Piston: Service and Repair

11 25 530 - Removing and installing/replacing all pistons (N73)





Special tools required:
- 00 9 120 00 9 120 Torque Angle Measuring Dial
- 11 7 280 11 7 280 Piston Ring Pliers
- 11 8 151 11 8 150 Insertion Tool
- 11 8 152 11 8 150 Insertion Tool





Necessary preliminary tasks:
- Remove engine Removing and Installing Engine.
- Remove left cylinder head Removing and Installing Left Cylinder Head (N73).
- Remove right cylinder head Removing and Installing Right Cylinder Head (N73).
- Remove oil pump Service and Repair.
- Remove upper oil sump Removing and Installing/Replacing Oil Sump Top Section (N73).





Important!
Re-install piston, connecting rod and bearing shells back in the same position and in the same installation location.
Conrods and conrod bearing caps are denoted with the same pairing letters.
Do not mix them up as this will cause engine damage.





Unscrew conrod bearing cover.
Set down conrod bearing caps in order.

Note:
All conrods are cracked.
Do not mix up conrods and conrod bearing caps, observe pairing letters.





Fit special tools 11 8 151 11 8 150 Insertion Tool and 11 8 152 11 8 150 Insertion Tool.
Remove conrod with piston from cylinder head side.

Important!
Do not damage cylinder wall when pulling out conrod with piston.





When removing and installing pistons with conrods, make sure piston cooling spray nozzles (1) are not bent.
Risk of damage.





Lift out retaining ring and press out piston pin.

Important!
Piston and piston pins are paired and must not be fitted individually.





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

Installation:
The conrods can also be replaced individually.





Prior to installation, 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.
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.





Due to the design of the piston head and the piston pin offset, the pistons are constructed specifically for each cylinder bank and must not be mixed up.

Important!
Risk of damage.





Cylinder bank 1 to 6.
Fit conrod with piston pin to piston in such a way that arrow on piston points to front and flat end of conrod points to rear.

Important!
Conrods for cylinders 1 to 6 and 7 to 12 must be fitted differently to the pistons.





Cylinder bank 7 to 12.
Fit conrod with piston pin to piston in such a way that arrow on piston points to front and flat end of conrod points to front.
Install conrod bearing.





Install retaining ring.





Insert special tools 11 8 151 11 8 150 Insertion Tool and 11 8 152 11 8 150 Insertion Tool in conrod.
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 boss.
Press together piston rings using special tool 11 7 280 11 7 280 Piston Ring Pliers.

Important!
Danger of piston ring failure.





Keep piston rings pressed with special tool 11 7 280 11 7 280 Piston Ring Pliers.
Install pistons with arrows pointing towards camshaft drive.

Important!
Danger of piston ring failure.
Only press pistons into place with finger force - do not knock in!





Fit conrod with special tool 11 8 151 11 8 150 Insertion Tool and 11 8 152 11 8 150 Insertion Tool on crank pin.
Remove special tools 11 8 151 11 8 150 Insertion Tool and 11 8 152 11 8 150 Insertion Tool.





Important!
Conrods and conrod bearing caps are denoted with the same pairing letters (1), do not mix them up.

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





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

Note:
All conrods are cracked.
Do not mix up conrods and conrod bearing caps.

Important!
During installation, make sure conrod bearing caps are correctly seated.
Risk of damage.

Install new conrod bolts.





Tighten down conrod bolts with special tool 00 9 120 00 9 120 Torque Angle Measuring Dial.

Installation:
Tightening torque 11 24 1AZ 11 24 Connecting Rods and Bearings.





Assemble engine in reverse sequence to its disassembly.

Installation:
Lubricate all crankshaft bearing positions with engine oil.

Note:
The pistons are available in two oversizes for subsequent treatment of the cylinders.