hacker-pschorr

So how many total is that out of how many 5-speeds we think were manufactured and do we know this was caused by starting the car with the clutch disengaged or just clutch action in general?

I'm not saying anything is or isn't possible, just curious. TBF with 5-speeds so so rare, why have we not seen more higher mileage 5-speed cars with TBF? Could there just be some other fluke that caused the wear in Dennis's car here?
Seems simple, put on a dial indicator and work the clutch, see how much the crank moves. I'll do this myself once I get a chance to swap cars on my lift.

Could this be a case where stronger pressure plates could make it worse? TBF is almost nonexistent with early cars, what makes the double disc clutch less likely to cause it?

As we know, if an automatic trans / TT job is installed with too much forward pressure from the get-go, that can exacerbate the issue. Is this also possible with a 5-speed?

I think we need some additional data before we start scaring the bejesus out of every 5-speed owner.

ptuomov

Yes. There are fluke (not systematically repeating across cars) failures everywhere, so I wouldn't get too excited about one or two failures.

An additional consideration is that observed failures that make sense based on logic are both more likely to be systematically repeating and to be fixable. As a purely hypothetical example, consider the following scenario in a five-speed car: The tuned mass damper becoming loose inside the torque tube, then moving with car accelerations to bang the bearings further away, the longer unsupported shaft section starting to resonate at some rpms, the shaft resonance and other forces causing the rear clamp to slip, the shaft pulling out of the fifth gear, and the fifth gear eventually breaking (NLA now for '87 S4 at least). This process of the shaft pulling out of the fifth gear could theoretically also put pressure on the thrust bearing on the rear side (the same as in automatics, but the opposite side of the front failure that may sometimes be caused by the clutch pressure in 5-speed cars) and cause the thrust bearing to fail. Is there something in this hypothetical chain of events that is outright implausible or impossible?

worf928

Mostly-different stuff in a 5 speed. The drive shaft is not attached in any way to the flywheel on a 5-speed.

The rear arrangement is the same for 5-speed and auto. However, unlike the front clamp on an auto, in the rear the shaft has a 'neck' (can't remember term of art right now) cut into it near the end that captures the rear pinch bolt. The bolt itself will prevent more than a few mm of migration of the shaft in the clamp. In a 5 speed since there's no front attachment of the shaft, there's no axial force on the shaft.

See above. I can't - quickly - think of a way for a 5-speed to do what you describe. If you remove the rear pinch bolt you could theoretically crow bar the shaft forward until the nose of the clutch intermediate shaft is pushing against the pilot bearing in the flywheel but you wouldn't be able to reclamp the shaft at the rear due to the 'neck' at the end and thus the shaft would just slide back a bit. You'd wear the crap out of the pilot bearing and loose pinch bolts (three on a 5-speed) will wear the crap out of the splines on the drive shaft and intermediate shaft.

worf928

I've got two clutches out right now and one empty engine bay showing the nose of the 5-speed drive shaft. You know were I am. So, if you want to stop by and ponder the vicissitudes of the 5-speed drive line guts lemme know.

ptuomov

Thanks for the offer.

I was thinking about a scenario where the driveshaft moves forward and pushes on the clutch intermediate shaft. This was a theoretical scenario, so it might not have any practical relevance (another way of saying it was all BS). The remaining used fifth gears are in storage, but from a photo I can see a bolt groove on the fifth gear and another bolt groove on the shaft. Do those grooves prevent any relevant amount of driveshaft movement?

So the 5-speed thrust bearing failure in the stock direction clutch would only happen on the front side of the bearing. Whatever driveshaft problems we'd see, those would manifest themselves with worn or broken fifth gear.

FredR

How about "detent"? '

Not to be confused with the derived from french political term "Détente" .

worf928

Assuming we're not talking about "Hulk? Smash!" I can't think of what forces would push the driveshaft forward on a 5-speed.

The 5-speed shaft is free to wind and unwind without putting axial force on the rear clamp. Perhaps if the shaft 'grows' as a result of winding and unwinding, this might, eventually cause forward pressure against the pilot bearing.

I won't opine on 'relevant' movement. The grooves do prevent more than a millimeter or three of movement. Three millimeters may be 'relevant.'

When installing a rebuilt TT it is usually the case that the drive shaft position needs to be 'adjusted' slightly to get the rear pinch bolt in. It is very tight fit.

For a stock 928 5-speed I think this is the case.

Definitely off the edge of my world except for loose pinch bolts.

I've taken apart two 5-speeds down to the thrust bearing. (Your old fire-damaged block may have been a third but I don't remember.) One 80k motor, one 120k motor. There was copper showing on the forward TB surface on both. I posted pictures of the 120k TB on the random picture thread. Below again. I could probably measure wear if I can be motivated to search through the boxes of parts-to-be-discarded.

worf928

Close. I would say that the coupler on fifth gear has a 'detent' to allow the bolt to pass. But, the 'neck' or 'groove' circumscribes the entire shaft (else the shaft would need to be clocked within one spline of the coupler which would be... irksome. Very.)

