M. Requin

A simple question from a confused mind: does the crankshaft move forward or rearward as end play decreases?

docmirror

Forward. But then again, the end play never really decreases unless a new bearing is installed, at which time it will be moving rearward as the end play total is reduced.

worf928

Depends on if automatic or 5-speed yes? For autos the crank gets pushed forward and for 5-speeds it gets pulled backwards. Of course, the former is a real issue and the latter not so much.

M. Requin

Ah, should have noted the question was about an automatic- thanks Dave.

1989porsche928

I really can't believe they would design a car that would catastrophically break itself and offer nothing to the victims. This had to have happened before the Warranty Expired and they most likely refused to do a recall.

Rob Edwards

The 'new car' warranty on the later 928s was only 2 years (and 7/70 on emissions-related stuff, only in California...). Would be amazed if Porsche ever warrantied a 928 motor because of thrust bearing failure within 2 years of manufacture.

FredR

Although the failure timeline is not entirely clear, if as I suspect TBF failures tend to manifest themselves after several years and/or some 40k miles onwards, it is not difficult to reason that the problem would be somewhat obvious by around 1992 or so- just at the time when Porsche were close to being bankrupt. A major recall of their flagship model [80% of 928's are autos] to rectify a deficiency like this would have been catastrophic for them- "the straw that broke the Camel's back" so to speak. By 1992 some 15k [Rob?] S4's/GT's had been produced so some 12k examples were automatics- possible lawsuits from owners who had to purchase new engines already and a global recall for those 12,000 examples to rectify at say $5k a pop [$60k million] for a problem that did not have a solution at the time- such would have potential to finish them off at the time I rather suspect.

What I find mind blowling is that they would release the GTS with the same inherent problem. In 1992 [I think] they issued a service bulletin to change out one of the coil to distributor HT leads because failure was leading to one group of 4 cylinders shutting down and owners were reporting drive shaft failure due to the harmonic pulses. If you have ever driven a 928 with 1 cylinder down it is as sick as a dog- exactly who would drive a 928 on 4 cylinders? Talk about "plausible deniability". It is well known that the later drive shafts fail and it is generally assumed it is because of the taper design [which doubtless is a contributory factor] but I suspect that is not the full story in that I suspect initial slippage of the clamp [the common 3mm variety] induces fatigue failure and pops the shaft either at the front or the rear. Once the clamp lets go [either front or rear] and the extension is greater the rearward face of thrust bearing is overwhelmed at some point given it was not designed to be a disc brake.

Quite a saga and easily avoidable. Having [hopefully] stopped the clamp from slipping for some 16 years [and no drive shaft snaps- touch wood] may indicate the above holds but of course for all I know my drive shaft [or heaven forbid my motor] could let go any time and then a rethink.

As with any engineering failure investigation one can only look at the various signs and symptoms thrown up to theorise the root cause of failure. If we can theorise such conditionality surely the vastly superior folks who designed and built them can?

Back in 1999 when my drive shaft failed Porsche told me that these cars were designed to run on 4 cylinders so as to get you home. I was also told by a local rep when I [disgustedly] challenged why such should fail at 84k km "I can break a drive shaft in 5 minutes"-- enough said?

Rgds

Fred

worf928

Seriously? Did you neglect to include emotions? Or, did you intend this as a serious, thoughtful comment?

Mongo

Could be worse... you could own a 996 with an IMS issue.

When it comes to the pinch collar bolt, just break it loose before you drain your car's oil the next time you do an oil change. The collar will either make a loud bang or slowly relax back into place as your oil poors out of the pan.

Apply loctite blue and tighten the bolt to 66 lbs/ft, then proceed with the rest of the oil change.

Simple as that to prevent TBF. Although some people pry the flywheel back, I got no movement at all when I tried this, and this still confuses the hell out of me as to why.

M. Requin

[My ed. Martin]

This was the somewhat hidden motive behind posting this thread. When I first did my front flex plate release and tighten, I followed this thread http://www.nichols.nu/tip598.htm, including the use of Loctite 270 and a paint "witness" mark. I also had to hand the article in Shark Bit, Paul Jager's collection of notes, by Richard Andrade and Dan Warner on this subject. Neither article mentions prying the flywheel back before tightening, so I didn't.

Fast forward. After all the conversation recently about the rear pinch bolt, I thought it was time to check it (derelict is my middle initial...). I had the 928 on a lift anyway so it was easy (and BTW even with the new Volvo MM's I had no trouble moving the heat shields out of the way). Here is what I found: first, the front flex plate was flat, and the paint marking showed no movement. Using a dial indicator I measured free play when i levered the flywheel rearward to be .008". [And a big thank you to Leon Speed, who noted that the rod on his magnetic dial indicator base was threaded 8mm and could fit in the bell housing mounting bolt holes- genius!]

The rear flex plate coupling was centered in the inspection port, BUT when I went to remove it (replacing both front and rear bolts) it was tight all the way out, indicating interference. When fully removed, the bolt showed thread damage and nicks by the shaft splines (pictures this PM) indicating the clamp had moved. The nicks were bright and clean, and did not indicate pounding over time. After I removed the bolt, and worked the clamp a little with a screwdriver, I could slip the new bolt in and tighten it with my fingers, indicating I hope that the damage had just begun.

Comparing free lengths of the front and rear pinch bolts I found them to be identical but slightly longer than the new ones (actual dimensions with the pics, later). I attribute this to production run differences.

So to the big question. Everything looks fine, EXCEPT when I pry the flywheel rearwards to check clearance. I am prying against the flex plate, which then rebounds to rest. IOW, the .008 clearance seems to be in the wrong direction. Before I start burning acetylene (500F on the coupling will not be a big deal, and if I have to do it I'll post pics) I would like input from anyone who has an opinion on this.

Captain_Slow

Martin - I have a new bolt. Can't remember if I bought it to replace the front or the rear. Are the front and rear pinch bolts identical? This thread is going to be a valuable one for many. Looking forward to the pics.

SeanR

All one has to do is look at his previous posts everywhere on Rennlist and then ya figure out he's either well versed on the ability to troll or ........ well, you know.

FredR

Martin,

What you have described reads pretty much OK to me. To check the crank float you need to push it all the way forward that you can, zero the gauge and then push it all the way back and if that gives you a reading of 0.2mm then you are fine.

When you set the clamp I presume you had no pre-load on the flex plate. Not sure if your model year has the pre-load condition or not. I suspect it is like the S4/GTS. If so I understand it does not really matter if the crank is at the full forward or full rearward position as the flex plate will easily absorb 0.2mm of axial travel.

Trust i have interpreted your message correctly.

Rgds

Fred

M. Requin

Hi Jon- ordered a couple of new ones before I started, so I can compare. I need to make measurements and pics, but it may be tomorrow AM before I can post them. The new ones are identical to each other in length as are the old pair, FYI.

Captain_Slow

Thanks Martin - I think I bought it to replace the rear bolt. I'll do the rear and order a new one for the front. I haven't observed any problems with the front bolt (original), but I've removed it and retorqued it three times (once when installing the P-Klamp, again when doing motor mounts, and one more time when doing trans mounts. Seems like cheap insurance to put in a new one.