While waiting for some parts I put together some thoughts on the IMSB problem from my point of view.
I don’t pretend to be an expert on the InterMediate Shaft (IMS) bearing (IMSB) problem in M96 Porsche engines, but I consider myself an informed observer who has spend a lot of time reading about others opinions and experiences as well as a bit of hands on experience. There is not much original here, but I haven’t seen where all this information is brought together and made available. With a bit of logical analysis.
First some facts.
1) The original bearing from the factory is a sealed ball bearing lubricated by the grease originally packed into it.
2) In many cases, after a certain mileage, the grease has been washed out by engine oil but there is not enough oil or perhaps not enough fresh oil to properly lubricate the bearing which can lead to failure. I base this statement on the many observations reported on various forums and technical sites.
I don’t think this is disputed.
3) It seems that process by which the grease is eliminated is the following.
The IMS is a hollow shaft of about 2” in diameter and over a foot long that comes from the factory filled with genuine Porsche… air. When the engine warms up the air expands and tries to exit the shaft, the only way it can do that is at the end containing the bearing, either through the bearing seals or around the inner or outer surfaces of the bearing.
When the engine cools down a partial vacuum is created and whatever is outside of the bearing is pulled in. Outside the bearing is oil, since most of the bearing is sitting in oil and air. Logically after enough cycles the seals will be compromised to the point that a small amount of oil will pass through the bearing, in both directions eventually removing all the grease and leaving some oil in the IMS.
Proof of this? Many observations of oil in varying quantities flowing out of the IMS after removing an IMS bearing and the lack of grease in the IMSBs.
Is this disputed? I have not seen other explanations.
(An aside; could this problem have been avoided if Porsche had drilled a hole in the IMS to prevent the pressure and vacuum build up?)
4) The oil in the bearing and in the shaft does not seem to circulate enough that it is renewed with fresh oil and it is questionable, at least in some cases, that there is enough to properly lubricate the bearing. I am speculating a bit here. Removing the outer seals on the bearing has been seen as a way to protect the bearing by allowing it to be constantly lubricated by crankcase oil.
5) The IMSB is bathed in oil. See post #53 of this thread.
http://986forum.com/forums/show-tell-gallery/73917-blue-boxster-resurrection-project-3.html
So removing seal allows lubrication by oil, not by oil mist as has been suggested by some. In a general discussion on bearing lubrication the LN Engineering site states that the oil level should be much lower than it is in an M96 engine, that a too high oil level can generate heat and is not optimal. But, elsewhere, they also suggest removing the bearing seals is better than doing nothing.
6) It seems that the double row bearing used up until 2001 is more reliable than the single row bearing used in later engines. I don’t think this is disputed.
There is also a debate on whether the fundamental problem with the IMSB was that it was too small for the application or that it should have been properly lubricated… or both. In my opinion the fact that the single row bearing fails more often would indicate that it is overstressed. The double row it would seem is usually OK if it doesn’t lose its lubrication.
I have personally inspected 3 IMSBs in place, all where double row bearings from model year 2000 engines. All three had had the grease washed out. All three turned smoothly with no indication of wear. Two had some oil in the bearing, so I removed the outer seals and left them in place. The third one had almost no oil and the “lubricant” that I found after pulling the seal looked like burned, hardened grease. There is a photo at the beginning of this thread.
I have removed this one and am in the process of replacing it. Unfortunately I have no way of testing the integrity of the bearing seals, that would be interesting.
In my opinion inspecting the bearing in place and removing the outer seal is a valid procedure for protecting the bearing. It has the advantage of costing nothing (if you are already there to change the clutch and/or the RMS) and you don’t run the danger of causing new problems. It is possible to mess up an IMSB removal and install.
I question some of the commonly (more or less) accepted “truths” about the IMSBs.
It is often said that IMSBs that survive for over 100,000 miles will (likely) not fail. I haven’t seen any statistics that would indicate this. But I wonder if with time the bearing seals deteriorate to the point that they allow enough oil to circulate into the bearing to keep it lubricated.
Engines with frequent oil changes and engines that do track duty supposedly have fewer IMSB failures. I have heard this but have not seen an explanation of why this would be true, nor have I seen statistics that support this.
Then there is the question of whether the IMSB is a maintenance part that should be changed every 30,000 or 50,000 miles. Porsche didn’t think so. I would suggest that with proper lubrication (removal of the outer seals) the bearing would in most cases last until a major rebuild is needed. I would have difficulty defending suppliers that sell a product that costs one or two thousand dollars to install and that has to be changed each time that the clutch is serviced.
These are but my opinions. Porsche must have the data on failures and forensic data to fully understand what has happened… but they aren’t sharing it.
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