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Where Am I In IMS Bearing Replacement?

IMS: three simple letters that strike fear in brave men and ladies who own Porsche cars.

So where am I on IMS bearing replacement? If your Porsche was built before 1997 or after 2009, you can resume peaceful sleep with no worries. If, however, your Porsche was built between 1997 and 2009, you should educate yourself about the danger these three letters pose for your beloved Porsche.

IMS stands for Intermediate Shaft. Over the years, many manufacturers have used intermediate shafts in their engine designs as an auxiliary drive for various sub-assemblies that need to rotate at a different rate than crankshaft speed, or in our example, the camshafts and oil pumps.

Porsche 911 models have used an intermediate shaft since their inception. On the air-cooled motors, and later GT3 and Turbo engines, the IMS was a forged steel part that rode in insert bearings with positive lubrication. This shaft had a multitude of machining steps in its manufacture, making it an expensive part to build. The current retail price for this part is $2,100. The upside of this part is that, in 35 years, I’ve not heard of a single failure!

When Porsche engineers began their clean sheet design on the new M96 motor for the new Boxster and 996 cars, one of the design prerogatives was, as in any design, lowering production costs. One of the many ways they lowered production costs was in redesigning the IMS. Their new design was ingenious. Instead of a complex shaft with many machining operations, Porsche engineers used a steel tube cam chain to drive the sprockets. And, rather than being splined, they were swaged onto this tube.

The oil pump drive was pressed into one end and a sealed ball bearing was pressed into the other end. The retail price of this shaft is $559.09; a substantial saving from the previous design.

The fly in the ointment of the new design turned out to be this sealed bearing. While it did a superb job of supporting the tube and controlling axial movement, the lubrication of the bearing, over time, became the root cause of the current IMS failures. In the environment that this bearing is normally used, the seal is made to keep dirt and debris out and the lubricant in. However, when used in this engine design, the sealed bearing is submerged in engine oil that can reach 250 degrees F. This very hot oil compromises the seals and washes away the lubricant.

Now the trouble begins… and without warning!

All photographs courtesy of BR Racing, LLC, Los Gatos, CA

Because it was designed to be lubricated by grease and not engine oil, once the lubricant is washed away, the bearing begins its failure mode. As the bearing begins to fail, metal fragments are spread throughout the engine. When the bearing totally fails, it allows the shaft to move, at which time the cam chains slip on their sprockets which causes the cam timing to change. This allows the valves to contact the pistons, causing terminal engine failure.

So, what to do? If your car is a 1997 – 2005, you should have the IMS bearing replaced. The replacement bearing is a ceramic hybrid that is designed to be lubricated by the engine oil. This new bearing is not permanent and is recommended to be changed at 50k miles. While not an inexpensive repair, it is approximately one-tenth the cost of an engine replacement.

If your car is a 2006 – 2008 there is bad news and good news. In these model years, Porsche used a newer design shaft with a substantially larger bearing. The bad news is that the engine has to be completely disassembled to replace this bearing. The good news is that there have been no reports of this larger bearing failing.

In conclusion, you must ask yourself, “Is it better to be safe than sorry?” The answer, of course, depends on your tolerance for risk. The risk, of course, is not knowing if and when the IMS will fail. Certainly, your investment is worth protecting. One advantage of replacing the IMS is that the resale value of your Porsche will be stronger against Porsches that have not performed the repair. The choice is yours.

[Ed. Note: This story originally appeared in the September 2013 issue of Challenge]

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