Piston: Service and Repair
11 25 530 - Removing and installing/replacing all pistons (N12)
Special tools required:
- 00 9 120 00 9 120 Torque Angle Measuring Dial
- 11 9 620 11 9 620 Assembly Tool
- 11 9 670 11 9 670 Bush
Warning!
Protective goggles must be worn when working on the gudgeon pin circlip.
Important!
If pistons, connecting rods and bearing shells are reused, they must be reinstalled in the same places.
connecting rods and connecting rod bearing caps are denoted with the same pairing letters; mixing them up will result in engine damage.
Piston and gudgeon 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 Service and Repair.
Note:
In event of heavy oil carbon residue:
Carefully remove oil carbon residue from cylinder wall.
Note:
Illustrations show N46.
Note:
Mark all pistons and conrods with a pen prior to removing.
Example N14:
Direction of arrow (1) points to camshaft drive.
Cylinder allocation (2) per cylinder.
Note:
Mark all pistons and conrods with a pen prior to removing.
Example:
Mark cylinder allocation (1) per cylinder.
Direction of arrow (2) points to camshaft drive.
Release connecting rod bolts (1).
Tightening torque, 11 24 1AZ.
Remove conrod bearing cap.
Important!
connecting rods and connecting rod bearing caps are denoted with the same pairing letters; mixing them up will result in engine damage.
Attach special tool 11 9 620 in conrod big end.
Press out connecting rod and piston to cylinder head side.
Warning!
Protective goggles must be worn for the next work step.
Warning!
Protective goggles must be worn.
Important!
Piston and gudgeon pins are paired and must not be fitted individually.
Lever out piston pin circlip with a screwdriver in direction of arrow.
If necessary, replace connecting rods.
Installation Note:
The gudgeon 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 gudgeon pin.
Piston diameter at measuring point A.
Adjust micrometer to cylinder bore of engine block. Set internal calliper 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.
Install all piston rings Service and Repair.
Install all bearing shells Service and Repair.
Coat piston and piston rings with oil.
Pre-install piston in special tool 11 9 670.
Screw special tool 11 9 620 into conrod.
Installation Note:
Check protective lugs on special tool 11 9 620 for correct position and damage.
Insert piston with connecting rod in cylinder.
Important!
Danger of piston ring failure.
Press in piston with finger pressure only, do not drive in (see arrows).
Insert piston (1) so that arrow on piston crown points to camshaft drive.
If reusing the pistons, assign cylinder allocation (2) to correct cylinder.
Press in piston with special tool 11 9 670.
Important!
Point of fracture (1) on conrod.
Conrod and conrod bearing cap are identified with pairing letters (2) 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.
Both pairing letters (2) must be together on one side.
Apply a light coat of oil to crank pin.
Assemble connecting rod and crank pin.
Remove special tool 11 9 620.
Fit bearing caps (2) so that pairing letters match up.
Check cylinder identification markings (1).
Direction of arrow (2) points to camshaft drive.
Install new connecting rod bolts (1).
Carry out torsion angle tightening of conrods with special tool 00 9 120.
Tightening torque: 11 24 1AZ. Mechanical Specifications
Assemble engine.