ptuomov

Mark Kibort apparently had the same kind of shaft failure on a running engine on a dyno, based on a PM.

DR

Found this one from 2011, also with 5th gear issue, not sure if it is that one...of course Mark could have more than one tranny replacement..LOL Just kidding..

Here is a quote from the thread...
Link to Thread: https://rennlist.com/forums/928-foru...-swap-out.html

Page 3 is where we finally find out what was wrong....once it is opened up, duh!!
https://rennlist.com/forums/928-foru...ml#post8323593

Hope that helps,

Dave

ptuomov

GregBBRD

Tough to know, definitively.

I've seen a large amount of worn torque tube shafts and worn splines inside 5th gear, from the two pieces moving against each other. There is generally no broken "leaves" from 5th gear, when this occurs. As I mentioned above, this is always accompanied by rust.....the metal powder that is ground off oxidizes very quickly. I've never been certain if this is an inadequacy of the clamp, or if someone left the clamp loose.

Another factor could be bearing migration or shaft harmonics. I can envision the driveshaft moving in a "wave" at that end of the shaft, in either scenario. No amount of clamping is going to keep the two parts (driveshaft and 5th gear) from moving in relationship to each other. I firmly believe that Porsche knew what they were doing when they installed the damper into the torque tube, near the rear, which helped absorbed these harmonics. As the energy transfer pieces wear (the rubber deteriorates) the harmonics of the shaft will increase. And of course, the current trend to just remove the damper is a terrible idea, in my mind. (Read what Porsche went through to engineer the "fast shaft" in Project 928.)

Broken "leaves" are rarely associated with rust. I think that this damage generally occurs on disassembly or re-assembly. (I can also see this happening from harmonics.) I believe that these "leaves" can be fractured and not completely separate until a later day.....although most breaks are "clean" without any evidence of initial cracking. (Although it is tough to tell much from a picture, it appears that the one in this post is "clean".

Kevin in Atlanta

The only other break I've seen that was identical to this was on an 90 S4 automatic rear flex plate. The customer complained of a rattle. Previous history on that car was a monster of a rear end hit that cracked the front upper bell housing, both transmission mounts and differential cover. In a testament to how stout our cars are it drove straight as an arrow. The drive shaft in the torque tube was quite loose and the splines were so worn that Mark held back the core charge.

Kevin in Atlanta

Novice question: Does 5th gear determine 928 Final Drive ratio?

dr bob

No. In top gear, the drive passes straight through to the pinion. It's the R&P ratio from there.

Kevin in Atlanta

And that is in the differential.

Thanks for the clarification, dr bob. (got it right this time.)

Kevin in Atlanta

Per the PO: "It worked fine before it broke. It broke after I merged onto the highway and shifted into top gear - I wasn't even driving it hard . I think think had about 110k miles on it when that happened."