Piston: Service and Repair
11 25 530 - Removing and installing/replacing all pistons (N51)
Special tools required:
- 00 9 120 00 9 120 Torque Angle Measuring Dial
- 11 4 491
- 11 4 492
- 11 4 493
- 11 4 494
- 11 6 261
- 11 8 330 11 8 330 Insertion Tool
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 Removing and Installing Engine.
- Mount engine on assembly stand 11 00 ... Mounting Engine on Assembly Stand.
- Remove intake air manifold.
- Remove cylinder head Removal and Replacement.
- Remove engine oil sump Service and Repair.
- Remove oil pump 11 41 000 Removing and Installing/Replacing Oil Pump.
Note:
In event of heavy oil carbon residue:
Carefully remove oil carbon residue from cylinder wall.
Note:
Illustrations show N46.
Do not release screw (1).
Oil spray nozzle (2) must not be maladjusted or bent (risk of damage).
Readjust Service and Repair if necessary.
Release conrod bolts (1).
Tightening torque, 11 24 1AZ. Mechanical Specifications
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.
Attach special tool 11 8 330 11 8 330 Insertion Tool in conrod big end.
Press out conrod and piston to cylinder head side.
Important!
Risk of damage to oil spray nozzle.
Preliminary work:
Clamp special tool 11 4 491 in a vice.
Secure piston (1) with conrod to special tool 11 4 491.
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.
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 Note:
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.
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.
Warning!
Protective goggles must be worn.
Insert piston circlip (2) into groove (1) of special tool 11 4 493.
Bring piston circlip into assembly position.
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.
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.
Note:
B 30.
Install all piston rings Service and Repair.
Install all bearing shells Service and Repair.
Coat piston and piston rings with oil.
Pre-install piston (2) in special tool 11 6 261 11 6 260 Mounting Sleeves with Case.
Attach special tool 11 8 330 11 8 330 Insertion Tool in conrod (2).
Installation Note:
Check protective lugs (1) on special tool 11 8 330 11 8 330 Insertion Tool for correct position and damage.
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 11 6 260 Mounting Sleeves with Case.
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.
Apply a light coat of oil to crank pin.
Assemble conrod and crank pin.
Detach special tool 11 8 330 11 8 330 Insertion Tool.
Fit bearing caps (2) so that pairing letters match up.
Install new conrod bolts (1).
Tightening torque: 11 24 1AZ. Mechanical Specifications
Adjust torsion angle of conrod with special tool 00 9 120 (see illustration).
Tightening torque: 11 24 1AZ. Mechanical Specifications
Assemble engine.