11 25 530 Removing and installing/replacing all pistons (N55)

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Special tools required:

WARNING: Safety goggles must be worn when working on the gudgeon 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 work:

Remove ENGINE .
Mount engine on ASSEMBLY STAND .
Remove INTAKE PLENUM .
Remove CYLINDER HEAD .
Remove oil sump. See REMOVING OIL SUMP (RWD) or REMOVING OIL SUMP (AWD) .
Remove OIL PUMP .

NOTE: In event of heavy oil carbon residue:

Carefully remove oil carbon residue from cylinder wall.

NOTE: Illustration shows N46.

Fig 1: Locating 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: Identifying 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).

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

Fig 3: Identifying 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.

WARNING: Risk of damage to oil spray nozzle.

Fig 4: Removing Connecting Rod And Piston Cylinder Head

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Preliminary work:

Secure special tool 11 4 491 in vice.

Secure piston (1) with connecting rod to special tool 11 4 491 .

NOTE: Illustration shows N52.

Fig 5: Securing Piston With Connecting Rod And Special Tool (11 4 491)

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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 6: Removing Piston Circlip Using 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 7: Pressing Connecting Rod Gudgeon Pin

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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.

Piston diameter at measuring point A.

Fig 8: 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.

For diameter of cylinder bore, piston installation clearance, and permissible total wear clearance: see ENGINE BLOCK, CYLINDER CRANKCASE and PISTONS WITH RINGS/PINS .

Installation note:

Fig 9: Checking Cylinder Bore Diameter

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If necessary, replace piston.

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 10: 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 installation 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 11: Sliding Special Tool (11 8 564) Onto Piston Pin Circlip

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WARNING: Safety goggles must be worn.

The aperture 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 illustration.

Fig 12: Inserting Piston Pin Circlip Using Plastic Hammer

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Piston pin circlip is correctly installed when opening (1) points downwards.

Fig 13: Identifying Piston Opening

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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 14: Installing Special Tool (11 8 590) With Connecting Rod

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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.
Press in piston in direction of arrow with finger pressure only, do not drive in.

Fig 15: Pressing Piston Using Special Tool (11 8 141)

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

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

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

Fig 16: Identifying Connecting Rod And Bearing Cap Pairing Letters