11 25 000 Removing and installing or replacing all pistons (B36A/B38A)

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Special tools required:

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

IMPORTANT:

The piston diameter depends on the engine version!

B38 AU12 Lower performance class for piston diameter 78 mm
B38 AM15 Medium performance class for piston diameter 82 mm

Necessary preliminary tasks:

Remove ENGINE with transmission
Remove transmission. See REMOVING AUTOMATIC TRANSMISSION or REMOVING MANUAL TRANSMISSION .
Mount engine on ASSEMBLY STAND
Remove CYLINDER HEAD .
Remove OIL/VACUUM PUMP

IMPORTANT:

If pistons, connecting rods and connecting rod bearing shells are reused, they must be reinstalled in the same places.

Individual connecting rod replacement is not permitted; they are classified by weight class.

Connecting rods and connecting rod bearing caps are denoted with the same pairing letters. Mixing up the components will result in engine damage.

Piston and gudgeon pins are paired and must not be fitted individually or confused.

Removal:

Release screws (1).

Remove reinforcement (2).

Fig 1: Screws And Reinforcement

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IMPORTANT:

Position crankshaft (1) vertically.

Proceed with care at the cylinder wall and oil spray nozzles (risk of damage)

Fig 2: Positioning Crankshaft

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Undo the connecting rod bolt (1).

Remove connecting rod bearing caps (2) and set down in neat order.

Fig 3: Connecting Rod Bolt And Bearing Caps

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Remove connecting rod bearings (1) and set down in neat order.

Fig 4: Connecting Rod Bearings

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Screw special tool 11 8 860 in at large connecting rod eye.

Fig 5: Screwing Special Tool (11 8 860) In Large Connecting Rod Eye

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Remove piston (1) with connecting rod.

Fig 6: Locating Piston With Connecting Rod

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NOTE: Fixtures (1) for special tool 2 360 584 .

Fig 7: Locating Fixtures For Special Tool (2 360 584)

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Prepare special tool 2 360 584 with fixtures (1).

NOTE: Ø for fixtures (1) = 12.5 mm.

Fig 8: Preparing Special Tool (2 360 584) With Fixtures

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

Position piston (1) with connecting rod (2) on special tool 2 360 584 .

Fig 9: Positioning Piston With Connecting Rod On Special Tool (2 360 584)

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WARNING: Safety goggles must be worn.

Lift off gudgeon pin circlip (1) from piston over groove (2) using special tool 11 4 492 .

IMPORTANT:

Piston and gudgeon pin are matched to each other.

They must not be mixed up.

Fig 10: Lifting Gudgeon Pin Circlip From Piston Over Groove Using Special Tool (11 4 492)

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Pay attention to installation position of piston and connecting rod.

Piston and connecting rod must not be installed with a rotational misalignment of 180 degree to each other.

Check if the gudgeon pin (1) has noticeable play in the piston.

Check whether gudgeon pin (1) can be removed without excessive force.

Fig 11: Checking Gudgeon Pin

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Result:

Gudgeon pin (1) is difficult to remove or gudgeon pin (1) has noticeable play.

Fig 12: Checking Gudgeon Pin

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Measure:

Replace the piston (2) and gudgeon pin (1).

Remove gudgeon pin (1) from piston (2) and connecting rod (3) in the direction of the arrow.

The gudgeon pin length must be determined in order to replace the piston:

Engine	Gudgeon pin length	Part number
B38 UL	Without measurement	8 618 505
B38 ML	55.00 mm	8 638 448
B38 ML	51.00 mm	8 657 760
B38 TOP	55.00 mm	8 618 504

Fig 13: Measuring Gudgeon Pin Length

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Installation:

Before installation, measure piston installation clearance. Measure piston diameter with micrometer screw at measuring point A from lower edge of piston and offset at 90° to the axis of the gudgeon pin.

Distance A: refer to technical data for PISTONS WITH RINGS AND PINS - END CLEARANCE, SIDE CLEARANCE .

Fig 14: Measuring Piston Installation Clearance

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Adjust micrometer screw to cylinder bore of engine block. Set internal measuring device at micrometer screw to zero. Measure bottom, center and top of cylinder bore in direction of travel and direction of rotation.

