Rear Main Seal Failure

 One of the problems, fortunately upgradeable, in the M96 engine is the design of the crankshaft which has an approximate 50mm distance between bearing 7 and the crankshaft seal surface. Additionally, the chain that drives the crankshaft is located at the back of the engine (looking toward front of the car). The pressures from these combined forces cause excessive vibrations on the crankshaft.

The picture to the right compares the end of an air-cooled crankshaft to the end of a water-cooled (M96) crankshaft. While both crankshafts have a flywheel connected to the end, the M96 motor has the added forces of the chain, a heavy flywheel-clutch combination (42lbs.), and a longer distance from its support bearing. All forces working against a smooth spinning crank.

Compared to the previous engine design, the M96 crankshaft has one less main bearing than the air-cooled versions, air-cooled with 8 main bearings and 7 on the M96. This design leaves the 996 engine susceptible to greater vibration and becomes the source for oil leaks through the RMS seal. It is also one of the reasons for premature bearing 7 wear. As the bearing begins to wear, RMS failure can occur at any time. Replacement of the seal is simple as the part is relatively inexpensive, but because it requires engine removal the labor can be cost-prohibitive when it becomes a regular occurrence. Bearing 7 has been known to wear excessively due to an out of round crank and leading to expensive crankshaft related repairs.

In the video below, you see an out of round crankshaft. This part was removed from a 996 with 60,000 miles. As the crankshaft rotates, the dial indicator should remain steady. Instead what you see is a crank that was twisted with evidence of how far out of round it is.


Aftermarket automotive engineers are working right now to resolve this problem and we expect an effective and permanent solution to this problem. For example, check out the next two pictures below. Aftermarket engineers have devised a solution to the crankshaft problem by inserting an additional main bearing. This creates much-needed support for the crankshaft seal surface and reduces the forces from the chain and flywheel on the crank. The additional support utilizes the existing rear main seal area and a different seal is utilized as the rear main seal. This new design has a channel to lube the area and allows for the rear main seal and bearing to share the same friction area of the crankshaft to reduce vibration on the seal and thus reducing the risk for oil leak. The engineers tell us that the additional support also yields a smoother running engine and longer-lasting seal and bearing.

The bottom picture shows the new design installed. You can see both the end of the crankshaft at the top and the intermediate shaft bearing at the bottom.