11 25 530 Removing And Installing/Replacing All Pistons (N51)

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Special tools required:

For the following special tools, refer to MAINTENANCE AND GENERAL INFORMATION - SPECIAL TOOLS -- 128I .

00 9 120

For the following special tools, refer to ENGINE - SPECIAL TOOLS (N51) .

11 4 491
11 4 492
11 4 493
11 4 494
11 6 261
11 8 330

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

IMPORTANT: If pistons, conrods and bearing shells are reused, they must be reinstalled in the same places.
Individual conrod replacement is not permitted; they are classified according to weight categories.
Conrods and conrod bearing caps are denoted with the same pairing letters; mixing them up will result in engine damage.
Piston and piston pins are paired and must not be fitted individually.

Necessary preliminary tasks:

Remove engine . See 11 00 050 REMOVING AND INSTALLING ENGINE (N51) .
Mount engine on assembly stand . See MOUNTING ENGINE ON ASSEMBLY STAND (N51) .
Remove intake air manifold. See 11 61 050 REMOVING AND INSTALLING INTAKE AIR MANIFOLD (N51) .
Remove cylinder head . See 11 12 100 REMOVING AND INSTALLING/SEALING CYLINDER HEAD (N51) .
Remove engine oil sump . See 11 13 000 REMOVING AND INSTALLING, SEALING OR REPLACING OIL SUMP (N51) .
Remove oil pump . See 11 41 000 REMOVING AND INSTALLING OIL PUMP (N51) .

NOTE: In event of heavy oil carbon residue:

Carefully remove oil carbon residue from cylinder wall.

NOTE: Illustrations show N46.

Fig 1: Locating Carbon Deposit Area In Cylinder Wall

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Do not release screw (1).

Oil spray nozzle (2) must not be maladjusted or bent (risk of damage) .

Readjust if necessary.

Fig 2: Identifying Screw

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Release conrod bolts (1).

Tightening torque. See 11 24 1AZ in 11 24 CONNECTING RODS AND BEARINGS .

Remove conrod bearing cap (2) in direction of arrow.

IMPORTANT: Conrods and conrod bearing caps are denoted with the same pairing letters; mixing them up will result in engine damage.

Fig 3: Removing Conrod Bearing Cap

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Attach special tool 11 8 330 in conrod big end. See Fig 4.

Press out conrod and piston to cylinder head side.

IMPORTANT: Risk of damage to oil spray nozzle.

Fig 4: Identifying Special Tool (11 8 330)

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Preliminary work:

Clamp special tool 11 4 491in a vice. See Fig 5.

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

Fig 5: Securing Piston With Conrod To Special Tool

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WARNING: Protective goggles must be worn for the next work step.

WARNING: Protective goggles must be worn.

Lever out piston circlip with special tool 11 4 492 in direction of arrow. See Fig 6.

Fig 6: Removing Piston Circlip With Special Tool

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If necessary, replace connecting rods.

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

Fig 7: Identifying Connecting Rod

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Installation:

The piston pin must be able to be pressed through the liner by hand with little force and must not display any significant play.

Measure piston installation clearance:

Measure piston diameter with micrometer at measuring point A from bottom edge of piston and offset at 90° to the axis of the piston pin.

Piston diameter at measuring point A.

Fig 8: Measuring Piston Diameter With Micrometer

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

Diameter of cylinder bore.

Piston installation clearance.

Total permissible wear tolerance .

If necessary, replace piston.

Fig 9: Measuring Diameter Of Cylinder Bore

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WARNING: Protective goggles must be worn.

Insert piston circlip (2) into groove (1) of special tool 11 4 493. See Fig 10.

Bring piston circlip into assembly position.

Fig 10: Inserting Piston Circlip Into Groove Of Special Tool

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WARNING: Protective goggles must be worn.

Guide lug and cutout on special tool 11 4 493 must point to the piston crown; only then can special tool 11 4 494 be correctly fitted. See Fig 11.

Fig 11: Identifying Special Tools (11 4 493 And 11 4 494)

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When special tools 11 4 493 and 11 4 494 are correctly positioned, the piston pin circlip must be driven in with a plastic hammer in the direction of the arrow.

NOTE: See Fig 12 illustration.

NOTE: B 30 .

Install all piston rings .

Install all bearing shells .

Coat piston and piston rings with oil.

Pre-install piston (2) in special tool 11 6 261. See Fig 12.

Attach special tool 11 8 330 in conrod (2).

Installation:

Check protective lugs (1) on special tool 11 8 330 for correct position and damage.

Fig 12: Attaching Special Tool In Conrod

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Insert piston with conrod in cylinder.

IMPORTANT: Risk of damage to oil spray nozzle.
Danger of piston ring failure.
Press in piston with finger pressure only, do not drive in (see arrows).

Insert piston (1) so that arrow (2) on piston crown points to camshaft drive.

Press in piston (1) with special tool 11 6 261. See Fig 13.

Fig 13: Inserting Piston On Piston Crown Points

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IMPORTANT: Conrod and conrod bearing cap are identified with pairing letters (1) and must not be mixed up.
Mixing them up or incorrectly fitting the conrod bearing cap on the big end will result in engine damage .

Fig 14: Identification Of Conrod Bearing Cap