Diameter of the cylinder bore,

Piston installation clearance,

Permitted total wear clearance,

For diameter of the cylinder bore, piston installation clearance, and permitted total wear clearance: refer to TECHNICAL DATA .

Clean and oil connecting rod bearing.

If necessary, replace CONNECTING ROD BEARING .

IMPORTANT:

Pay attention to installation position of piston and connecting rod.

Piston and connecting rod must not be installed with a rotational misalignment of 180 degree to each other.

NOTE: It must be possible to the press gudgeon pin (1) through the connecting rod (small connecting rod eye) with little force; it must not reveal any noticeable play.

Position piston (2) on connecting rod (3).

Install gudgeon pin (1) in piston (2) and connecting rod (3).

Fig 15: Installing Gudgeon Pin In Piston And Connecting Rod

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

Position connecting rod (2) in piston (1).

Position piston (1) with connecting rod on special tool 2 360 584 .

Fig 16: Positioning Piston With Connecting Rod On Special Tool (2 360 584)

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Pre-tension gudgeon pin circlip (1) in special tool 2 360 584 .

Fig 17: Pre-Tensioning Gudgeon Pin Circlip In Special Tool (2 360 584)

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NOTE: Make sure the tool is positioned correctly.

Fig 18: Special Tool (2 360 584)

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IMPORTANT: Insert the circlip (1) in a way that the opening (2) points towards the piston crown.

Fig 19: Inserting Circlip With Opening Toward Piston Crown

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IMPORTANT:

Drive in gudgeon pin circlip in direction of arrow.

Risk of damage on piston and gudgeon pin!

Drive in gudgeon pin circlip in the direction of the arrow by means of slight hammer blows .

Fig 20: Driving-In Gudgeon Pin Circlip

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Align contact points of PISTON RINGS (offset at approx. 120°, but do not arrange over gudgeon pin boss).

Coat piston (1) and piston rings with oil.

Prepare piston (1) with piston rings on special tool 11 6 240 .

Screw special tool 11 8 860 on at large connecting rod eye.

NOTE: Make sure CONNECTING ROD BEARING (2) is in proper installation position.

Fig 21: Preparing Piston With Piston Rings On Special Tool (11 6 240)

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IMPORTANT:

Oil spray nozzles (2) must not be maladjusted or bent.

A bent oil spray nozzle cannot be adjusted and must be replaced.

Release screw (1) and remove oil spray nozzle (2).

Fig 22: Locating Oil Spray Nozzle And Screw

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NOTE: Pay attention to the protective edge for the cylinder wall.

Insert piston so that the arrow points in direction of cylinder 1.

Position piston with connecting rod in special tool 11 6 240 and insert into cylinder.

Fig 23: Positioning Piston With Connecting Rod In Special Tool (11 6 240)

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IMPORTANT:

Risk of piston ring breakage .

Risk of damage to oil spray nozzle.

Only press pistons in with finger force - do not knock in!

Slide connecting rod eye onto crankshaft.

Remove special tool 11 8 860 from large connecting rod eye.

Fig 24: Screwing Special Tool (11 8 860) In Large Connecting Rod Eye

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Insert CONNECTING ROD BEARING SHELL (1) into connecting rod bearing cap (2).

Fig 25: Inserting Connecting Rod Bearing Shell Into Connecting Rod Bearing Cap

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Clean and oil connecting rod bearing.

If necessary, replace CONNECTING ROD BEARING .

Mount connecting rod bearing caps so that pairing letters (1) match.

Fig 26: Locating Connecting Rod Bearing Caps Pairing Letters

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Parts: Replace connecting rod bolt (1).

Tighten connecting rod bolts (1).

Tightening torque: 11 24 1AZ .

Fig 27: Connecting Rod Bolt And Bearing Caps

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Parts:

Replace reinforcement (2).
Replace screws (1).

Install reinforcement (2).

Tighten down screws (1).

Tightening torque: 11 41 3AZ .

Fig 28: Screws And Reinforcement

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Required follow-up work:

Check engine function and check for tightness