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An intermediate shaft has been used ever since Porsche developed the aircooled 911 engine, starting in 1965 with the 2.0.
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 aircooled six-cylinder Metzger 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 same design has been retained with the water-cooled Turbo, GT2, and GT3 models as their engines are based off the same 964 engine case with the same internals as the earlier aircooled engines. This intermediate shaft features plain bearings that are pressure fed engine oil for lubrication and never fail. If these bearings wear out, and 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.
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, a single row ball-bearing with a significantly reduced load capacity was used. By model year 2002, 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. 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.
From 1997 to 1999, Porsche use a dual row intermediate shaft bearing which has proven to be as robust as the larger single row used from 2006 through 2008 model years.
Starting in 2000, Porsche began phasing out the dual row bearing and went to a smaller single row, with significantly less load capacity. From 2002 through 2005, all engines used this smaller intermediate shaft bearing until they went to the larger, third revision for the 2006 model year, which increased the load capacity back to what the original dual row bearing could support. The larger model year 2006 and later bearing also increased the diameter, which increased the bearing and ball speed, further improving the bearing. However, this change has not been enough to resolve the IMS failure issues.
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.
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 we have 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 failure, 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 affect bearing life. That’s why some bearings last 3,000 miles and others have lasted over 200,000 miles. Here is a good link about bearing failures and some of the common causes:http://www.maintenanceworld.com/Articles/manufacturing/bearing-failure.htm
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 pro-active 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 removing the grease seals off the factory intermediate shaft bearings for years 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 as 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 pro-active 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.
So what can be done to minimize the likelihood of an IMS failure?
Although a greatly debated subject, most experts agree that more frequent oil changes every 6 months or 5,000 miles is a good first step. Secondly, actually driving your Porsche more often and avoiding higher gears to keep the revs above 2500-3000 rpm is another good step to take to improve the life of the ball-bearing in the intermediate shaft. Although there is limited data, the general trend is that lower mileage vehicles with infrequent oil changes or driven light-footed (as in run at low speed/engine rpms) are most likely to suffer a failure rather than those cars that are driven hard and well-maintained. Lastly, use of premium engine oil such as Motul’s 8100 X-Cess 5w40 (which is a Porsche approved oil) or Joe Gibbs DT40 can also provide added protection to all critical engine components.
Along with more frequent oil changes, the addition of a magnetic drain plug coupled with close inspection of the oil filter and magnet at these shorter intervals may help owners identify a failure in its early stages, but later models using the single row bearing can fail with little warning. One alternative to bearing replacement is the IMS Guardian, which is an early warning system utilizing a magnetic chip detector designed to monitor the engine for ferrous debris and identify a possible failure between oil changes, providing owners with additional feedback about the bearing. When inspecting the filter and magnetic drain plug (or IMS Guardian), ferromagnetic debris from the intermediate shaft bearing can be identified easily, appearing like silver glitter. Larger debris than this is indicative of a complete failure.
When contemplating replacement, understand that there are several versions used by the factory.
Model years 1997-1999 used a dual row IMS bearing. Model year 2000 and 2001 engines are cross-over years where a dual row or single row IMS bearing could have been used, requiring visual inspection to identify which you have. Model year 2000 through most of the 2005 models will use a single row IMS bearing. However, some 2005 model year vehicles will have the larger, non-serviceable bearing as found in the 2006 through 2008 model years. Additionally, if the engine is not original to the car, for example a car that might have a replacement engine, it will have whatever type bearing was currently in production for the model year in which the engine was built. One easy sign that you have a remanufactured factory engine that has been replaced is if there is an X in the engine number located on the lower edge of the engine’s sump.
There are several commercially available products to service the IMS bearing available from LN Engineering LLC and IMS Solution LLC, as well as a do-it-yourself kit from Pelican Parts, among others, utilizing a similar ball-bearing arrangement. The LN Engineering IMS Retrofit uses a custom open (no grease seal) ceramic hybrid bearing (US or Japan sourced components) whereas the Pelican upgrade kit uses an OE NSK sealed conventional bearing. However, beware of kits using inferior import (Chinese) bearings!
LN Engineering currently offers a dual row IMS Retrofit as well as a single row IMS Retrofit, where the engine does not have to be disassembled to replace the factory sealed ball-bearing. IMS Solution LLC’s oil fed plain bearing arrangement similar in functionality to the intermediate shaft bearing used in the past aircooled models and most recently in the GT2, GT3, and Turbo models based off the GT1 aka Metzger engine, which eliminates the ball-bearing design completely. Typically replacement of the IMS bearing takes 10-14 hours (with exception of some Tiptronic 911 models) and is usually serviced at the same time the clutch is being replaced, or every 50,000 miles (for 4 years, like a timing belt on many modern engines).
LN Engineering also offers a MY06-08 IMS Retrofit kit for the larger bearing found on model year 2006 through 2008 M97 engines, but engine disassembly is required for installation. LN Engineering also offers an IMS Upgrade service where you send in your intermediate shaft and it is upgraded to a triple row bearing (with exception of MY06-08). Additionally, the main drive sprocket is pinned to prevent slippage of the drive gear, which is a known problem area.
Considering all three revisions of the IMS found in the M96 and M97 are known to fail, we know the use of a ball-bearing was a manufacturing constraint and a poor choice. That said, even the addition of a ceramic hybrid ball-bearing with its superiority over conventional ball bearings does not eliminate completely the chance of failure. With zero failures of our dual row IMS Retrofit, triple bearing IMS Upgrade, or even with our upgraded ceramic hybrid bearings used in the model year 2006 through 2008 intermediate shafts, we have almost a perfect track record, however there have been about a dozen single row IMS Retrofit kit failures. However, we still have a 99.9% success rate even considering some of these failures are due to improper installation or are collateral damage from other failures (there are dozens of confirmed modes of failure for the M96 and M97 engine that can damage or cause your IMS bearing to fail).
Knowing the single row bearing is undersized for this application, we recommend replacement every 4 years or 50,000 miles.
Fast forward to 2009, Porsche’s newest lineup of Boxster, Cayman, and 911 models feature their newest 9A1 engine, which no longer utilize an intermediate shaft to drive the camshafts, but rather drive them directly off the crankshaft.
Remember, not driving your car or worst yet, not driving it like Porsche intended can make the problem worse.
For some, choosing to roll the dice and hope they do not experience a failure is the right thing to do. Odds are in your favor that you’ll never experience a failure. That is little consolation for those having experience a failure out of warranty. Others choose to purchase extended warranties or have some sort of preventative or proactive maintenance done to protect their investment. First and foremost, owners should continue to drive and enjoy their cars and not let fear dictate their actions or inactions. Being informed and aware is the first step to happy Porsche ownership.