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Here is an instruction show you guide on how to inspect piston for MTU 12-16V 4000 engine.

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Procedures:
The graphite layer on the piston skirt must not be mechanically removed or treated with strong cleaners!

Treat carbon deposits on piston crown and piston ring grooves with decarbonizer.
Remove remaining residues from piston crown and piston ring grooves by brushing.
Note: Use only a soft brush (e.g. perlon brush) to avoid damaging piston surface.
After cleaning, blow out oil chamber and oil bores with compressed air and check for particular cleanness.

Using the surface crack-testing method with fluorescent penetrant dye, check piston for cracks; replace if necessary.
Carefully inspect piston ring grooves for cracks.
Checking piston skirt and top land
Check piston skirt surface condition for wear, pitting and scoring.
The piston skirt wear pattern must cover most of the surface as the friction scores must be easily identifiable in this area.
Note: The friction scores on the circumference are artificial depressions which are filled with oil and result in improved lubrication.
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In the event of contact erosion on the piston edge,replace the complete piston.
Remove scoring or scuffing on top land of piston if local and not extending as far as first ring groove.
Replace piston if damage is more extensive, i.e. if scoring or pitting extends as far as the first annular groove (arrow).
In the event of extensive polishing wear on piston skirt and concentrated scoring, replace piston.
Replace pistons exhibiting even minor, localized scuffing on piston skirt.

Inspecting piston ring grooves
Check groove width with final measurements.
Note: Alternatively, a new piston ring (2) can be inserted into groove and axial clearance measured with feeler gauge (1).
If the limit values as per Tolerance and Wear Limits
List are exceeded, or measurements are below limit values for groove for oil control ring, replace piston.

Inspecting piston pin boss
Adjust bore gauge and measure piston pin bore.
If limit values as per Tolerance and Wear Limits List are exceeded, replace pistons.
Check sliding surfaces of bronze bushing for wear and scoring; replace piston as necessary.

Inspecting piston pin
Using magnetic crack-testing method and fluorescent magnetic powder, check piston pin for cracks;if cracks are detected, replace piston pin.
Remove minor traces of wear and scoring by polishing.
Replace piston pins exhibiting scuffing, indentations and severe wear.
Measure outside diameter with outside micrometer and determine pin clearance see Tolerance and
Wear Limits List.
If limit value is exceeded, replace piston pin.

Inspecting piston rings
Replace piston rings during W6 overhaul.
Clean piston rings of combustion residue if necessary.
Replace sharp-edged piston rings, piston rings with burring and chipping and piston rings with axial ring wear on ring lower edge.
Condition or wear of chrome layer (x) is a deciding factor when reusing or replacing rectangular ring (1),taper-face compression ring (2) and DSF ring (3).
Chrome layer must not show signs of scoring, cracks or heat discoloration.

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