Groove (Circular) is probably the thing...

FredR

Dave,

I have used the word "detent" to describe the rear pinch bolt restraint many times without much further thought.

My general understanding of the term is "a device to stop or restrain movement of one system component relative to another component in the same system". Whether I have understood this correctly or not remains to be seen. Falling asleep at the moment- might look into that tomorrow just out of interest.

worf928

I think we all know - at this point - what we mean. I enjoy knowing the most-correct terms for things. However, I’m not the grammar police. But I don’t fear them as they rarely conduct unwarranted searches and generally don’t make a practice of civil forfeiture.

M. Requin

"Annular groove" may be the term you are looking for.

Bill Ball

I'm not trying to scare anybody with a 5 speed into thinking TBF is likely, although I see Greg B has reported it and it CAN happen.. I'm not the first to report the observation of visible wear to the front face of the thrust bearing. See Dave C's pics above. I think that IS common. I think crank endplay should be checked "periodically" in 5-speeds. I only check it once a year or so in my automatic, now less so with the PKlamp installed and no movement of the TT shaft. My endplay has never changed from what I first measured when I bought the car 20 years and 216K miles ago, maybe because I was pretty religious about releasing any accumulated preload once or twice a year. So, what should 5-speed owners do? Maybe check endplay once sometime soon, then anytime the clutch is exposed since it's dirt simple to do WAIT. Can't hurt to verify everything is hunky dory (sp?).

hacker-pschorr

Sorry Bill, I didn't mean to come off as dismissing anything. I'm more or less thinking out loud here since this is potentially something we need to get ahead of if it is in fact becoming an issue?

Since the discussion of the automatics has been beaten to death, why don't we get going on beating this one too?

Let's come up with a list of every potential way a 5-speed could damage the thrust bearing.

1. Starting the car with the clutch pedal depressed
2. Sitting at red lights with the clutch pedal depressed (this is currently my favorite)
3. Some inherent flaw with the installed bearing and / or assembly of the engine
4. Issue with the torque tube expanding / contracting as we've discussed with he automatics, putting unnecessary pressure on the crank
5. Same as #4 but maybe the issue is caused by aging / failed / moved torque tube bearings
6. Clutch failure that somehow puts more pressure forward under normal use but doesn't affect operation in any way the driver can notice
7. Modified cars (larger tires, bigger engines, boosted motors etc..) that put more stress on the drive-train causing #4 to happen
8. A complete random fluke not to be bothered by. I like the idea of measuring the crank whenever it's convenient, but if it's out of spec or close to...now what? What caused it & what can be done to limit further wear?

I'm skeptical on #1, I think we would be seeing way more of these, especially with the early cars where 5-speeds were produced in far greater numbers.

Addressing #2
I recall way back in my early years of being around 928's (mid 80's or so) it was a "known" thing to never sit for an extended period of time with the clutch pedal depressed as this would prematurely wear out the release bearing. This came up when shopping with my father for his first 928, since this was pre-S4 days or they were so new my father wasn't considering them. Clutch issues were really the only issue we were regularly warned about to watch out for back then. The "tick-tick-tick" sound of a failing release bearing was all too common. That was ingrained in my head and to this day I never sit at a light in any manual car in gear with the clutch depressed.
I've always pondered how many 928 owners do the same.

I don't know where I'm going with this, random thoughts I decided to toss on the fire.

worf928

I do not see how you can believe in #2 and disbelieve #1. The physics are the same except that for #2 you have ~2 BAR of oil pressure whereas in #1 there is 0 BAR of oil pressure. Hard to argue that there isn't more engine-run time occurring at stop lights. So, if you had to choose between #1 and #2 choose to do #1. Not that we have to since ...

... with no clutch lock-out I suspect that most 5-speed owners start with the clutch engaged, foot not on the pedal. We could poll this.

I'm skeptical of #2, not that it causes wear, but that most do this in a 928. See OT thread on He-Man clutch pedal force. If 928 5-speed owners routinely disengaged the clutch at stop lights a lot more of us would have left legs and left calves twice as large as the right (and tendonitis in the left knee for '90+ owners that haven't removed the anti-submarine pad under the cluster.)

Strosek Ultra

I have been working on engines for more than fifty years and I have never seen a TBF on a manual car. As a matter of fact the only TBF I have seen is on a used 928 engine (automatic) I bought years ago. Right now I have two BMW six, one 3,5L big six and one 3,8L high performance, both manual, apart. Both engines show copper on the thrust bearing but the crank end play was within specs. I am sure you are aware of the 928 factory thrust bearings being larger than the ones from Glyco.
Åke