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Check them peeps. Got a 47k mile car here that I have been taking care of for 3 years and has run like a top. No odd noises, the front flex plate has never shown any bowing and on two checks didn't show any deflection that needed tending to so was never worried about thrust bearing failure.
A few weeks ago the owner was driving it and all of a sudden the car started to make some very unsettling noises, could not go forward or backwards so he parked it until I could get it in the garage. Once here I pulled the front bell housing cover and all looked good, yanked the heat shields and the plug for the rear pinch bolt hole and could see that the torque tube bearing had migrated to the rear and was sitting on the coupler and the trans imput was not moving. I've had two that did this and both of them were broken torque tube shafts. Expected this to be the same but after taking things apart a few things caught my eye. paint marks on things that normally didnt have paint marks, loose torque tube bolts and the torque tube was 1mm away from the transmission bell housing.
At that point I felt a bit sick, someone had been in here before and had fvcked up. Checked the pinch bolt...........loose.
Got the rear suspension out, trans and TT out and separated them and this is what I found.
Not only do the TT bearings need replacing, but the entire shaft does and guess what? This one is not worth the core charge so the entire thing goes in the trash. Someone made an expensive mistake.
Semi-retired, as of Feb 1, 2023.
The days of free technical advice are over.
Free consultations will no longer be available.
Will still be in the shop, isolated and exclusively working on project cars, developmental work and products, engines and transmissions.
Have fun with your 928's people!
While you are under there check the front one too. When I bought the 88 first thing I did was check the thrust play and release the flex plate tension. Once I got the allen bit on the pinch bolt it had about 20 ft lb of torque. I felt kinda sick. Lucky me, Rog had a new pinch bolt in stock and I was on the road quickly.
I'd go further than checking them .. replace them .. they're cheap insurance against this happening.
Here are a few pics of damage to bolts, and splines, as the shaft slides, and rams the bolt into the splines.
The red rust is symptomatic of the damage when you pull them out, along with difficulty in removing them, because they're jammed against the splines.
After the bolt has loosened/stretched, you get rotational slop of the splines, which will eventually lead to what Sean posted .... holy moly! that is the ultimate damage.
I don't have the baseline measurements of the damaged bolts when new, but the bolts I have removed have all been longer than new ones, which leads me to conclude that they stretch, and therefore should be routinely replaced.
Wow Sean thats a sheared splined shaft,
might you have a picture of the input shaft.
Was the pinch bolt hard to remove?
I'll get some pictures in the morning, I am glad that the trans side is stronger than the shaft side. No, the rear pinch bolt was not hard to remove, it felt finger tight.
Originally Posted by docmirror
While you are under there check the front one too. When I bought the 88 first thing I did was check the thrust play and release the flex plate tension. Once I got the allen bit on the pinch bolt it had about 20 ft lb of torque. I felt kinda sick. Lucky me, Rog had a new pinch bolt in stock and I was on the road quickly.
This is sad to see on the torque shaft.
Front side was the first place I went to as it was the easiest one too look at, all was good there.
Wow Sean thats a sheared splined shaft,
might you have a picture of the input shaft.
Was the pinch bolt hard to remove?
Stan,
I may be completely wrong here but I suspect Sean's original photo may be a little deceptive n that it is a wide angle lens used close up at an acute angle- this distorts the perception of depth somewhat.
If I understand what has happened correctly the bolt has loosened and the clamp eventually lost grip causing the shaft to spin inside the clamp. If you look at the shaft surfaces close to the bolt they appear to be covered with a layer of rust whereas towards the end of the shaft the splines are polished- presumably from spinning inside the split sleeve collar that opens axially rear to front. If this analysis is correct then the inside of the sleeve may also be damaged.
The interesting bit now is whether the bolt "loosened" causing some vibration resulting in the rear most of the two support bearings to start moving or- did the rear support bearing fail due to age hardening of the casing and thus "migrate" rearwards causing loss of radial support for the shaft which in turn caused the shaft to vibrate and thus induce stresses in the bolt- that in turn caused it to deform plastically [i.e. stretch] and thus lose tension?
It will be interesting to see what the consensus is on this one and whether or not the owner had some "strange vibrations" prior to losing drive.
This is not an uncommon failure when the pinch bolts of the couplers are not tightened to their correct torque values.
The failure begins with the splines beaten to a knife edge and then they strip as Sean's picture show.
This problem also happens with 944 drive shafts when the rear pinch bolts of the coupler are not tightened correctly.
Attached is a picture showing the transmission end of two 944 drive shafts, one showing healthy splines on the right and the left one showing the knife edged splines, before they strip.
At this point we would also check the input splines of the transmission coupler to see what they look like. We would probably replace the rear flex plate/coupler just in case.
By the way, we also carry replacement drive shafts for both the 928 and 944 platforms.
I have been warning about the rear coupling bolt stretch for years & years & years, if its not checked periodically , then the rear coupling gets loose( bolt stretch) & because its just one bolt & hidden ( auto trans version ) behind heat shields , no body knows to check the tightness of this one bolt
And yes this is the point ( area) where the T Tube shaft migration starts , after all you can not get the shaft to move forward without it pulling out of this rear coupling ( all Very simple stuff)
Thats why we have not had an issue with this area for many many many years because we keep checking ( re torqueing the rear coupling bolt) on major service intervals( and replace the bolt every few years or so ) & yes on the 928 manuals( all ) and 928 Auto 4 speed onwards we nearly always see the rear coupling index bolt loose on cars we have never seen before & the bolt was not loose in the sense it had unwound , it had just stretched( like all bolts do under duress & time) & with the removal of this bolt ,sometimes one can see the teeth marks from the splines cutting into the flank of the bolt , its just a stretched bolt because they ALL do it ( see Photo's that Dave 928s put up ) I have seen plenty of these over the decades
Note } The 968 ( last of the 944 series has 4 of these "931" part numbered bolts ) on a sliding sleeve that joins two solid splined shafts thats why they give no issues at all , but the 928 Auto 4 speed onwards & the 928 manual ( all ) has a very sloppy design of having the solid splined shaft going into a very weak " CUT" in 3 places input shaft( What Were They Thinking?) with the single split coupling over the top with one lousy bolt , no wonder they stretch this one bolt that is hidden out of the way
This is really the only weak area ( in engineering terms) that I have been concerned with for decades , its strange , because Porsche had an early solution all along , the 3 speed auto version is just like the 944 , meaning it has two solid splined shafts meeting & over the top there is a quite robust female splined sliding sleeve( same sleeve that is used on all manual trans 944/951 models) with two of the 931 part numbered index bolts , this was not a bad design , but the manual trans 928 ( all ) & the 928 4 speed auto trans ( all ) was a strange backward step
But this poor design can be managed ( like we have done for years & years & years ) we just look after the rear coupling & this also takes care of T T shaft migration
Its so simple I am just amazed at the weird theories that abound about this ( under engineered ) design
Regards
Bruce Buchanan
Buchanan Automotive , Working on Porsche Cars since 1977
Last edited by JET951; 06-18-2015 at 07:34 PM.
Reason: spelling mistake
06-16-2015, 09:10 PM
Rear Pinch Bolts, check 'em.
