The purpose of the intermediate shaft is to drive the camshafts indirectly off the crankshaft. By using an intermediate shaft, the speed of the chains are reduced, which is better for chain life. This basic design was used throughout the entire lifespan of the air-cooled six-cylinder Mezger engine used through 1998. The inclusion of an intermediate shaft which drives the camshafts indirectly off the crankshaft has been a mainstay of the horizontally-opposed flat 6 engine utilized by Porsche.
The most common deficiency with the M96 engine and its revisions through 2008 is the failure of the ball-bearing found in the intermediate shaft. The intermediate shaft found in the M96 and later M97 engine was revised three times. The earliest design incorporated a dual-row ball- bearing used through model year 1999 and in some 2000 and 2001 models. Starting in model year 2000, Porsche began phasing out the dual row bearing and went to a smaller single row, with significantly less load capacity. From model year 2002 -2006, all engines used this smaller, lower capacity bearing. Starting with the 2006 model year, the design was again revised to use a much larger single row bearing with the same load capacity of the early dual-row ball-bearings. This bearing increased in diameter, which increased the bearing and ball speed, further improving the bearing. However, starting with the 2006 model year, Porsche in its third revision of the intermediate shaft bearing, changed over to a design that is not serviceable, leaving later model years with no recourse for addressing this issue with preventative maintenance, which failures still frequent. In all revisions, a sealed ball-bearing was used, rather than allowing for engine oil to lubricate and cool the ball-bearing. There is no recommended service interval for these bearings nor provisions for their replacement from the factory. However, with model year 1997 through 2005 engines, the intermediate shaft bearing thankfully is serviceable and with preventative maintenance, costly repairs can be prevented.
By far, the single row ball-bearing used starting in model year 2000 through 2005 are the most problematic. Based off projections calculated by a fellow Porsche enthusiast and retired bearing engineer from Timken, they figure a 90% survival rate of the single-row 6204 ball-bearing used in the IMS at 90,000 miles* – resulting in a staggering 10% failure rate (called the Ll0 life)!
*Assuming an average speed of 60mph in top gear. This coincides with the high number of failures we see in these model years. Regardless, all M96 and M97 engines from 1997 through 2008 can suffer intermediate shaft bearing failures at any mileage and at any time.
For those with MY06-08 intermediate shafts that cannot have their bearings replaced, we recommend to remove the grease seal from the existing ball bearing, which allows for engine oil to lubricate the bearing, but only when doing another procedure like a clutch, flywheel, or rear main seal replacement. However, this bearing is still problematic and should be upgraded anytime the engine is apart or if replacing the intermediate shaft with a new replacement shaft. That said, we are advising our customers that proactive disassembly of their engines is not required nor recommended unless performing other upgrades.
Beginning with MY09, the M97 engine was retired and was replaced with the 9A1 engine which features improvements such as direct fuel injection and more notably, the lack of an intermediate shaft or IMS bearing.
Further technical information
The [Mezger intermediate shaft] design has been retained with the watercooled 996 Turbo, GT2, and GT3 models as their engines are based off the same 964 engine case with the same internals as the earlier air-cooled engines. This intermediate shaft features plain bearings (no ball bearings) that are pressure fed engine oil for lubrication and never fail. If these bearings wear out, the engine may develop a slight knocking noise due to increased running clearance, but this condition will never result in a catastrophic engine failure.
When the M96 engine was developed, for cost savings, one cylinder head casting was made. As such, for interchangeability, the camshafts could no longer be driven off one end of the IMS. This required that the chains be driven off opposite ends of the intermediate shaft. On the rearmost side of the intermediate shaft (closest to the flywheel), you have the main sprocket the drives the intermediate shaft off the crank as well as a smaller sprocket and chain that goes to one of the cylinders heads to drive the cams. Clear on the opposite end of the intermediate shaft there is another sprocket to drive the cams for the opposite cylinder head. This end of the intermediate shaft has a plain bearing surface integrated into the front oil pump console that is pressure fed oil for lubrication directly from the oil pump. As a result, this end of the IMS always performs flawlessly and never shows wear.
