Erkka, disengaging the clutch pulls against a spring(pressure plate) which is attached to the flywheel. Unless I am missing something important, this will pull the crank to the rear against the thrust bearing. I agree that 1.5 tons is incorrect -- maybe the clutch has a 2000lb clamping force, but due to leverage the throwout bearing has less force applied to it than that. But I don't see how you can assert that disengaging the clutch does not pull the crank against a thrust bearing surface...

 

That seems counterintuitive to me, there must be an oil quality component to thrust bearing failure, it may only contribute 10%, where the remaining 90% is preload, but it must have some effect.

 

Why Paul? If you push a crank against a journal wall with the constant force that our pressure plates exert when the TBF condition is in-process (flex plate is loaded), how can any oil save you?

 

Try removing the oil altogether, I'm sure it will fail quicker than with some oil. By extension the sheer strength of the oil film must have some effect.

 

When the clutch pedal is pressed it does place a force on the thrust bearing, but only for the time when it is pressed and most of the time it's only seconds before the engine is running, replinishing the oil film. The force placed on the thrust bearing in an automatic with the movement issue would be continuous. So you shut off your hot engine in the automatic with the movement issue and it sets with hot thin oil with this pressure applied for the next 12 hours. How much oil film is left upon start up? My point being maybe it can be related to both issues?

 

So Paul if I instal let's say "wunder-oel" .... you know, the very best oil man can produce. You believe this will solve out TBF issues? The original poster said that it did.

 

Not at all, but the oil quality has to affect the time taken for the thrust bearing to fail. Like I said, perhaps it contributes 10%, or may be 20%. The OP said he thinks Oil is the main cause, I'm saying it has to be a contributing factor.

 

True Dave, I was replying without really reading what Sean was saying. It's true that clutch will push crank towards the rear when slave is pushing the arm.

Still I don't think clutch is really a problem to crank thrust bearing. Reason why I believe so is that on my car TT rear clamp was loose. This allowed center shaft to migrate forward. End result was worn rear side of thrust bearing. Luckily it hadn't worn completely yet when previous owner destroyed the engine. Othervise I wouldn't have good crank. So center shaft managed to wear thrust bearings rear side. Time it took must be much shorter than what clutch had time to wear front side, which was still like new. And there was nothing pushing center shaft forward except what ever force affects it when it's rotating. I'd say this same force is reason why front clamp creeps forward on later style automatic TT's. Having good oil layer supporting thrust bearing will definitelly help but it can't prevent TBF.

 

At first I thought for sure this was an elaborate sales pitch.

I've had many 928's over the years.
Used Castrol 20w-50 in all of them.
No tbf's. although there was preload on both the 88 and 87(within spec - wasn't aware of the issue when I owned the previous cars),
Always good oil pressure,
0 smoke,
All retained good compression,
0 bearing related failures,
small leaks,
Ran most of them up to 200,000 miles.

Maybe something to it?

 

erkka,
ok it probably wont be 1.5 tonne, infact the standerd pressure plate has about 1.25 tonne force. but there is stit a lot of force being pulled when you press down on the clutch and can you imagine how many times you press on the clutch when you drive the car? when the slave pushes the clutch pivot arm that pulls back on the thrust bearing which is pulling the fingers on the pressure plate back. and the pressure plate is bolted to the flywheel and the flywheel is connected to the crank. i think there is going to be a lot more force on the thrust bearing when you press the clutch in then the torque tube shaft can provide pushing forward.

heinrich,
you tell me that The problem has been examined, discussed and reexamined here and elsewhere for literally decades, it is stil happening today. you tell me there are two thoughts of why this problem occurs, well if it has been examined, discussed and reexamined here and elsewhere for literally decades where is the solution, the fix. how much force do you believe can be created by the torque tube pushing forward? i cant see this as a coinsidence. every 928 that was running on thin oil that we tested had more wear then the ones running on 25-50. every single one of them. im not saying the auto TB wears out quicker im saying it is equal with autos and manuals, but it really should wear the manual out quicker.

my father has an 86 928, first of the quad cam, he saw the car get sold new when orked at a porsche dealership back in 86. he worked on this car everytime it came in for a service, changed the oil every 10 000 kms with valvoline turbo V,( now known as gp50) when he left the porsche dealership, this 928 was one of our first customers( dad was very good friends with the owners). dad has owned this car for 3 years now it has 265 000kms, and had the oil changed every 10 with the same oil since new. we checked the cupling bolt for the first time when dad bought the car. this car has the fourth least worn thrust bearing( via checking with dial guage) out of all the 928s that come into our shop.


remember im just trying to give you infomation, it is up to you if you take it in or not. i love 928s and i dont want to see them have these problems. i guess i could have just left this subject alone because this problem is giving us so much work. in the future when you hear of these problems on a car that you know just ask what sort of oil it is running on?

sean.

 

As many know, Scots 4.7 euro US combo motor is running now and we are breaking it in on 10-40 weight dyno oil

I noticed the oil pressure when warm now is near 3 bar at 2500rpm, and its not until 3000rpm does the pressure approach 5bar. could this be an oil viscosity issue, or an oil pump pressure issue?

when do you think we should change out the 10-40 oil and what is the normal breakin miles?
new rings, new bearings, and thats it. valves , cams, etc are all the same out of the running engine .

mk

 

Very good points Sean but still...

Number of automatic cars with TBF is many magnitudes larger than manual cars. In fact has anyone ever heard of manual car with TBF? Or TBF where front side was worn, not rear.

Reason why there is not easy fix is that factory never took it seriously enough. All they did was widen thrust bearing a little when they shouldn't have never removed circlip from automatic TT's front end. There is or at least was aftermarket fix available in format of redesigned front flex plate clamp but it's too expensive and time consuming to install for most owners.

 

Ah, I see. I agree that wear from this would be minimal, but it is yet another reason to keep one's foot off the clutch when not actually shifting gears. Idling at stoplights with the clutch in is a bad idea for plenty of other reasons.

I think it's apparent that the main reason TBF is so rare in 5 spds is that the shaft really can't push on the crank continuously. So with the crank free to move fore and aft a bit, there is always an oil film on the thrust face. I think that part of the problem with AT cars is that the pressure on the thrust face can be continuous, so oil doesn't flow freely over the thrust face. That has to be especially harsh at startup, when you have all the oil squeezed out and metal to metal contact with significant force.