Inspecting Cylinders With Inspection Lamp (Borescope)

PRECEDING WORK
Removing and installing spark plugs.

OVERVIEW
Front engine compartment panel at bottom (step 1):
Take off
Fit on.
Respective piston (step 2):
Set to BDC (socket wrench 001 589 65 09 00).
Cable mounting at engine components partition (step 3):
Unbolt, push forward when testing cylinders 6 and 12,
Bolt on.
Motoskop and lens probe (step 4):
Connect
Disconnect.
Cylinder contact surface (step 5):
Examine.

PROCEDURE
1 Take off front engine compartment panel at bottom.





2 Turn piston of the cylinder to be examined into BDC position with socket wrench 001 589 65 09 00. When performing this step, the marking numbers on the vibration damper must be aligned with the TDC pointer (see below).





NOTE:
The markings on the vibration damper at 60, 180 and 300° have been fitted additionally as of July 1992 (phased-in). If the engine is not provided with these additional markings, determine the respective BDC position by means of the spark plug hole.





3 Model 140: Unbolt cable mounting at engine components partition (arrows) and push forward when examining cylinders 6 and 12.





4 Connect Motoskop lamp and lens probe. Motoskop TW 12 V to vehicle battery. Motoskop TW 220 V to mains supply.
5 Examine cylinder contact surface.

ASSESSMENT
The cylinder contact surfaces are electrolytically treated after boring and honing. This causes the aluminum to be set back to the extent that the piston rings and the chromed or iron-coated light alloy pistons are in contact only with the approximately 0.02 mm to 0.05 mm large silicone crystals, which produce a wear-resistant contact surface. No honing traces are visible on these cylinder contact surfaces. When assessing cylinder walls with striations or streaks, it is often difficult for the workshop to decide whether major damage already exists, necessitating engine repairs, or whether the markings are of no significance. The following information is intended to assist you in coming to a professional and proper decision regarding the condition and continued use of the crankcase. If it is not possible to assess this with a reasonable degree of certainty when performing a visual inspection with the inspection lamp, the cylinder head must be removed.

NOTES:
1. Normal state
Mat grey surface, no honing pattern
2. Glossy pressure points, smooth areas
Individual bright points, e.g. in the middle of the cylinder or in the area of the cylinder head bolt ribs. Crankcase can continue to be used.
3. Brown discoloration on cylinder surfaces
Oil varnish is present over extensive areas of the cylinder wall (normal above the piston ring zone) and indicates that the engine has been driven within a very high temperature level. Crankcase can continue to be used.
4. Visual streaks, traces of rubbing
Starting from the first piston ring and tapering out after about 30 mm. Traces of dry rubbing which cannot be felt caused by fuel ablution of the oil film, e.g. as a result of frequent cold starting when driving short distances. These traces of rubbing which tend to occur in the area of the cylinder head bolts and on the delivery side do not represent a problem if they are smoothed. The piston rings are not damaged. Crankcase can still be used.
5. Ring-shaped impressions
Visible in the top and bottom reversal area of the piston rings, are not a problem. Crankcase can continued to be used.
6. Individual continuous scratches
Caused by impurities, e.g. as a result of back pulsation of soot particles from the exhaust. Crankcase can continued to be used.
7. Roughened streaks, friction seizures
Starting from the first and second piston ring and not tapering out until bottom part of cylinder. Traces of rubbing as described in item 4, progress to form friction seizures. Piston rings may be damaged. If can be felt, cylinder wall unusable.
8. Piston seizures
Cylinder contact surface is usually felt to be rough over the entire length. Material wear and traces of rubbing can be felt on cylinder contact surface and piston skirt. Cylinder wall unusable.
9. Bright, reflective, polished cylinder surfaces all round
Usually starting from the first piston ring, in the upper area of the cylinder, with a measurable partial cylinder wear. Cylinder wall unusable.

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