Porsche IMS Bearing Replacement
For anyone who owns or is shopping for an M96-powered Porsche like a Boxster or 911 996 model, it’s important to understand the IMS bearing issue.
Since the intermediate shaft bearing is located at the flywheel end of the engine, just below the crankshaft, you could theoretically use a stethoscope and listen from under the car, touching the bottom of the engine case close to the transmission or gearbox, to listen to the IMS. The sound you do not want to hear is a random knocking, similar to the noise of a metal coffee can half full of rocks being shaken lightly. This is akin to the sound a bad water pump or belt idler/tensioner pulley bearing might make. From a more realistic listening position, such as near the right or left rear wheels, the noise will sound like something loose — a slight metallic rattle obscured by the other mechanical noises, which could include noisy lifters or chain tensioners. This sound indicates metal on metal contact with excessive clearance. If you had the IMS bearing in your hands, the inner and outer race would feel loose. It is also likely that any bearing this far gone will also display other symptoms, but with failure imminent, continued engine operation at this point is not recommended.
If you hear a ticking noise that sounds like a noisy lifter, more than likely you have another issue entirely — cylinder bore scoring. Arguably, this is a worse diagnosis since you have to rebuild the engine to correct for this, whereas the IMS bearing can be replaced as part of preventative maintenance as long as it hasn’t failed catastrophically.
Before a failure occurs, worn IMS bearings typically will leak oil from the IMS bearing flange, but this is often misdiagnosed as a leaking Rear Main Seal (RMS), another common issue with the M96. It’s crucial to notice this rattle or oil leak as it can be a precursor to the potentially serious problem of an IMS bearing failure. In a perfect world, the center stud will break, resulting in a large oil leak that can’t be ignored, well before the bearing completely fails, but this rarely is the case. Similarly, a rough idle coupled with debris in the oil filter are additional symptoms to watch for, but again, it’s too late if your engine has gotten to this point. If the IMS bearing fails, the rattle will become intense and is known as the “Death Rattle.” This term is used because when rotating engine components break, especially at higher RPMs, the resulting damage is often catastrophic.
Background
The 1997 Boxster was the first Porsche to feature the newly designed M96.20 engine. Prior to this, all Porsche six-cylinder horizontally opposed boxer engines were based on the Ferdinand Piech/Hans Mezger design, first introduced in the model 901 (which later became the 911) in 1963. This design remained the basis for all oil/air-cooled engine variations through the M64 versions used in the Normally Aspirated (NA) and Turbo 993 models, ending in 1998.
The 901 Mezger design utilizes a gear drive between the crankshaft and the Intermediate Shaft (IMS). The IMS directly drives the supply and return oil pumps within the crankcase and, via two chains (one for each side of the engine), drives each cylinder bank’s single overhead camshaft. The basic crankcase layout of the M64 engine family was carried over to the water-cooled, dual overhead cam engines: the M96.70 (996 Turbo), M96.70S (996 GT2), and M96.79 (996 GT3). This IMS design would later be used by LN Engineering and Flat 6 Innovations as an inspiration for the IMS Solution.
Because the M96.70, M96.70S, and M96.79 versions have different internal designs, they do not suffer from the IMS bearing problems found in the M96 engines installed in the Boxster and Carrera models.
The M96.20 engine marked a revolutionary change in Porsche’s design philosophy. It was compact and powerful for its size, capable of meeting increasingly stringent global emission standards, and could be manufactured at a much lower cost than its oil/air-cooled predecessors. The water-cooled M96 engine features a vertically split, open deck crankcase with integral cylinder sleeves. It houses a two-piece aluminum bearing case, also split vertically to match the crankcase, which contains the crankshaft. The valve train includes dual overhead camshafts per cylinder bank and four valves per cylinder. The M96 employs an ingenious self-contained wet sump oiling system that operates similarly to a dry sump system.
The early M96 engine is known as a five-chain engine. Chain one transfers rotation from the crankshaft to the IMS. Chains two and three provide the rotational drive from the IMS to the exhaust cams in the left and right cylinder banks. Chains four and five transfer the rotational drive from the exhaust cams to the intake cams. Each chain has a tensioner (chains four and five use an adjuster) that maintains enough pressure on the chain via a chain rail to ensure it does not slip timing. The five-chain engine was used in the Carrera (996) up to the 2001 model year and in the Boxster (986) up to the 2002 model year.
In the 2002 model year, the 996 Carrera was redesigned to operate with three internal chains, with the 986 Boxster following suit in 2003. In this updated layout, chain one continues to drive the IMS from the crankshaft, while chains two and three now each provide rotational drive to the exhaust and intake cams of their respective cylinder banks. This redesign also coincided with the introduction of VarioCam Plus in the 2002 996 Carreras and 2003 for the 986 Boxsters. VarioCam allows for cam timing adjustments to improve performance and emissions based on engine load and RPMs. The introduction of VarioCam Plus added the capability to adjust valve lift during operation, providing additional performance when needed. The three-chain design has remained in use through subsequent Carrera and Boxster versions of the M96 and continued into the M97 engine until the introduction of the MA101 (3.8 Liter) and MA102 (3.6 Liter) engines in 2009, when Porsche eliminated the IMS completely, choosing to drive the camshafts directly off the crankshaft instead.
