Piston: Service and Repair
11 25 530 Removing and installing/replacing all pistons (S63)Special tools required:
Warning!
Protective goggles must be worn when working on the gudgeon pin circlip.
Important!
If pistons and connecting rods are reused, they must be reinstalled in the same places.
Piston and piston pin are matched to each other and can only be replaced as a pair.
Connecting rod and connecting rod bearing cap are cracked.
Identification is effected by means of identical pairing letters on the connecting rod big end.
Mixing up the components will result in engine damage.
Connecting rod bearings must always be replaced.
Necessary preliminary work:
^ Remove engine
^ Mount engine on assembly stand.
^ Remove both cylinder heads.
^ Remove oil sump.
^ Remove oil pump.
Unscrew connecting rod bearing cap.
Note:
Connecting rods and connecting rod bearing caps are denoted with the same pairing letters.
The stamped dates are always arranged in opposite directions.
Set down connecting rod bearing caps in order.
Note:
To install and remove the connecting rods, it is essential for the crankshaft to be exactly in alignment with the cylinder bore (see dashed line).
Position crankshaft at central bolt.
Insert special tool 11 8 152 into connecting rod.
Screw special tool 11 5 440 into connecting rod with bolt (1).
Remove connecting rod with piston from cylinder head side.
Important!
Piston and piston bolts are paired and must not be fitted individually.
Lift out retaining ring and press out piston pin.
If necessary, replace connecting rods.
Note:
The connecting rods can also be replaced individually.
The gudgeon pin must be able to be pressed through the liner by hand with little force and must not display any significant play.
Prior to installation, measure piston installation clearance: Measure piston diameter with micrometer at measuring point A from lower edge of piston and offset by 90° to piston pin axis.
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.
Important!
The connecting rods of cylinder banks 1 to 4 and 5 to 8 are mounted to the pistons differently.
Note:
The pistons and connecting rods of cylinders 1 to 8 are identical.
Arrow on piston crown (cyl. 1 to 4) points upwards, bolt connection on connecting rod points at an angle to right.
Arrow on piston crown (cyl. 5 to 8) points upwards, bolt connection on connecting rod points at an angle to left.
Note:
For purposes of clarity, pistons are shown removed.
The connecting rods are correctly mounted on the piston when the bolt connections on the connecting rods are parallel to each other (see arrow).
Install retaining ring.
Installation:
Piston circlip (1) is correctly installed when the opening points upwards.
See graphic.
It must still be possible for the piston pin to moved easily.
Insert special tools 11 8 151 and 11 8 152 into connecting rod.
Install connecting rod bearing.
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 pin boss.
Note:
Picture shows S85.
Keep piston rods compressed with special tool 11 7 280.
Install piston so that arrow points to camshaft drive.
Important!
Danger of piston ring failure.
Press in piston only with finger force (do not knock in!).
Important!
Danger of piston ring failure.
Press in piston (1) only with finger force (do not knock in!).
The direction arrow on the piston crown must point to the camshaft drive (direction of travel towards front).
Note:
Graphics show N63.
Attach crankshaft journal to connecting rod.
Remove special tools 11 5 440 and 11 8 152.
Note:
For purposes of clarity, the pairing letters (1) are shown on the removed connecting rod.
Important!
Connecting rods and connecting rod bearing caps are denoted with the same pairing letters (1), do not mix them up.
Fit bearing cap so that pairing letters match up.
Apply light coat of oil to connecting rod bearing shells.
Fit bearing cap so that pairing letters match up.
Install new connecting rod bolts.
Tighten down connecting rod bolts (1) with special tool 00 9 120.
Tightening torque 11 24 1 AZ.
Assemble engine.
