I know, the IMS bearing is the most overpopulated blog in Porsche history. However, I’m doing a full engine rebuild and have several ideas to reduce its failure rate without major reengineering or cost. Wanted to run these ideas past the community, and also Wayne Dempsey, for thoughts and feedback.
As a little bit of background, I have several mechanical engineering degrees from top 10 schools and practiced as an automotive engineer for almost a decade. I know more than average Joe when it comes to engineering design and root cause analysis, but also value field expertise. I’m doing a full engine rebuild of the 2001 Boxster S 6-sp with w/70k miles, all new parts are from Pelican.
I discovered a great thread on this topic in GrassRootsMechanic.com.
https://grassrootsmotorsports.com/forum/build-projects-and-project-cars/the-frugal-mechanic-meets-a-porsche-boxster/120457/page2/. Inside he (mrap1000) is suggesting to install an unsealed roller bearing and knocking a small pinhole on the other end of IMS next to the oil pump to promote minor oil flow through the IMS and then across the unsealed bearing. It sounds highly logical however one weakness may be that little to no oil flow will come through the end of the shaft next to the oil pump given the inherent design of the oil pump. With 200,000 boxers on the road globally, I suspect only one boxer has this design, so the sample size is just 1 and it’s hard to assess the effectiveness of the fix. But I applaud him for his ingenuity an engineering prowess!!
Coupled the above, I couldn’t help but notice that when I removed my I am IMS shaft bearing it was full of old oil. I wasn’t unable to measure as it came out by surprise, but I suspect it was more than 50% full. Online I learned this is common for most IMS repairs.
One downside the design is the constant pressurization and vacuum that forms inside the intermediate shaft as the engine goes to heating and cooling cycles. Based on the well proven ideal gas law the pressure inside this intermediate shaft will increase approximately 6 psi from 70 degrees to 180 degrees (thermostat temp), and I suspect the actual shaft temperature is higher than 180 as it is at the very center of the engine, so conceivably the increasing pressure could be 7 to 8 PSI. While pressures don’t seem very high, they are over 50% of the pressure of a normal football, and any small holes will absolutely allow air and oil leakage. As a consequence, every time the engine goes through a heat cycle you’re forcibly pumping oil through the IMS bearing and washing out the higher viscosity grease that the seals are designed to keep in the bearing. When the engine cools the shaft will create a vacuum and air and oil will come back into the shaft. After 100,000 miles the average engine has gone through 2000-5000 of these heat cycles. That’s a lot of pumping across the bearing.
There are two simple ways to fix this heat cycle pumping action: 1) Fill the half shaft completely full oil before assembly thus removing all the air, or 2) Drill a small pinhole somewhere on the shaft to allow the air pressure to release without going through the bearing.
Pro/Cons of each:
Option 1. Fill shaft with oil.
PRO: Removes all the air. Also provides an oil bath on the backside of internal side of the bearing which will further cool the bearing from heat generated from rolling friction.
CON: The weight of the oil will create a slight pressure on the inside of the bearing and promote leakage path during non-operation. In other words, the oil will slowly seep out. When the oil leaks it will likely be replaced with air you are back to the same problem you started with (the pumping problem)…
Option 2. Drill small pinhole in shaft to release air pressure / vacuum during heat cycles.
PRO: Air is able to “breath” without passing through the bearing during heat cycles.
CON: The actual shaft could fail as they are a thin walled design and a small hole could lead to stress risers, cracks, and ultimately failures. I’ve done web search for known shaft failures but could not find any without bearing failures. However, there is so much bearing failure content online isolating just a shaft failure is a needle in a haystack exercise.
Here is the huge caveat. Neither fix can be accomplished with a normal IMS repair. This can only be accomplished during a full engine rebuild.
For option 1 and the oil fill… to fill the shaft with oil you must have the immediate shaft in a vertical position to fill up with oil. This means during engine rebuild with the shaft out, or with the engine removed and the entire engine stood vertically (not sure how to do this easily, engine stands do not rotate this direction). Or, if you’re really brave, stand the car on its front bumper during an IMS repair ;-) !!!
For option 2 and the drilled hole…. this is engine rebuild territory only, as you need drill the from the outside of the shaft. I suspect there may be a very small right-angle drill that could fit inside of shift thorough the bearing hole, but I have never seen one. (Maybe available in the orthopedics medical profession ??)
I’m a few weeks away from final assemble of my torn down engine. Will try to post pics later.
Many thanks to anyone who can share some engineering expertise on this subject.
(Sorry Wayne, not sure how we could monetize this solution!)