The IMS Bearing
The M96 intermediate shaft was manufactured in two configurations: with a dual row 5204 series or single row 6204 series ball bearing with steel ball and races housed in a permanently greased and sealed unit. The model year of the car and engine determined which IMS bearing version was used. Early M96 engines from model years 1997 through 1999 were fitted with a dual row IMS bearing, while engines produced for model years 2002 through 2005 have a single row IMS bearing. The type of IMS bearing used in engines produced for model years 2000 and 2001 is more difficult to determine, as the switchover point from dual to single row bearings was not clearly defined. Most 2000 model year engines will have a dual row bearing, and most 2001 engines likely came with a single row bearing; however, to be 100% sure you have to check.
Additionally, many M96 engines have been replaced under warranty or by owners due to issues with the original factory-installed engine. If your engine was replaced in or after the 2002 model year, there is a good chance it has a single row IMS bearing, even if your car is pre-2000. These engines used whatever IMS bearing was current for that model year. To add to the complexity, starting with the 2006 model year, remanufactured M96 engines were fitted with the later M97 intermediate shaft with the non-serviceable 6305 series IMS bearing. The good news is that this newer bearing has shown fewer problems so far. However, if this bearing does develop an issue, the engine must be disassembled to replace the IMS bearing, as the newer IMS bearing is larger than the opening in the back of the engine case and cannot be extracted. If you have a newer engine with the non-serviceable 06–08 IMS bearing, the best thing you can do is have the grease seal removed off the original bearing to allow the engine oil to properly lubricate the bearing.
The best way to determine which IMS bearing you have is to inspect the IMS bearing flange and the nut size on the bearing support stud. The IMS bearing flange is located in an opening in the crankcase just below the crankshaft. To see and access the IMS bearing flange, you need to remove the gearbox and flywheel or the transmission and flex plate. The IMS bearing flange is held in place by three bolts, with a nut in the middle securing the IMS bearing’s threaded center support stud. The IMS bearing is located directly behind this flange.
The easiest way to tell which bearing you have is to measure the depth of the dish in the IMS cover: A single row flange measures 19.27mm deep and a dual row flange measures 13.34mm deep. Additionally, if the IMS bearing flange has already been removed, a single row bearing has a visible snap ring retaining the bearing, whereas a dual row does not. The M96 IMS bearing uses an 8mm diameter support stud with a 13mm hex nut. In contrast, the later non-serviceable M97 IMS bearing support stud is 10mm with a noticeably larger 22mm hex nut.
The Design
The IMS in all M96 engine versions is a hollow tube, closed on the internal crankcase end and open with a machined end (to fit the IMS bearing) on the side facing the transmission/gearbox. The IMS has three swaged-on (press fit when heated, not keyed or indexed in any way) drive sprockets: two on one end and a single sprocket at the opposite end of the shaft. The early IMS used a light bicycle-type chain, while later versions utilized a different gear and a heavier multi-link internal tooth chain. The IMS also drives the engine’s oil pump. It’s important to note that the IMS cannot be removed without completely disassembling the engine.
The outer race of the IMS bearing is press-fit into the end of the hollow IMS. The inner race of the IMS bearing has a threaded support stud that extends through the center of the IMS bearing flange, where the support stud is bolted into place. Therefore, the inner race of the IMS bearing remains stationary while the outer race revolves around the inner race as the IMS turns. The IMS bearing support stud and flange locate and hold the IMS and the chain drives connected to it in position, making the IMS bearing critical to the operation of the M96 engine. Interestingly enough, the opposite end of the IMS uses a plain bearing design, rather than a ball bearing.
The IMS Problem
The exact cause of IMS bearing failures is not known. Generally, most cases of bearing wear and premature failures in specific applications result from a combination of issues: bearing choice, overloading, and lack of proper lubrication. Typically, when the grease seals are removed from a used IMS bearing, no grease is found — only small amounts of engine oil. Due to the IMS bearing’s location below the crankshaft, hot engine oil eventually penetrates the bearing’s grease seals as they wear, washing out the grease. The IMS bearing was designed to operate with internal grease, however Porsche probably should have just omitted the grease seal in the first place since the intermediate shaft is partially submerged in engine oil during normal operation. Without the grease and with only small quantities of lower quality engine oil entering the IMS bearing, there is less lubrication protection, leading to more metal-to-metal contact, especially under low operating speeds. Long drain intervals, along with fuel and moisture intrusion in the motor oil, further decrease IMS bearing life.