Unfortunately, due to how the crankcase was designed, there are no oil passages from which pressurized engine oil can be used to lubricate a plain bearing on the side closest to the flywheel. The IMS in the M96 (and subsequent M97) engine is located directly beneath the crankshaft carrier and is connected to the crankshaft by means of a chain. The IMS has a sealed ball bearing on one end (closest to the flywheel) and a plain bearing on the other end, which resides in the oil pump/coolant console. Additionally, it is this ball-bearing that handles the majority of the load on the intermediate shaft.
According to information published about the Eisen IMS Class Action Lawsuit, the single row IMS bearing used in 2000 through 2005 model years is reported to have an 8% failure rate, versus less than 1% with the dual row IMS bearing. The 8% failure rate cited by the settlement documents is not far off of the calculated L10 life LN Engineering has been using for the last five years! With half the load capacity, it is clear to see that the reduced load capacity of the single row IMS bearing is a significant contributing factor to the increased number of failures.
In addition to ball-bearing failures, the intermediate shaft can suffer other failures including but not limited to slippage of drive sprockets which can lead to valve timing related failures. Again, as a cost saving measure, the sprockets were not secured to the intermediate shaft. The center bearing support stud was manufactured with an o-ring groove cut into it, reducing its cross- sectional thickness to less than the diameter of the threads used to secure the IMS bearing assembly. As such, it is common to have the center bearing support stud break during operation, resulting in a large oil leak, and is often an indicator of a failing or failed IMS bearing.
It is not known exactly why these bearings fail, but there are many contributing factors including over-loading. Poor lubrication, long drain intervals, high fuel and moisture content in the engine oil, high oil temperatures, and even operational speeds can also affect bearing life. That’s why some bearings last 3,000 miles and others have lasted over 200,000 miles.
One of the most common sources of trouble in bearings is wear and pitting caused by foreign particles and is responsible for 70% of all early bearing failures. With six years and tens of thousands of IMS Retrofit procedures having been carried out since 2008, we have enough experience to confirm that foreign object debris can and will compromise the replacement IMS bearing. We have confirmed documented failures of the [LN Engineering] IMS Retrofit bearing after it was installed as part of a reactionary procedure when the original bearing had already started to fail or had failed completely.
Now for some technical information from our bearing engineering team: “There are many reasons for bearing failure, and usually each failure is due to a combination of causes, not just a single cause. Spalling occurs as a result of normal fatigue where the bearing has reached the end of its normal life span but this is not the most common cause of bearing failure. Spalling detected in bearings can generally be attributed to other factors. A common cause of bearing failure is due to contamination from tiny metallic particles left over from a previous mechanical failure. These particles are suspended in the oil and if they are allowed to get into the bearing, the result is tiny dents in the hard steel raceway. Often the dent is surrounded by a microscopic raised area (or ‘lip’), and when the ball rolls over this lip, the ultra-high point loading exceeds the strength of the steel and it fractures, leaving behind a jagged depression (or ‘pit’). Once this cycle begins, wear is greatly accelerated and the bearing will fail prematurely.” One thing is for sure; once you have experienced an IMS bearing failure, there is no turning back. A complete engine disassembly is required to replace the intermediate shaft and in most cases, complete rebuild or engine replacement is your only option.
When an IMS failure occurs, or more specifically the ball-bearing or bearing support fails, the intermediate shaft is damaged beyond the point of being serviceable, but moreover, debris from the failure contaminates the entire engine, requiring a complete teardown and rebuild to recover from such a failure. In worst-case scenarios, the cam timing can also be thrown off, causing valve to piston contact, and in some cases, even lead to a failure that requires replacement of the engine. In that case, the engine will not be accepted as a core, requiring the purchase of another core or to pay an ever-increasing core charge from Porsche on top of the cost of the replacement engine. Aside from the proactive approach of replacing the IMS bearing prior to such a failure, prevention and early detection are some of the steps that can be taken to try to minimize the risk of a costly engine failure.
One attempt at extending the life of the original IMS bearing is removing the grease seals off the factory intermediate shaft bearings to better improve their lubrication and cooling, as is the case with model year 2006 through 2008 engines, as that’s the only preventative measure that can be taken that is cost effective. The other option is to tear down the engine completely to access the intermediate shaft, for which upgraded IMS bearings are available for installation at this point.
