I never said disbelieve, I'm skeptical that's the primary reason.

Why? It takes mere seconds for the car to start up and usually you let the clutch out to get moving or put it in neutral. I cannot recall ever starting the car and just sitting there with the clutch depressed. If this is a common thing, it combines 1 & 2 at the same time. Double whammy.

If you live in Los Angeles and spend 60 minutes in stop and go traffic every single day each way, and you sit there with the clutch depressed for possible minutes on end.....on a hot July day with oil pressure pinging around 1psi, what is that doing to the thrust bearing?

In my opinion it's more likely the vast majority of 928's have been started multiple times over the past 40 years with the clutch pedal depressed versus the number that may have spent a significant amount of time in severe stop traffic with an owner who rides the clutch on hot days. Yet we don't really see that many TBF's with 5-speeds. The known number is actually statistically irrelevant as far as I'm concerned, but here we are discussing it.

Again, early vs late cars too.
The lack of TBF with early cars has made it "known" the cir-clip configuration makes it unheard of for those automatics, but wouldn't shield the 5-speeds from the damage. Yet with the vast numbers of early 5-speed cars, we don't see those either.

Another theory that reinforces your idea. Did the 928 ever get a clutch switch which requires the clutch to be depressed when starting? If yes, and we only see failures in years starting with that model year.....bingo.

However, what makes me the most skeptical about the startup theory is almost every car made over the last 20 years has a clutch lock-out switch requiring it be depressed upon startup. The 928 is not unique in the design of putting pressure on the crank when depressing the clutch. Yet we don't see TBF as a common thing with any other car with hundreds of thousands of miles on them. Be it VW, Honda, or other more modern Porsche's. Would the 928 be unique due to an inherent flaw in the thrust bearing design?

This also shoots holes in #2 since we also don't see thrust bearing issues with other cars that spend god awful amounts of time in traffic with owners I'm sure sit there with the pedal on the floor.

Yes I know there are theories all over the internet about this, at the same time Honda's and VW's with 300,000+ miles is not uncommon and every single start the clutch pedal was depressed.

 

Sure. Since we (the rennlist collective) have seen few 5-speed TBFs it doesn't seem like there's a primary reason. It's speculation as to relative contributions to TB wear.

I suspect most folks put it in neutral and make use of the lack of a clutch lock-out.

One difference between #1 and #2, though is the oil film. Lot's of 928s - at least my 'client fleet' - see few starts per year. With weeks between starts there's plenty of time for the oil film to be almost non-existent. There is, of course, the variable of the little 'pockets' in the OE bearing that will hold oil forever.

I think that's an extreme example as most aren't going to have the leg stamina to keep the peddle down for minutes over-and-over again. Me? When I get accidentally caught in stop-and-go I'm spending most of my time in neutral with the clutch engaged with folks behind me going ape $h1+ because I won't close a 6 ft gap.

1 PSI? You mean 1 BAR. Nevertheless, 1 BAR hot idle pressure is a cause for concern period. It's not enough to keep the lifters going on a 928. Engine will be clickity-clacking itself into an early grave.

But, set that all aside as we're trying to parse out minutiae.

It's safe, I think to say, that starting with the clutch disengaged is bad and sitting, idling, with the clutch disengaged is bad too. I'm sure there's some equality between the two such as 1 dry start has the same wear as x minutes idling. But, without data or extensive experimentation we can only speculate.

This is likely population specific. In my area, 928s don't get driven in stop-and-go traffic except by accident. I'd wager that the ratio of dry starts to idle time is much higher for the 'fleet' up here.

I've never once encountered a 928 with a clutch lockout and I've driven nearly every model year from '78 to '95.

But, that comparison population are daily-drivers. They rarely sit long enough for the oil film to fall off surfaces. Essentially, they never get a dry start.

I'd wager that a 5 speed 928 with 400k miles driven several times per week would show equal or less TB wear than a 40k mile 5-speed that's driven twice per year.

 

The 5 speed driveshafts can migrate forward due to a loose or stretched rear pinch bolt. Luckily in my case no damage was done and endplay is within spec.

 

Since you are claiming ignorance (I think you are far smarter than that, but I will play along) -

I can't imagine any scenario where the intermediate shaft could push inward on the crank. Not for an extended period of time. The clutch is free to slide back and forth on the short shaft. So, unlike the clamp on the flex plate, any 'push' would relieve itself immediately. If I understand correctly, part of the auto issue is that the shaft slips in the clamp, but then 'stays slipped' (so to speak). When the clamp is loosened, the flex plate moves itself back to unload the forces. In a manual, the shaft is never clamped to the fly wheel.
Also, if there was some sort of way the TT shaft could push forward and 'stay pushed.' against the motor, the pilot bearing would take the brunt of the force. I would guess that the pilot bearing would be destroyed by those forces long before any damage would be done to the thrust bearing.

I don't. I've actually started my 944 a lot more without pressing on the pedal than the 928. The 944 had a crank sensor issue (the connector actually), that would give me a fairly regular no-start. A good wiggle on the connector would get it started. So I got used to reaching in and cranking it rather than sitting down in the seat (saved the effort of getting back up).

I've driven manual cars much of my life. I didn't own an auto from 1992 to 2015. My work trucks have been manuals for 19 years (I had to raise a pretty big fuss to get a manual a few months ago - my company isn't buying any more manuals - ARRRRGGGHH!!!!!).
All except the 944 and 928 had pedal lock outs. When I first started driving truck, the big trucks didn't have them, but they do now.

So the habit of 'clutch to the floor, turn the key' is ingrained pretty deeply. Since this thread became active again, I've been trying to not push the pedal down when I start the cars, but I've been forgetting most of the time.
Working on it, but it's not easy to change a habit that's been in there that long.

 

Yeah, I was posting an obviously clueless post there about the front of the manual driveshaft. I was focusing on the rear clamp and the fifth gear which is NLA for g28.13.

 

What does it mean if you cant get it to move in either direction with both the front and rear pinch bolts removed?

 

if you cant get what to move?
Can you add detail to this question of exactly what your doing please.