11 25 530 Removing and installing/replacing all pistons (N74)

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

IMPORTANT: If pistons and connecting rods are reused, they must be reinstalled in the same places.

Piston and piston pin are matched to each other and can only be replaced as a pair.

Conrod and conrod 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.

Conrod bearings must always be replaced.

Fig 1: Locating Stamped Dates On Connecting Rod Bearing Cap

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Necessary preliminary tasks:

Remove engine
Mount engine on assembly stand.
Remove both cylinder heads.
Remove oil sump.
Remove oil pump.

Unscrew connecting rod bearing cap.

NOTE: Connecting rods and connecting rod bearing caps are denoted with the same pairing letters.

The stamped dates are always arranged in opposite directions.

Set down conrod bearing caps in order.

NOTE: To install and remove the conrods, it is essential for the crankshaft to be exactly in alignment with the cylinder bore (see dashed line in illustration).

Position crankshaft at central bolt.

Insert special tool 11 8 152 into conrod.

Screw special tool 11 5 440 into conrod with bolt (1).

Remove conrod with piston from cylinder head side.

Fig 2: Removing Conrod Using Special Tool 11 5 440 And 11 8 152

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IMPORTANT: Piston and piston bolts are paired and must not be fitted individually.

Lift out retaining ring and press out piston pin.

Fig 3: Lifting Out Retaining Ring

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If necessary, replace connecting rods.

NOTE: The conrods can also be replaced individually.

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

Fig 4: Pressing Gudgeon Pin

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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 by 90° to piston pin axis.

Measuring point A.

Fig 5: Measuring Piston Diameter Using Micrometer At Measuring Point A

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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.

Fig 6: Measuring Piston Installation Clearance Using Micrometer

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Diameter of cylinder bore.

Piston installation clearance.

Total permissible wear tolerance.

IMPORTANT: The conrods of cylinder banks 1 to 4 and 5 to 8 are mounted to the pistons differently.

NOTE: The pistons and connecting rods of cylinders 1 to 12 are identical.

Arrow on piston crown (cyl. 1 to 6) points upwards, screw connection on connecting rod points at an angle to right.

Arrow on piston crown (cyl. 7 to 12) points upwards, screw connection on connecting rod points at an angle to left.

NOTE: For purposes of clarity, pistons are shown removed.

Fig 7: Identifying Piston Crown And Connecting Rods

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The conrods are correctly mounted on the piston when the bolt connections on the conrods are parallel to each other (see arrow in illustration).

Fig 8: Identifying Connecting Rods

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Install retaining ring.

Fig 9: Installing Retaining Ring

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Installation:

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

See illustration.

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

Fig 10: Identifying Piston Circlip

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Insert special tools 11 4 440 and 11 8 152 into connecting rods.

Fig 11: Inserting Special Tools 11 4 440 And 11 8 152 Into Connecting Rods

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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 piston pin boss.

NOTE: Picture shows S85.

Fig 12: Identifying Piston Rings Contact Points Offset Angle

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Keep piston rods compressed with special tool 11 7 280 .

Install piston so that arrow points to camshaft drive.

IMPORTANT: Danger of piston ring failure.

Fig 13: Compressing Piston Rods Using Special Tool 11 7 280

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IMPORTANT: Danger of piston ring failure.

Press in piston (1) only with finger force (do not knock in!).

Fig 14: Pressing Piston With Finger Force

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

Fig 15: Identifying Direction Arrow On Piston Crown

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Attach crankshaft journal to connecting rod. Remove special tools 11 5 440 and 11 8 152 .

Fig 16: Removing Special Tool 11 5 440 And 11 8 152

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

IMPORTANT: Conrods and conrod 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 17: Locating Conrod Bolts

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Apply light coat of oil to connecting-rod bearing shells.

Fit bearing cap so that pairing letters match up.

Install new conrod bolts.

Join connecting rod bolt (1).

Tightening torque: see 1AZ in 11 24 CONNECTING RODS AND BEARINGS

Installation:

Check entire angle adjustment.

For better overview of screw connection quality.

After joining, use an oil-resistant marker to apply a stroke (1) on the bolt head (see illustration).

Fig 18: Identifying Oil-Resistant Marker Stroke On Bolt Head

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

Tightening torque: see 1AZ in 11 24 CONNECTING RODS AND BEARINGS

Fig 19: Tightening Connecting Rod Bolts Using Special Tool 00 9 120

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Assemble engine.