marine928

Thanks to Sean for again bringing this to the forefront. Many of us newer owners <4 years are aware of the front flex plate issue due to all the press on it. It seems that all the focus is on correcting that issue and I don't recall the rear bolt being addressed as serious. PPI does not mention it, or maybe I overlooked it.
I for one, replaced all my components with Constantine's products a couple of years ago. I also replaced the "rivets" with bolts in the rear plate as well and tightened the rear pinch to 80ft lbs. Overtighting a bolt probably contributes to bolt stretch and is more than likely just as bad as under tightening especially in this application.
Having C's components gave me a false sense of security so I have been lax on checking the front flex plate. ( did check 6 months after install and it was perfect).
Based on this post I am going to check EVERYTHING front and rear. PITA but worth it.
My question is what about the bolts that replaced the rivets. Any ideas on the possibility that they could loosen up. We are dealing with multiple bolts with lock washer and red lock tite.

Lon

FredR

That torque figure looks worryingly high for a bolt of that size- hopefully it came from a reputable source and is well matched to the bolt metallurgy.

Overtorquing a bolt may not plastically deform it immediately but one has to be mindful of "creep" [cold deformation] that may be induced as a consequence.

Now that our cars are getting somewhat old highly stressed components are more likely to be vulnerable to this phenomena and some items more susceptible than others.

Rgds

Fred

bureau13

Dave, thank you. That's perfect, I'm pasting that verbatim into my ever-growing to-do list!

marine928

Thanks Fred, I am going to recheck everything next weekend. So what bolt should we purchase to replace the original rear pinch bolt.

Lon

FredR

Roger does them [see his earlier post]. I think replacing this bolt every so often is a good policy. Remember that these things in most cases are still original and there is only one of them.

Porsche advised me some 16 years ago to renew and over tighten the front bolt by 10% when we could not get the front clamp to hold- needless to say it did not work but the Loctite on the splines [not the bolt] did.

Presumably there is some mechanical margin in the bolt design but whether using that up is a good idea may be another thing altogether.

Rgds


Fred

marine928

Thx Fred,

I'll p.m. him and have one sent.

Lon

Black Sea RD

Hi Fred,

From memory, didn't Porsche also advise your mechanics to change out the front clamp/flexplate piece with a new one, besides the 10% over value of the torque for the front pinch bolt?

Cheers,

Bertrand Daoust

Thanks for posting this Sean.
I'm sure it will be very use full in the future for anybody looking for some info on this subject.

Don't worry, Rennlist is the best place to post great info like this.

... Someone once told me I was an imbecile because I didn't want to pay 35$ for shipping for a t-shirt, so...

Keep up the good work my friend.

Captain_Slow

+1 re what Bertrand said. Sean, Stan, Greg, and a few others on here are the only ones who work on (many) 928s enough to observe failure trends first hand, and provide examples like this.

FredR

Hi Constantine,

I cannot remember the specific sequence of events but invariably they recommended changing everything.

When the drive shaft failed [1999] there were no signs of slippage at the flexplate but I suspect it had occurred and was the prime cause of failure- slippage- longer length- pressure on the flex plate- induced vibrations - leading to fatigue failure. One of their chumps actually had the audacity to tell me there was nothing wrong with the transmission design and that he could break such a shaft in 10 seconds! Well I suppose you could if you were an utter tw*t - but there again he was from L.....! I came very close to physical violence with him for implying it was my driving style that caused the problem.

We inspected the flex plate carefully for signs of damage- there were non and the same unit is still in service in my current driveline. With the new torque tube installled they could stop the clamp migrating along the shaft. Amazing how the parent company denied there was any problem but within 10 minutes of being approached for a solution Stuttgart came back with the replace the bolt and over torque by 10% solution- and that did not work either. I took Earl's Loctite 290 solution to them and it is still working for me today albeit second application after I lost the 90 S4.

Regards

Fred

Texbuckeye

Sean,
Thank you for getting it all fixed up. I knew the risk something like this could pop up when I bought the car. I had no inkling of any problem until it broke. It is shifting better than it ever did now so I am happy.

docmirror

DO NOT! Check your rear pinch bolt!!!

No, no, no.

Call Roger, order it, and replace it then have it torqued to 10% over factory(or take it to Sean, hehe).

Mine was evaluated today and after 27 years, it only had ~20ft lb of torque left and it had stretched about .4-.6 mm. Just replace it darn thing. Pay $12 now, or $4000 later.

Dave928S

^^^ THIS!!

I found all the old bolts that I referred to in an earlier post, and can confirm that measurement trends of those, back up my previous measurement of 0.45 mm extra length of old bolts compared to new ... which parallels what Doc reports.

Average of new bolts 54.60 mm ... old bolts 55.05 mm.

Is it stretch? ... as the old saying goes 'if it quacks like a duck, and waddles like a duck ... it's probably a duck'

Can you post a pic of the old bolt if you have it Doc? Anyone else have pics or measurements?

depami

Thank you docmirror for confirming what many suspected. I guess after 27 years it's more than done it's duty.

And thank you Sean for sharing this extreme failure example. Sorry I blew up.

docmirror

Sean took a comparison pic, old and new. I think he'll be along tomorrow.

Man, get those things checked. I saw the bill from the last car that had a failure. Really cheap insurance here.