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11 25 530 Removing and installing/replacing all pistons (N51)

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Special tools required: 

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: 

NOTE: In event of heavy oil carbon residue:

Carefully remove oil carbon residue from cylinder wall.

NOTE: Illustrations show N46.
Fig 1: Cylinder Walls
G04751123Courtesy of BMW OF NORTH AMERICA, INC.

Do not  release screw (1) of oil spray nozzle (2).

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

Readjust  if necessary.

Fig 2: Oil Spray Nozzle And Screw
G04751124Courtesy of BMW OF NORTH AMERICA, INC.

Release conrod bolts (1).

For tightening torque refer to 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.

Danger of engine damage! 

Fig 3: Conrod Bearing Cap And Bolts
G04751125Courtesy of BMW OF NORTH AMERICA, INC.

Attach special tool 11 8 330 to connecting rod.

Press out conrod and piston to cylinder head side.

IMPORTANT: Risk of damage  to oil spray nozzle.
Fig 4: Special Tool (11 8 330)
G04751126Courtesy of BMW OF NORTH AMERICA, INC.

Preliminary work: 

Clamp special tool 11 4 491 in vice.

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

Fig 5: Special Tool (11 4 491), Piston And Removal Direction
G04751127Courtesy of BMW OF NORTH AMERICA, INC.
WARNING: Protective goggles  must be worn for the next work step.
WARNING: Protective goggles  must be worn.

Lever out piston pin circlip with special tool 11 4 492 in direction of arrow.

Fig 6: Special Tool (11 4 492) And Removal Direction
G04739769Courtesy of BMW OF NORTH AMERICA, INC.

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.

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.

Fig 7: Locating Piston Pin
G04751129Courtesy of BMW OF NORTH AMERICA, INC.

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
G04739771Courtesy of BMW OF NORTH AMERICA, INC.

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
G04739772Courtesy of BMW OF NORTH AMERICA, INC.
WARNING: Protective goggles  must be worn.

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

Move piston pin circlip (2) into installation position.

Fig 10: Special Tool (11 4 493), Piston Circlip And Groove
G04751132Courtesy of BMW OF NORTH AMERICA, INC.
WARNING: Protective goggles  must be worn.

Guide lug and cutout on special tool 11 4 493 must point to piston crown. Only then can special tool 11 4 494 be correctly fitted.

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 illustration.
Fig 11: Special Tools (11 4 493) And (11 4 494)
G04751133Courtesy of BMW OF NORTH AMERICA, INC.
NOTE: For vehicles with B30  engines.

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.

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

Installation: 

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

Fig 12: Special Tools (11 8 330), (11 6 261), Piston And Connecting Rod
G04751134Courtesy of BMW OF NORTH AMERICA, INC.

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 in Figure

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

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

Fig 13: Special Tool (11 6 261), Piston, Arrow And Installation Direction
G04751136Courtesy of BMW OF NORTH AMERICA, INC.
IMPORTANT: Connecting rod and big end bearing cap are marked with identical pairing letters (1) and must not be mixed up.

Mixing them up or incorrectly fitting the big end bearing cap on the connecting rod will result in engine damage. 

Fig 14: Connecting Rod And Big End Bearing Cap Pairing Letters
G04751137Courtesy of BMW OF NORTH AMERICA, INC.

Apply a light coat of oil to crank pin.

Assemble conrod and crank pin.

Detach special tool 11 8 330.

Fig 15: Special Tool (11 8 330)
G04751138Courtesy of BMW OF NORTH AMERICA, INC.

Fit conrod bearing caps (2) so that pairing letters match up.

Install new conrod bolts (1).

For tightening torque refer to 11 24 1AZ in 11 24 CONNECTING RODS AND BEARINGS .

Fig 16: Conrod Bearing Caps And Bolts
G04751118Courtesy of BMW OF NORTH AMERICA, INC.

Adjust torsion angle of conrod with special tool 00 9 120 (see Figure).

For tightening torque refer to 11 24 1AZ in 11 24 CONNECTING RODS AND BEARINGS .

Fig 17: Special Tool (00 9 120)
G04751121Courtesy of BMW OF NORTH AMERICA, INC.

Assemble engine.

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