Notices
928 Forum
Sponsored by:
Sponsored by: 928 Specialists

Review: Thrust Bearing/Crank Endplay Check at 125k

 
Old 02-20-2019, 06:28 AM
  #16  
FredR
Addict
Rennlist Member
 
FredR's Avatar
 
Join Date: Jun 2005
Location: Oman
Posts: 5,814
Default

Originally Posted by chart928s4 View Post
Purest hearsay, but I don't think that the crank end play and the shaft snapping have much to do with each other. I have one data point:
The two have nothing directly to do with each other- quite the opposite or so I suspect- and that was the entire point of the discussion. When the clamp slips the shaft is put into tension- at some point one of two consequences occurs- either the shaft eventually snaps or the thrust bearing goes- I have never come across an example where both occurred simultaneously.

The theory is that there may be one common root cause and that is the slippage of the clamp and then the question becomes why did the clamp slip on some examples and not on others. If the taper shaft design were the root cause then all examples would fail but they do not and far from it or so it seems. One possibility is that a bearing migrates and that throws everything out of kilter as a consequence but in my case that did not happen as the bearings were exactly where they were supposed to be.

The trouble is no one has been able to categorically define the failure mechanism but clamp slippage is invariably in the mix..Stop the migration and neither problem occurs or so it seems.
FredR is offline  
Old 02-20-2019, 09:13 AM
  #17  
Mrmerlin
Petie3rd
Rennlist Member

This Post Sponsored by:
Dow Corning® & DeoxIT®
 
Mrmerlin's Avatar
 
Join Date: May 2004
Location: Philly PA
Posts: 22,609
Default

OP from your post I do not see you mention that you removed the rear pinch bolt and inspected it,
or verified that the drive shaft is properly centered in the rear pinch bolt clamp.

If this task was not done then you should go back and do it,
tightening the front pinch bolt is the last step of the inspection.

NOTE you will probably find the rear bolt is loose.
NOTE before tightening the bolts make sure the drive shaft cutout is centered in the input shaft of the transaxle.
Then use a drop of blue loctitie on the bolt threads and torque it to 66 ft./lbs.
Then pry the flywheel to its rearmost position then install the front pinch bolt.
Mrmerlin is offline  
Old 02-20-2019, 04:55 PM
  #18  
Jason89s4
Addict
Rennlist Member
Thread Starter
 
Join Date: Jan 2019
Location: Dallas
Posts: 124
Default

MrMerlin,
I did not perform any inspection or measurements involving the rear pinch bolt. It is on my lengthy to-do list, which I try to work through in order of priority. Therefore, I first performed what seem to be the most reported observations and preventative procedures regarding clamp slippage detection at the flex plate, and the associated endplay measurements.

I do have a question regarding your comment on the rear pinch bolt likely being loose or the driveshaft not being centered. Isn't this most likely if rear pinch bolt had been disturbed at some point? (eg. during a driveshaft or transmission service.) In the instance of a rear pinch bolt has not been touched since it left Germany, have there been findings of concern in the rear, such as those well-documented at the front?
Thanks, Jason
Jason89s4 is offline  
Old 02-20-2019, 06:51 PM
  #19  
Mrmerlin
Petie3rd
Rennlist Member

This Post Sponsored by:
Dow Corning® & DeoxIT®
 
Mrmerlin's Avatar
 
Join Date: May 2004
Location: Philly PA
Posts: 22,609
Default

The rear pinch bolt gets disturbed when the engine is started.
After many cycles the bolt will stretch and thus get loose,
if your bolt is not damaged on the shank then its safe to reuse it with a drop of blue loctitie.

At this point you should retrace your steps and inspect the rear pinch bolt ,
with an auto this means moving the rear heat shields apart to have a look at the rear pinch bolt .

Since your taking the time to document your work,
it would also be wise to include the fact that have indeed inspected the rear pinch bolt as part of the crank end play inspection into your first post,
as this is a very important part of the drivline inspection.
I will edit this post when you update your first post to include this work.
Mrmerlin is offline  
Old 02-21-2019, 01:52 AM
  #20  
FredR
Addict
Rennlist Member
 
FredR's Avatar
 
Join Date: Jun 2005
Location: Oman
Posts: 5,814
Default

Originally Posted by FredR View Post

When the clamp slips the shaft is put into tension- at some point one of two consequences occurs- either the shaft eventually snaps or the thrust bearing goes- I have never come across an example where both occurred simultaneously.

