Piston: Service and Repair

Removing and Installing or Replacing All Pistons
(Engine Removed)

Removal:
Remove Cylinder Head.

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.





Remove oil pump.
If oil pump and oil scraper are two parts:
Remove oil scraper.





M52 B25/B28
S52
M52TU B25/B28
M54 B25/B30

Note: Figure shows crankshaft removed.

Injection nozzles for cooling the pistons are installed on the underside of the cylinder between the bearing seats.

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





In event of heavy oil carbon residue:
Carefully remove oil carbon residue from cylinder wall.





Unscrew conrod bearing cover.





Insert special tool 11 3 480 in conrod.
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.
The assembly sequence differs between removal and installation.





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





Measure piston installation clearance:
Measure piston diameter with micrometer at distance A from the underside of the piston and at offset angle of 90° to piston pin axis.
Measurement 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.





Replace conrod bearing shells.

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

Fit conrod 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.





Install snap ring so that opening is opposite 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 piston so that arrow points to camshaft drive.
Entire circumference of tightening strap must rest firmly on 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.
Fit bearing caps so that pair numbers match up.
Install new conrod bolts.





Tighten down conrod bolts with special tool 00 9 120 or special tool 11 2 110.
Tightening torque, refer to Specifications, Mechanical.





Install oil pump.
If oil pump and oil scraper are two parts:
Fit oil scraper.
Assemble engine.