What is the technical cause of RMS?

I am not clear on what causes the RMS leak. I read a lot of posts about something being out of spec, but no one mentions just what is out of spec. 1. is a problem with the seal itself? 2. is the outside area in the case around the seal too big? 3. Is something wrong with the crankshaft? 4. Is the center of the crankshaft end not exactly lined up with the center of the hole in the case where the crankshaft exits? If so, is this a problem with the case hole being offcenter or the crankshaft located offcenter in its bearings? Just curious. Steve 99 996 - no leaks 76 930 - some leaks, not rms 64 356 - drip pan required

My understanding is it's #4.

#4 The round seal is in a bore in the crankcase. The round end of the crankshaft goes through the middle of the seal. If the crankshaft is off-centered then the seal lip cannot make 360 degree contact with the shaft. Think of a cork in a wine bottle. If the opening is shaped like an oval instead of round then the cork will not seal. This is a poor example as the seal and crankshaft are round and that does not change when it leaks, but it was the only example I could think of.

Tool pants: I understand now that the center of the crankshaft and the center of the case bore are located further apart from each other than is within spec, and the crank is tight against the seal on one side and loose on the other, creating an oval shape in the inner surface of the seal, thus allowing oil to pass. But is the crankshaft misaligned in the cases, making it off-center, or is the bore in the end of the case off-center from where the crank passes thru it, either by mis-design or by misaligned joining of the case halves? Any chance a re-designed and very pliant seal could accomodate the center line offset? Steve

That is a good way to explain it. The end of the crankshaft is tight against the lip of the seal in one area but loose in the oposite area, thus providing oil a path to escape. What I think happens is that the crankshaft moves after a while. The Boxster/996 engine design is unique and unlike the older air cooled and GT2/Turbo/GT3 engines. The crankshaft does no spin directly on bearing shells installed in the case like the other cars. Instead, the crankshaft is supported by a separate bearing block or cradle inside the cases. If the cradle moves inside the cases then the crankshaft has to move as well. The shop manual says that if the crankshaft is not centered then the cases are to be taken apart and the crankshaft realigned, and that is checked with the measuring mandrel to determine if it is centered. Then last year Porsche issued a TSB and said if it is not centered then just replace the engine. Why, I suspect it is simply cheaper for Porsche to replace an out of spec engine then to pay a dealer for 2-3 days of work for a rebuild, and so the customer is not without a car for an extended period. If you have worked with oil or grease seals then you know there is a round spring that forces the lip of a seal into contact with a round shaft. When I started in 1999 some mechanics were shortening the spring to create more force, or try 993 seals. Then in Feburary 2000 Porsche came out with the new style seal and issued a TSB. The TSB does not really explain in detail how the new seal is different. My speculation is that the lip is made of a material that is more pliable or the spring tighter. I will have to ask the mechanics I know if they have any old style seals around, and compare it to the new.

So, if one has an engine that is out of warranty, and the new style seal does not work, one could take the engine to a very competent Porsche engine building shop, split the case, realign the crank bearing cradles, and reassemble it with basically the very same parts other than seals and gaskets. Assuming at most we are talking about 40 hours labor at say $100 per, that's four grand plus R&R. A lot cheaper than a new engine from Porsche, I would assume? Also, does Porsche sell assembled short blocks for the 996? Thanks again for your intelligent and informative answers, Steve

"Also, does Porsche sell assembled short blocks for the 996? More Importantly, are they warrantied?

Here is a page from the Boxster tech intro booklet. The 996 is the same design. I do not know anyone personally who has taken the motor apart to do what the shop manual says. Most of the mechanics I know work at my local dealer. I do not think they would ever do it. The shop rate last time I checked was $125 per hour. If they tried it and it still leaked then what would they do? They would rather replace the motor and if it leaks again then the risk is on Porsche. They do rebuild the air cooled flat sixes. We have 3 local independent shops that put on tech sessions for us. You always see the air cooled motors apart in their shops for a rebuild. Never a 996/986. I would have to ask if they ever tried. The only one I personally know who has taken a motor apart is the shop manager at my dealer and he also happens to be PCA tech advisor. He did it on his own time on a Saturday. It was a motor that had sustained catastrophic damage unreleated to the RMS issue so it was going back to Porsche on a pallet anyway. He does not like the design.