.
A correction/clarification to my earlier note [thanks Dave]- the shaft is under tension during the "pullout phase" when the shaft is wound up at max torque loading- thus it creates a situation where the shaft wants to pull out from the clamp. Under lighter loads when cruising along the shaft is then under compression due to the elongated shaft and the thrust bearing is then absorbing a thrust load it was never intended to bear and sooner or later something gives.

In the situation when the drive shaft snaps one sees delamination of the outer part of the taper section, this effectively reduces the cross sectional area of the shaft and at some point under full load the remaining metal shears completely as it gives up the ghost.
FredR is offline  
Old 02-21-2019, 03:22 AM
  #21  
mike77
User
 
Join Date: May 2012
Location: Edinburgh
Posts: 598
Default

I thought it was best to do the rear first to allow the driveshaft to migrate backwards to its original position? If the flexplate was under load and it has been unclamped then surely the driveshaft has been allowed to migrate forward and been reclamped in its new position?

I'm no expert on this procedure so I may well be wrong but this is my recollection from discussion here.
mike77 is offline  
Old 02-21-2019, 07:01 AM
  #22  
Adk46
Addict
Rennlist Member
 
Adk46's Avatar
 
Join Date: Aug 2014
Location: Adirondack Mountains, New York (For NYC residents: "near Canada")
Posts: 1,950
Default

Random remarks:

It just occurred to me that the larger diameter shafts would have reduced "wind-up" shortening under load. That might have been Porsche's intent, or one of them, with an unintended consequence, it seems. The shortening may depend on D^3 while the longitudinal stiffness depends on D^2 - you want a greater reduction in the first than an increase in the second - but I don't know. Mix in the effects on vibration modes, bearing quantity, locations, and damper and let the beer flow.

After viewing a photo appearing here years ago, I concluded that "delamination" failures were from multiple fatigue cracks initiating at the base of each spline. I shouldn't brag, but crack behavior is something I know about. ZZZZzzzzzzzzz.

If I had ten failed and ten good driveshafts, I'd get hardness tests done on them. That's a simple test that can reveal variations in heat treatment and material. I'd also ask their owners how often they needed new tires.
Adk46 is offline  
Old 02-21-2019, 10:45 PM
  #23  
chart928s4
Addict
Rennlist Member
 
chart928s4's Avatar
 
Join Date: Feb 2017
Location: Massachusetts
Posts: 263
Default

Originally Posted by Adk46 View Post
I'd also ask their owners how often they needed new tires.
LOL. I see what you did there, Curt. One factor I forgot about.

Bolts on the clamp, splines on the shaft, nuts behind the wheel.
chart928s4 is offline  
Old 02-22-2019, 11:38 PM
  #24  
GregBBRD

Rennlist
Basic Site Sponsor

 
GregBBRD's Avatar
 
Join Date: Feb 2005
Location: Anaheim
Posts: 9,676
Default

Originally Posted by Jason89s4 View Post
All,
This post is a detailed review and commentary on the testing of the crankshaft axial movement and endplay to diagnose any possible thrust bearing failure. This post does not present anything "new" regarding the procedure or the reason for it, as there are dozens of threads on the topic. However, based upon the obvious continued interest in the subject--be it due to engineering curiosity or conspiracy theory--I figured I'd post my results.

For those that just want the results and don't care about the details, here is my summary up front:
Objective: Measure flexplate deflection and crankshaft axial movement, for detection of possible crankshaft endplay outside specified parameters due to crankshaft thrust bearing wear or failure.
Subject: 1989 Porsche 928 S4, Automatic. 124,824 miles. Well maintained, stock drivetrain. No known driveshaft or crankshaft repair.
Measurements: Initial flexplate (loaded) deflection: 4.5mm. Axial crankshaft endplay: 0.21mm (0.0083inch). (Average of several tests with digital calipers and dial indicator).
Conclusion: Within factory specifications (<0.4mm/0.016inch).

Discussion: First, let me say that this forum and its contributors are an incredible asset to folks caring for the 928 and keeping it alive! While I have performed crankshaft measurements on other vehicles, I literally learned everything that I know about the 928 TBF history and this particular test from this forum, its contributors and also links that they provided. Thanks.

