11 25 530 Removing and installing/replacing all pistons (N20, N26)

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Special tools required:

WARNING: Safety goggles must be worn when working on the piston pin circlip.

IMPORTANT: If pistons, connecting rods and bearing shells are reused, they must be reinstalled in the same places.
Piston and gudgeon pins are paired and must not be fitted individually.
Individual connecting rod replacement is not permitted; they are classified by weight class.
Connecting rods and connecting rod bearing caps are marked with the same pairing letters; mixing them up will result in engine damage.

Necessary preliminary tasks:

Remove ENGINE .
Mount engine on ASSEMBLY STAND .
Remove INTAKE PLENUM .
Remove CYLINDER HEAD .
Remove oil sump . See 11 13 000 REMOVING AND INSTALLING, SEALING OR REPLACING OIL SUMP (N20, N26) or 11 13 000 REMOVING AND FITTING, SEALING OR REPLACING THE OIL SUMP (N20 ALL-WHEEL DRIVE VEHICLE) .
Remove OIL PUMP .

NOTE: In event of heavy oil carbon residue:

Carefully remove oil carbon residue from cylinder wall.

NOTE: Graphic N46.

Fig 1: Cylinder Wall Oil Carbon Residue

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IMPORTANT: Do not release screw (1).

OIL SPRAY NOZZLE (2) must not be maladjusted or bent.

If necessary, readjust (risk of damage) .

Fig 2: Oil Spray Nozzle And Screw

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Release connecting rod bolts (1).

Tightening torque: 11 24 1AZ .

Remove connecting rod bearing cap (2).

IMPORTANT: Connecting rods and connecting rod bearing caps are marked with the same pairing letters; mixing them up will result in engine damage.

Fig 3: Connecting Rod Bearing Cap And Bolts

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Screw special tool 11 8 590 into big end of connecting rod.

Press out connecting rod and piston to cylinder head side.

IMPORTANT: Risk of damage to oil spray nozzle.

Fig 4: Special Tool (11 8 590)

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WARNING: Safety goggles must be worn for the next operation.

WARNING: Safety goggles must be worn.

To lever out piston circlip (1), support with special tool 11 4 492 on piston (2).

Lever out piston circlip (1) with special tool 11 4 492 in direction of arrow.

Fig 5: Turning Piston Circlip With Special Tool (11 4 492)

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IMPORTANT: Connecting rods are divided into weight classes and are only available as a set.
Old and new connecting rods must not be installed in mixed combinations.

Installation note:

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 6: Installing Gudgeon Pin

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Measure the 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.

Piston diameter at measuring point A.

Fig 7: Measuring Piston Diameter Using Micrometer

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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.

Permissible total wear clearance . See BORE and PISTON AND PIN .

Installation note:

If necessary, replace piston.

Fig 8: Checking Cylinder Bore Diameter

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Clamp special tool 11 5 341 in a vice.

NOTE: Piston and gudgeon pin are optimized to each other.

Screw in suitable special tool 11 5 342 with dia. 10.7 in special tool 11 5 341 .

Fig 9: Holding Special Tool (11 5 341) In Vice

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Secure piston (1) with connecting rod to special tool 11 5 341 .

Fig 10: Securing Piston With Connecting Rod With Special Tool (11 5 341)

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IMPORTANT: Safety goggles must be worn.
Lift out circlip and press out piston pin.

Fig 11: Removing Circlip

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WARNING: Safety goggles must be worn.

IMPORTANT: The opening of the piston pin circlip must be installed on the piston in the 6 o'clock position .
Risk of damage!

Fig 12: Inserting Piston Circlip Into Special Tool (11 8 562) Groove

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Insert piston circlip (2) into groove of special tool 11 8 562 .

Bring piston circlip (2) into working position (1).

WARNING: Safety goggles must be worn.

Slide on special tool 11 8 564 up to piston pin circlip (1)

Special tools 11 8 562 and 11 8 564 are prepared.

Fig 13: Special Tools (11 8 562 And 11 8 564)

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WARNING: Safety goggles must be worn.

Opening on special tool 11 8 562 must point to piston crown; only then can special tool 11 8 564 be fitted correctly.

If the special tools 11 8 562 and 11 8 564 are not positioned correctly, the piston pin circlip must be driven in using a plastic hammer in direction of arrow.

NOTE: See Fig 14.
Graphic with special tool 11 4 491

Fig 14: Positioning Special Tools (11 8 562) And (11 8 564)

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Install all PISTON RINGS .

Install all BEARING SHELLS .

Coat piston and piston rings with oil.

Pre-install piston (2) in special tool 11 8 141 .

Screw on special tool 11 8 590 in connecting rod (2).

Installation note:

Make sure protective lugs (1) on special tool 11 8 590 are positioned correctly and check for damage.

Fig 15: Installing Piston Rings

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Insert piston with connecting rod in cylinder.

IMPORTANT: Risk of damage to oil spray nozzle.
Risk of breakage of piston rings.
Only press pistons into place with finger force - do not knock in!

Insert piston so that arrow on piston crown points to camshaft drive.

Press in piston (1) with special tool 11 8 141 .

Fig 16: Piston

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IMPORTANT: Connecting rod and connecting rod bearing cap are marked with pairing letters (1) and must not be mixed up.
Swapping or incorrectly fitting the connecting rod bearing cap on the big end will result in engine damage .

Fig 17: Connecting Rod Bearing Cap Pairing Letters

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Apply a light coat of oil to crankshaft journals.

Attach connecting rod to crankshaft journal.

Screw off special tool 11 8 590 in anticlockwise direction.

NOTE: Graphic N54.

Fig 18: Special Tool (11 8 590)

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Fit bearing caps (2) so that pairing letters match up.

Install new connecting rod bolts (1).

Join connecting rod bolts (1).

Tightening torque: 11 24 1AZ .

Fig 19: Connecting Rod Bearing Cap And Bolts

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

Tightening torque: 11 24 1AZ .

Fig 20: Tightening Connecting Rod Bolts With Special Tool (00 9 120)

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Assemble engine.