Crankshaft Main Bearing: Service and Repair

11 21 531 - Replacing all crankshaft main bearings (N51)





Special tools required:
- 00 2 590 00 2 590 Measurement Aid
- 11 4 251
- 11 4 252
- 11 4 470 11 4 470 Tool





Important!
Aluminium-magnesium materials.

No steel screws/bolts may be used due to the threat of electrochemical corrosion.

A magnesium crankcase requires aluminium screws/bolts exclusively.

Aluminium screws/bolts must be replaced each time they are released.

Aluminium screws/bolts are permitted with and without

colour coding (blue).

For reliable identification:

Aluminium screws/bolts are not magnetic.

Jointing torque and angle of rotation must be observed without fail (risk of damage).





Necessary preliminary tasks:
- Remove crankshaft Service and Repair.





Check setting of oil spray nozzles, adjusting if necessary:

Attach special tool 11 4 251 11 4 250 Gauge with Adapter Plate to screw connection on main bearing.

Special tool 11 4 252 11 4 250 Gauge with Adapter Plate must be pre-installed at the seventh main bearing block.

Tightening torque: 11 11 5AZ. Mechanical Specifications

Note:
See illustration.





Remove crankshaft (1).





Note:
Pilot bearing shell (1) at the fourth bearing block is a thrust bearing.





Remove bearing shells (1).





Installation Note:
Bearing shell (1) with lubricant groove must be fitted in crankcase upper section.

Bearing shell (2) without lubricant groove must be fitted in crankcase lower section (bedplate).





Bearing classification (1) on crankcase lower section (values from 1 to 3).

Bearing classification (2) of connecting rod bearings Service and Repair(b/r).





Bearing classification (1) in crankcase upper section as per table (values of A/B/C).

Installation Note:
When all the letters and numbers have been determined, the bearing shell colour must be allocated, see table.

Important!
Engine damage will result if a small bearing play is determined.

The colour combination Yellow and Red must not be fitted.

Possible colour combinations, see table.









Clean sealing faces (1).

Important!
Do not clean sealing faces with a metal-cutting tool.

Clean sealing faces (1) with special tool 11 4 470.





Insert all bearing shells (2 and 3).

Note:
Bearing shell at the fourth bearing block is a thrust bearing.





Determine bearing play with special tool 00 2 590.

Installation Note:
All measuring points must be free from oil and grease.

Use used screws to determine bearing play.

Set up crankcase lower section Service and Repair with bearing shells.





Remove lower crankcase.

Read off bearing play at width of flattened plastic thread and measurement scale.

Crankshaft bearing clearance radial.

Installation Note:
Remove plastic thread.

Apply a light coat of oil to bearing shells and crankshaft.





Install crankcase lower section Service and Repair.

Assemble engine.