The following page contains a description of the processes and techniques involved in changing the connecting rod bearings in the M30 engine.  The process is the same for most engines, the only difference is the way the oil pump is attached.

Changing the connecting rod bearings has the potential of greatly extending the life of the engine.  Once the engine has covered 150 thousand miles it becomes all but necessary to perform this repair.  Should the opportunity be presented, this is also a worthwhile endeavor for engines with slightly fewer miles on them.  The costs of these repair are negligible.  A set of connecting rod bearings for an M30B35 will cost around $77.  A new oil pan seal is also needed, adding around $16 to the project.

The connecting rod bearings show signs of wear already at 100 thousand miles.   Often the first layer of the 3-layer bearing is worn down by this point.  If the engine was exposed to more rigorous driving conditions, the entire first layer and even some of the second may already be worn.  The dangers associated with a worn connecting rod bearing range from a simple annoying rattle,  to a tremendous vibration caused from within the engine that can shake the entire engine assembly and car, causing serious damage to both the engine and the motor mounts.  Simply replacing the connecting rod bearings before they wear seriously enough to do damage, can extend the life of the crankshaft by another 100 thousand miles. 

Engines that have already clocked a large number of miles (over 150 thousand), and already are experiencing the symptoms described above, may need more repairs than a simple connecting rod bearing replacement.  
 

The Sport-Connecting Rod Bearings for the M30 have a harder surface, and provide greater axial movement, much like the ones used in the M-engines.   A sport bearing set for the M30 will cost around $135.55.  Because the friction produced by these bearings is slightly less than that produced by new factory bearings a few horse power may be gained over the stock, or previous rating.   The sport bearings should be used if at all possible (they can be purchased from: http://www.motorrevision.de/ **).

	First, the oil pan must be removed.  Once removed the oil pump can be accessed.   This too must be removed in order to gain access to the front few cylinders.
	Now the crankshaft must be turned to raise (technically lower, as the engine is upside down) the first piston to a position that allows access to the hex-screw. Once in this position, the connecting rod can easily be disassembled and the bottom half can be removed.
	In order to remove the other side of the bearing (that which is further in toward the engine and the piston), the crankshaft is kept turning until it releases the connecting rod.
	Now the connecting rod can be pulled past the crankshaft, and the rest of the bearing can be removed.
	The left pictures shows the new and the old bearings.  Before the bearing is installed, the connecting rod is to be rubbed with a clean rag to remove all old oil.  The new bearing can be placed onto the connecting rod at this point.
	After the bearing is placed on the correct points of the connecting rod it is to be moistened with clean oil.  At this point the connecting rod is pushed down, and lower into the engine, so it can again be attached to the crankshaft.
	The other half of the bearing is also installed after the connecting rod has been cleaned in the areas to contact the bearing.  Again clean oil is rubbed onto the bearing just prior to completing the installation.
	Before installing the bearing, the attachment points of the connecting rod pieces are to be cleaned.  It is extremely important that the assembly be put back together using all the correct positions and attachment points.
	After the nuts and bolts have been properly installed they are tightened to the prescribed pressures (52-57 Nm).

That marks the end of the first cylinder.  This process is to be repeated for each of the following cylinders.  Before beginning on the second, the crankshaft is to be turned a few rotations.  A single jerk will be felt, but thereafter the crankshaft should be relatively easy to turn.  This same test should be repeated after every cylinder, in order to diagnose any problems with the installation.   This check also makes it possible to correct any problems before it becomes unclear which cylinder is causing them.  As a side note, the effects of the sport-bearings are felt only when compared directly to new factory bearings.  The reason for this is quite simple: a worn bearing already posses the increased axial movement provided by the new sport-bearings.  This improvement is equally masked under driving conditions.  The upgrade to the sport-bearings serves only to eliminate the horse power losses that would result from the use of new factory bearings. 

tino@e30.de
 

Prices provided by: http://www.motorrevision.de/ ** on 01/2003 

**Available in German Only