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Originally Posted by denverpete
...As to the RMS - I don't think mileage has much to do with it. It seems like a gremlin that just pokes it's nasty little German head up randomly....
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Hi,
All good points, but as far as RMS goes, the majority which fail prematurely do so under 30k mi., usually under 24k mi. There have been failures at higher mileages, but these are rare. So much so, that there well may be other issues at play in these instances.
But, the Main Seal on any car is a wear item. The Front and Rear Seals on your Chevy Smallblock or Honda 4 will
eventually wear out.
These seals are essentially a rubber (sometimes synthetic, leather or cardboard depending on the car) lip which is precision engineered to rub (read direct contact) against the circumference of the crankshaft as it spins, this is what provides the seal and keeps the Oil in. Think of it as similar to a rubber doughnut with a shaft running through the middle which presses against the doughnut around it's whole inside diameter.
The crankshaft theoretically spins on a stable axis, but eventually, due to wear in the bearing shells, it's axis changes slightly (or it will wobble slightly) and it will exert more pressure, and therefore wear, to one part of the seal and away from the opposite part. When this happens, the seal is compromised and the seal leaks.
One of the keys to a long-lived seal is to have it centered on the Crankshaft's spin axis, and this is where Porsche has had some difficulty. The M96 engine block halves are cast using a proprietary casting method developed by, and adopted from, Audi. The advantage of this type of casting theoretically removes any need for post- cast machining the block to center bore the Crankshaft opening. This means less $$ to produce the engine and more profit for the manufacturer. But, it also eliminates the possibility of post-machining of the Block halves because no excess material is present, as in conventional casting, to machine away; this is why a bad block cannot be saved.
But, the tolerances are not as good as if the block were precision center-bored. If the block is really off the seal will not center-align with the spin axis of the Crankshaft (Porsche measures the block on failed RMS to determine whether to refit a seal or the engine entirely) . The original Seals were not able to tolerate much variance by flexing sufficiently to maintain a proper seal. The redesigned seals and those from the Cayenne block, do seem to be more flexible and capable of accomodating greater variances better than the original seal. This is why for many, it's a one-time repair. But, for others, the variance is too great and it will consistently fail or require engine replacement. Of course, should a Crankshaft be inaccurately machined and slip through Quality Control, it too would cause a premature failure of the seal, but these are probably as rare as the same occurance in a Honda. Also, should there be a burr on the Crankshaft, this too can tear the seal and cause it to fail. The seal will also sometimes scar the Crankshaft by wearing a groove into it. The original seal is set 11mm deep into the Block, but Porsche recommends seating a replacement seal 13mm deep. This way, it comes into contact with a fresh, smooth part of the Crankshaft.
Also, each of the materials used in making the seal have their own lifespan. Rubber and other materials harden with time and heat cycling and the seal becomes less pliable. At this point, it is less able to endure the stress placed on it and it can wear or crack, again loosing it's integrity and a leak develops. Due to greater manufacturing tolerances and better materials, these seals now last a very long time, somewhere in the neighborhood of 150k-250k mi., but still not forever...
Happy Motoring!... Jim'99