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

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 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: Graphic shows N46.

Fig 1: Locating Oil Carbon Residue On Cylinder Wall

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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).

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: 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.

IMPORTANT: Risk of damage to oil spray nozzle.

Fig 4: Installing Special Tool (11 8 590) Into Connecting Rod Big End

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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: Removing 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.

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 tolerance, see ENGINE BLOCK/CYLINDER CRANKCASE and PISTONS WITH RINGS/PINS .

Fig 7: Measuring Piston Installation Clearance

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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 To 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 Piston Pin 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!

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

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

Fig 12: Inserting Piston Circlip Into Special Tool Groove

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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: Identifying 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 illustration with special tool 11 4 491: Fig 14.

Fig 14: Installing Piston Pin Circlip Using Special Tools

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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: Identifying Piston And Special Tools

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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: Inserting Piston With Arrow On Piston Crown Pointing Toward Camshaft Drive

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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: Identifying 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 counter-clockwise direction.

NOTE: Graphic shows N54.

Fig 18: Installing Special Tool (11 8 590) Into Connecting Rod Big End

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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: Identifying 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 Using Special Tool (00 9 120)

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Assemble engine.