Piston: Service and Repair

Remove cylinder head, refer to Engine, Cylinder Head Assembly, Service and Repair.
Remove oil pump, refer to Engine, Engine Lubrication, Oil Pump, Engine, Service and Repair.





Note:
^ Injection nozzles for cooling the pistons are installed on the underside of the cylinder between the bearing seats.
^ This illustration shows crankshaft removed.

Installation:
Screw threads are coated with screw securing adhesive (Loctite). Replace screws.

Removal of pistons is described separately from installation.
Assembly sequence for removal and installation is different.

Caution:
^ Re-install piston, connecting rod and bearing shells back in the same position and in the same installation location.
^ Connecting rods and bearing covers are designated with the same pair number: do not mix them.





M52/S52
Remove oil scraper.





Unscrew conrod bearing cover.





Insert special tool 11 3 480 in connecting rod.
Remove conrod with piston from cylinder head side.





Lift out retaining ring and press out piston pin.

Caution: Piston and piston pin are paired and must not be fitted individually.

Installation:
Installation of pistons is described separately from removal.
Assembly sequence differs between removal and installation.





It should be possible to press piston pin into bush by hand using only slight pressure. Clearance should then be minimal.





If necessary, check conrod bearing for parallelity and torsion stress.
A = Inspection gap
B = Permitted parallelity deviation at distance A
C = Permitted distortion on each side
Measurements A, B and C, refer to Engine, Specifications.
Replace conrod bearing shells, refer to Connecting Rod Bearings, Service and Repair.





Before installation, measure piston installation clearance.
Measure piston diameter with micrometer at distance A from the underside of the piston and at offset angle of 900 to piston pin axis.

Measurement A, refer to Engine, Specifications.





Adjust micrometer on cylinder bore of engine block and set plug gauge 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, refer to Engine, Specifications.
Piston installation clearance, refer to Engine, Specifications.
Permissible total wear tolerance, refer to Engine, Specifications.





Caution:
^ Piston and piston pin are paired and must not be fitted individually.
^ Fit connecting rod with piston pin to piston in such a way that both of the visible pair numbers on the installation direction arrow on the piston point to the right.





Fit the retaining ring with its aperture opposite the recess.





Insert special tool 11 2 470 in conrod.





Lightly coat pistons and piston rings with oil.
Align contact points on piston rings (offset at approx. 120°, but do not arrange over piston pin lug).
Press together piston rings with special tool 11 2 260.





Install pistons with arrows pointing towards camshaft drive.
The tightening strap must locate firmly right around circumference of engine block.

Caution:
^ Danger of piston ring failure.
^ Press in piston only with finger force (do not knock in!).





Attach crankpin to connecting rod.





Apply light coat of oil to connecting-rod bearing shells.
Place bearing caps in position, making sure that matching numbers are paired.
Install new conrod bolts.





Tighten down conrod screws with special tool 00 9 120 or special tool 11 2 110.
Tightening Torque, refer to Engine, Specifications 11 24 1 AZ.





M52/S52
Fit oil scraper.

Assemble engine.