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 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 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 internal 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 factory 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, including thrust control.
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 without engine dis-assembly, 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 located in the wet sump 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. The recommended interval for the Classic Dual Row & Single Row Pro IMS Retrofits is 6yr/75,000mi. The IMS Solution is a permanent fix with no service interval.
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. The IMS Class Action Lawsuit filed against Porsche revealed the factory Dual Row was much stronger than the Single Row used from 2000-2005.
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 completely.
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 8 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 and that oiling alone is not the cause or solution to IMS issues.