hon-
ing pattern is still visible. Smoothened areas appear brighter and more brilliant
than the surrounding running surface.
Findings not critical.
Polished area
Polished areas are on the running surface and show local removal of the honing
pattern. Grooves from the honing process are not visible any more.
New cylinder liners must be fitted in the following cases
Polished area:
covers more than 20% of the entire piston running surface,
covers more than 30° of the circumference and extends over more than 50% of
piston stroke,
is wider than 15 mm over the entire piston stroke.
Discoloration
This is caused by oxidation (surface discoloration through oil or fuel) and temper-
ature differences around the liner. It appears rather darker within the honed
structure in contrast to the bright metallic running surface. The honing pattern is
undisturbed. Discolorations extend in stroke direction and may be interrupted.
Findings not critical.
Corrosion fields / spots Corrosion fields / spots result from water (condensed water) with the valves in
the overlap (open) position. They are clearly visible due to the dark color of the
honing groove bottom.
This corrosion is not critical unless there is corrosion pitting.
Black lines
Black lines are a step towards heat discoloration. They are visible as a clear dis-
coloration from TDC to BDC in the running surface and the start of localized dam-
age to the honing pattern.
Cylinders with a number of black lines around the running surface have limited
service life and should be replaced.
Discolorations (Heat)
These are caused by a disturbance in the liner / ring tribosystem. Usually they
run over the whole ring-travel area (TDC/BDC), starting at the first TDC-ring and
becoming more visible from the second TDC-ring onwards and less pronounced
from TDC-ring 1. The honing pattern is usually no longer visible and displays a
clearly defined (straight) edge to the undisturbed surface. The damaged surface is
usually discolored. The circumferential length varies.
66 | Task Description | MS150026/02E 2013-11
TIM-ID: 0000003285 - 003