There's also another thing that I am bit confused about and have some concerns with. The dealer engineer said my gearbox was so damaged that it was the cause of the repeat RMS problem I have. Phil Hindly at Tech9 disagrees. I tend to disagree too. But if either the end bearing at the gearbox clutch end, or the thrust bearing of the clutch itself were out of wack, could they not cause the same effect. That is, could the gearbox cause the crankshaft to wobble by a few thou and thus cause the seal to leak or the weakening of the seal such that it eventually leaks? My gearbox has been wining in 1st and 2nd since it was about 24,000 miles old. It's now done 38,000 miles and the noise is exactly the same. It doesn't sound bad to me and it hasn't got any worse. It changes gears fine and never misses a change. In fact, it was the engineer who pointed the wining out to me. I didn't notice it myself. The thing is, Phil Hindly also doesn't think it's hat bad either. He said it is repearable "when I am ready". Here's the catch: I wonder if the cause could also be transmissions and or clutches. I mean everyone seems to pass over a wining gearbox and Porsche AG engineers seem to be stumped on a fix for the RMS. Could they be looking in the wrong place? The gearbox and/or clutch being the culprit. Just a hunch.

Tool Pants - Please check you PM. Thanks

Cars with tiptronics are also experiencing the problem. Wouldn't that suggest that it's not the gearbox/clutch? I have absolutely no idea how the tiptronic works so I could be way off base.

I do not know how gears inside a transmission can move a crankshaft so that it is not centered in the seal. The transmission gets it power from the crankshaft via an input shaft that sticks out in the bell housing.

The clutch pressure plate exerts a few thousand pounds of pressure on the rear crankshaft main journals and bearings. In fact, if you ever inspected the journal and bearing closest to the flywheel on any car that has a manual transmission, you will normally see more wear/clearance that you would on a car equipped with an automatic tranny. Therefore, it is possible for the crankshaft to move a few .01 mm.

There are not thousands of pounds of pressure needed to release a 996 pressure plate. The torque output of the engine is not even close to that. You would need a 6 foot long clutch pedal to generate that type of force and Arnold's leg in order to release the clutch, or a power assisted device like the power assisted clutch the turbo has. But you are correct that force is placed against the end of the crankshaft in order to release the clutch. If it moves then it is going to force the crank into the case, not side-to-side, and the crank would still be centered. Then at a local tech session at an independent Porsche shop we looked an x-ray view of the engine and wondered if the weight of the dual mass flywheel and the clutch at the end the crank was somehow causing the crank to become off-centered. A combination of factors, perhaps, we thought. Pushing against the crank and the weight factors do not apply to a tip. Tips leak as well. Reports of them is not as common as most cars have a manual transmission. So there goes that theory out the door. 4 years of development and 7 years of production for the Boxster. I'll call Zuffenhausen and ask to speak to a power plant engineer. This page is from the 996 tech intro booklet Published by Porsche.

LOL I was talking about static pressure. You are correct that it would take the hulk and a 12 foot lever to depress. Static pressure refers to the amount of base pressure, in pounds, exerted to the pressure ring by the internal springs when the clutch is engaged. Here are some typical pressure ratings for a Borg Warner presuure plate: DIAMETER STATIC PRESSURE 10.5� 2400 11� 2400 12� 2500 Racing clutchs will have upto 6000 PSI

LOL I was talking about static pressure. You are correct that it would take a hulk and a 12 foot lever to depress. Static pressure refers to the amount of base pressure, in pounds, exerted to the pressure ring by the internal springs when the clutch is engaged. Here are some typical pressure ratings for a Borg Warner presuure plate: DIAMETER STATIC PRESSURE 10.5� 2400 PSI 11� 2400 PSI 12� 2500 PSI

The presssure plate static (clamping) pressure is not applied to the crankshaft and thrust bearings. The force applied to the clutch fingers is much less than the clamping pressure. This is because of the leverage of the pressure plate release fingers inside the pressure plate.