With model year 1997 through 2005 engines however, the bearing is indeed accessible. Any IMS bearing replacement is intended to be installed as a proactive measure in preventative and regular maintenance. Once an engine has suffered a failure, replacement of the intermediate shaft bearing is no longer an option. Installing a replacement IMS bearing in an engine that has already suffered a failure will result in a subsequent failure due to collateral damage, including but not limited to debris contaminating the new bearing.
Oiling is just one part of the IMS failure problem. As supplied by Porsche, the intermediate shaft uses a sealed ball bearing. The original bearing was grease filled, considered a “permanent” lubricant. However, the original bearings and seals used are rated only to 250F and over time, the seals become hard, allowing the grease under normal operation to be “washed” out of the bearing during operation.
Additionally, as the intermediate shaft itself is completely submerged at times, the shaft will actually fill up with engine oil. With long drain intervals, contaminants in the oil find their way into the bearings and intermediate shaft. These worn out seals do allow oil in but they also limit the exchange of fresh oil.
Once the bearing begins to wear, the seal will actually fail completely, allowing for oil to wash out the permanent grease but not allowing for fresh oil to exchange in and out of the bearing during operation. It is at this point that you can begin to find debris in the filter from the seal and from the bearing itself.
The IMS Retrofit uses an open bearing (no grease seal), allow engine oil to freely enter and exit the bearing. The ceramic hybrid bearing used is designed to function on splash lubrication from engine oil alone, but is submerged in oil as well, depending on operating conditions due to its location in the engine’s “integrated dry sump.”
The IMS tube itself will end up storing a sizable amount of oil that will centrifugally be fed to the bearing during operation as it leaks past the rear seal on the new IMS bearing. This is another reason why frequent oil changes are a must – as this oil gets trapped and is one of the sources for lubrication of the new bearing!
Forced oiling of the ball bearing does not provide any benefit as the bearing does not require or benefit from additional oil. Lubrication alone is not the problem. “Rolling element bearings depend on the continuous presence of a very thin – millionths of an inch – elastohydrodynamic film of lubricant between rolling elements and raceways, and between the cage, rings and rolling elements. Lubricant-related failures can be avoided by selecting a grease or oil that generates a sufficient film to keep bearing elements separated. A good lubricant also provides good boundary lubrication. Lubricant failures can be detected by the presence of discolored (blue/brown) raceways and rolling elements. Excessive wear on rolling elements, rings and cages follows, resulting in overheating and subsequent catastrophic failure.” – Applied.com
Some claim without forced oiling of the ball bearing there is inadequate lubrication. Unfortunately, there is no evidence of this with thousands of stock or ceramic hybrid replacement bearings LN Engineering has inspected. Furthermore, the bearing operates submerged in oil or partially submerged in normal operation, for example at idle, when bearing load is highest and need for lubrication is greatest. In fact, it’s possible to over-lubricate a bearing, which causes the balls to slide rather than to roll. “Too much oil or grease in a bearing or its housing causes an effect called churning resulting in a sharp temperature rise and often premature lubricant and/or bearing failure.
Circulating oil, however, should not be used in high-speed applications, such as machines tool spindles, because of unacceptable friction losses as the lubricant moves through the bearing. In these applications, only a small amount of oil is needed, and a spray-mist system is generally preferred.” – Petronomics.com
For those choosing to retain the original conventional ball bearing, it has been known for years that simply removing the grease seal will promote improved bearing life along with use of an oil with higher viscosity or higher film strength.
Another issue that forced oiling of a ball bearing does not solve is over-loading of the ball bearing. The single row ball bearing used starting in 2000 through 2005 was replaced with a larger bearing in 2006 with double the load capacity, restoring the original load capacity the dual row bearing first used in the M96 engine(1997-99). Insufficient load capacity leading to wear and fatigue is the primary problem with the single row bearing used in 2000 through 2005, not lubrication! It is for this same reason that the class action lawsuit settlement regarding the IMS only covers vehicles with the single row bearing as found in 2000 through 2005 model year vehicles!; http://www.imsporschesettlement.com/