Interestingly, there are fewer reported problems with M96 engines or their IMS bearings in track cars compared to their more sedately street-driven counterparts. The general theory is that track-driven cars receive more frequent servicing, such as an oil change after each run session or event. Additionally, the IMS bearing is much happier at higher operating speeds when a phenomenon known as Elastohydrodynamic (EHD) lubrication can occur inside a bearing. This is where the viscosity of a lubricant increases at the point of surface contact under extremely high pressure, providing improved oil film retention between those surfaces. EHD lubrication does not occur under light loading conditions or low speeds. Therefore, running engines at higher RPMs, typically above 2500–3000 RPM, usually results in longer IMS bearing life. The loads and wear are higher in the IMS bearing at lower speeds. Coincidentally, owners with high-mileage cars exhibiting no IMS problems tend to be former air-cooled Porsche owners who are accustomed to keeping the revs up to ensure proper cooling of their air-cooled engines.
As the engine operates, oil becomes contaminated with fuel, combustion byproducts, and water from condensation. These elements build up and are held in suspension by the oil, ultimately reducing its lubricating effectiveness. This, combined with the longer drain intervals recommended by automobile manufacturers and the reduction of anti-wear additives such as Zinc Dialkyl-Dithio-Phosphate (ZDDP) and friction modifiers like Molybdenum Disulfide (Moly), creates a condition where internal engine components can wear at an accelerated rate. Bearings can also be severely affected by very small amounts of water mixed into the lubricating oil, which can easily reduce bearing life by 25 to 50% or more. Cars driven only short distances without reaching full operating temperature to burn off the water content are at the highest risk for developing problems. So even if the car is not being driven or even if the engine isn’t being run, the IMS bearing is in service 24/7. This is also why we recommend changing your Porsche’s oil with Driven DT40 every 6 months or 5,000 miles. Not only does this help the IMS bearing but it also is one of the best things you can do to help prevent bore scoring.
Over time, the materials in the IMS bearing will fatigue and start breaking down. The ball bearings will begin to lose surface material (spall), causing pitting of the internal balls and races. As wear continues, operating roughness, vibration, and noise will increase, leading to higher heat and more fatigue. At this stage, if the IMS bearing flange is removed, there would be excessive movement of the IMS bearing center support stud due to internal wear in the bearing. Oil can start leaking around the IMS bearing flange, which might be misdiagnosed as a rear main seal (RMS), gearbox main shaft, or transmission torque converter seal leak. Any seepage in the bellhousing area should be diagnosed as soon as possible.
If left unchecked, this wear and fatigue will eventually cause the IMS bearing to fail completely. The ball bearing separator and outer race will disintegrate, and the resulting debris will be distributed throughout the engine by the oiling system. Without the ball bearings and separator to maintain separation between the inner and outer races, significant movement of the IMS will occur, resulting in a loss of proper camshaft timing, valve to piston contact, and foreign object debris to be spread through the entire engine. An engine that has experienced an IMS bearing failure should either be replaced or rebuilt.
What Can Be Done
A simple step can be taken during your next Porsche service to better assess the condition of your IMS bearing and overall engine health. Ask the technician (or do it yourself during scheduled maintenance) to remove the oil filter, cut off the ends of the filter element, and then cut the filter lengthwise so the filter element can be laid flat. Turn the filter material over to inspect the outside surface where debris would be deposited as oil enters and passes through the filter element. Carefully check the oil filter element for any debris, specifically plastic and metal particles. If you find any pieces of plastic (likely from chain rails) or shiny metal debris that looks like glitter, this could indicate a problem with the IMS bearing or other internal components. At this point, you should drop the sump to further inspect the engine for any other debris or contamination.
Forewarned, the single row 6204 series IMS bearing, unlike the earlier dual row bearing, gives little to no warning before it fails catastrophically. In fact, it was disclosed during the Eisen class action lawsuit that these bearings had a failure rate of 8% under warranty, which is surely higher now over a decade later.
The good news today is that IMS bearing replacement is possible. These replacements feature a different design and added strength to the areas of the IMS and bearing that have previously failed. When installed before the original IMS bearing fails according the manufacturer recommendations, these upgraded intermediate shaft bearings can potentially eliminate IMS problems altogether.
As Porsche intended the IMS bearing to last the lifetime of the engine, they did not assign a part number, service interval, or specify the method to replace the intermediate shaft bearing, let alone offer a product that could be used to replace the bearing without requiring engine replacement. The IMS Retrofit, co-developed by LN Engineering and Flat 6 Innovations, made this procedure possible.
Owners of Boxster and 911 996 models with a dual row or single row IMS bearing may consider upgrading to the IMS Retrofit or the IMS Solution, the latter being the only truly permanent solution to intermediate shaft bearing failures. The IMS Solution uses an oil pressure fed plain bearing, eliminating the need for rolling element bearings.
Special thanks to Tony Callas and Tom Prine, Callas Rennsport, for sharing their body of work that was the basis for this article.