11 25 530 Removing and installing/replacing all pistons (N63) - from 3/2011

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Special tools required:

WARNING: Safety goggles must be worn when performing repair work on the piston pin circlip.

IMPORTANT: If pistons and connecting rods are reused, they must be reinstalled in the same places.
Pistons and gudgeon pins are matched to each other and can only be replaced as a pair.
Connecting rod and connecting rod bearing cap are cracked.
Identification is effected by means of identical pairing letters on the connecting rod big end.
Mixing up the components will result in engine damage.
Connecting rod bearings must always be replaced.

Necessary preliminary tasks:

Fully remove front axle together with engine.
Remove engine from front axle. See applicable procedure for drivetrain configuration: 11 00 598 REMOVING AND INSTALLING ENGINE ON FRONT AXLE (N63O1) WITH GA8HP70Z TRANSMISSION , 11 00 598 REMOVING AND INSTALLING ENGINE ON FRONT AXLE (N63O1) WITH GA8HP70Z ALL-WHEEL DRIVE VEHICLE TRANSMISSION , 11 00 598 REMOVING AND INSTALLING ENGINE ON FRONT AXLE (N63O1) WITH GA6HP26Z ALL-WHEEL DRIVE VEHICLE TRANSMISSION or 11 00 598 REMOVING AND INSTALLING ENGINE ON FRONT AXLE (N63O1) WITH MANUAL GEARBOX .
Mount engine on assembly stand. See 11 00 MOUNTING ENGINE ON ASSEMBLY STAND (N63) .
Remove both cylinder heads.
Remove oil sump . See 11 13 020 REMOVING AND INSTALLING/REPLACING LOWER OIL SUMP SECTION (N63) and 11 13 010 REMOVING AND INSTALLING/REPLACING UPPER OIL SUMP SECTION (N63) .
Remove OIL PUMP .

To release the connecting rod bolts (1), the crankshaft (2) must be moved to a suitable position.

Release connecting rod bolts (1).

Fig 1: Identifying Crankshaft, Connecting Rod Bolts, And Pairing Letters

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Screw the special tool 2 249 164 into the connecting rod.

Fig 2: Screwing Special Tool 2 249 164 Into Connecting Rod

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NOTE: For installation and dismantling of the connecting rods, it is essential for the crankshaft to be exactly in alignment with the cylinder bore.

Position crankshaft at central bolt.

Press out the connecting rod and piston with special tool 2 249 164 towards the cylinder head side.

Fig 3: Pressing Out Connecting Rod And Piston Toward Cylinder Head Side

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IMPORTANT: Pistons and gudgeon pins are paired and must not be fitted individually.

Lift out circlip and press out piston pin.

Fig 4: Removing Circlip From Piston

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If necessary, replace connecting rods.

NOTE: The connecting rod can also be replaced individually.
The gudgeon pin must be able to be pressed through the bush by hand with little force and must not display any significant play.

Fig 5: Pressing Gudgeon Pin Through Bush

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Prior to installation, measure piston installation clearance: Measure piston diameter with micrometer at measuring point A from lower edge of piston and offset at 90° to the axis of the gudgeon pin.

MEASURING POINT A .

Fig 6: Measuring Piston Installation Clearance

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Adjust micrometer to cylinder bore of engine block. Set internal measuring device on micrometer to zero. Measure bottom, center and top of cylinder bore in direction of travel and direction of rotation.

DIAMETER OF CYLINDER BORE .

PISTON INSTALLATION CLEARANCE .

TOTAL PERMISSIBLE WEAR CLEARANCE .

Fig 7: Measuring Cylinder Bore Diameter

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Installation note:

Piston circlip (1) is correctly installed when the opening points upwards.

See Fig 8.

It must still be possible for the gudgeon pin to be moved easily.

Fig 8: Identifying Correct Installation Position Of Piston Circlip

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Insert the special tools 2 249 164 into the connecting rod.

Fig 9: Inserting Special Tools Onto Connecting Rod

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INSTALL CONNECTING ROD BEARING .

Lightly coat pistons and piston rings with oil.

Offset the contact points of the piston rings by approx. 120° to each other but do not position above the gudgeon pin boss.

NOTE: Illustration S85.

Fig 10: Identifying 120° Offset Of Piston Ring Contact Points

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Keep piston rings compressed with special tool 11 7 280 .

Fig 11: Compressing Piston Rings Using Special Tool (11 7 280)

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Install piston so that arrow points to camshaft drive.

IMPORTANT: Risk of breakage of piston rings.

Press in piston only with finger force (do not knock in!).

Fig 12: Installing Piston Using Minimal Force

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The direction arrow on the piston crown must point to the camshaft drive (direction of travel towards front).

Connect the crankshaft journal and connecting rod.

Fig 13: Removing/Installing Connecting Rod And Piston

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Remove the special tools 2 249 164 .

NOTE: For purposes of clarity, the pairing letters (1) are shown on the removed connecting rod.

IMPORTANT: Connecting rods and connecting rod bearing caps are denoted with the same pairing letters (1), do not mix them up.
Fit bearing cap so that pairing letters match up.

Fig 14: Identifying Connecting Rods Pairing Letters

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Apply light coat of oil to connecting rod bearing shells.

Fit the bearing cap so that the pairing letters (3) match up.

Install the connecting rod bolts (1).

Fig 15: Identifying Connecting Rod Bolts

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Secure connecting rod bolts (1) with special tool 00 9 120 .

Tightening torque 11 24 1 AZ .

Fig 16: Tightening Connecting Rod Bolts Using Special Tool (00 9 120)

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