I took delivery of this 2-owner vehicle almost a month ago, and I had the opportunity to review all of the extensive maintenance records prior to my purchase. (I even was able to talk to the mechanic for the car the last six years.) There were no records of anyone checking the crankshaft endplay for possible thrust bearing failure, nor any records of any major service to the engine, crankshaft, torque tube or transmission. Maintenance was largely either preventative or routine R&R of worn or non-functioning parts. So, upon delivery I was very anxious to perform the TBF tests. I drove the car a total of about 50 miles before putting it on the lift for this test and many more over the last two weeks.

Accessing the flywheel/flexplate area was fairly uneventful. As expected, I had to unbolt the exhaust from the manifolds so it could drop down enough to be able to remove the two bellhousing bolts furthest back. The "worst" part of this was loosening the bolt on the engine that holds the short bracket to the manifold. It was on there good. For better or worse, I only had to struggle with that on the right side..the left side bracket was broken in the middle. When removing the exhaust bolts, the bracket bolt and the bellhousing bolts, I saw no evidence that any had been wrenched before. Also, I followed the advice of at least one of the forum members, and I just didn't replace the two back bolts. That bellhousing cover is so light, I'm not worried about it going anywhere.

With the bell housing removed, I observed a very clean inside area, despite the fact that the plug on inspection hole on the bottom was missing. Looking at the flexplate clamp and driveshaft, I could see a long portion of the splines compared to other pictures I've seen in this forum. There was no sign of any slippage along the splines. After using a prybar to carefully rotate the engine (here is where I feel like I will be banned if I don't mention "rotate the engine clockwise only, never counterclockwise") I lined up the pinch bolt for loosening. Again, no indication of any previous loosening of the pinch bolt--it still had the red factory paint dab lining it up with the clamp. I could see that the flexplate did have some load deflection, and I measured it to be 4.5mm. Not excited about that, but I've read about worse.

I loosened the pinch bolt, and while I did not actually visually see it or feel it, the flexpate and clamp did move once unloaded and returned to "flat." I could also now see less of the splines on the driveshaft. (I'm guessing about 4.5mm!) I used a prybar to first move the flywheel back (toward the rear of the car). I had read that you would hear and feel a "clunk" or "thump" letting you know you had moved it rearward enough, and I did. I took a measurement using a digital caliper across the edge of the flywheel and the edge of the upper part of the bellhousing. I then used the prybar to move the flywheel forward, again with a tactile "thump" when it had moved. I took another measurement and calculated the difference. I repeated this back-and-forth test three or four times, until I felt comfortable with my readings. (I attributed any very small deviations between measurements to being a human.)

Then, I set up a dial indicator on the flywheel and performed the back-and-forth test measurements another 3 or 4 times. (Why use both the caliper and the dial? Heck, I have both in my garage and I've already gone this far, so why not?) Satisfied with the measurements, I moved the flywheel back to the rear (unloaded), tightened the pinch bolt to 66 ft/lbs. (110% of original factory specs), and marked the location of the clamp along the splines with white paint. I did not use any thread lock.

The results are as stated at the beginning of the post. Unless I change my mind (based on something posted on this forum, I'm sure!) I don't think I will go with a different clamp or loctite, but just make a practice of inspecting, measuring and releasing the pressure on the flexplate every 5,000 miles or so, and just enjoy driving the car for a while instead of being underneath it.
Thanks,
Jason
4.5mm of deflection?

And minimal wear to the thrust bearing.

Buy a lottery ticket, you're a very lucky man!
__________________
greg brown




714 879 9072
[email protected]

There's always different ways to do a particular job.

The trick, in life, is to know which of those ways are stupid....
GregBBRD is offline  
Old 02-23-2019, 12:09 AM
  #25  
Jason89s4
Addict
Rennlist Member
Thread Starter
 
Join Date: Jan 2019
Location: Dallas
Posts: 124
Default

Originally Posted by GregBBRD View Post
4.5mm of deflection?

And minimal wear to the thrust bearing.

Buy a lottery ticket, you're a very lucky man!
Will do!
It had me a bit worried as that was the initial measurement, but as you saw in the post, I subsequently measured endplay with both calipers and a dial a number of times, and we're good.
Geza at post #3 had very similar numbers to mine at very similar mileage.
FYI, flexplate deflection at return to service was nominal. (So close to zero I did not have a shim small enough to measure).
Thanks, Jason
Jason89s4 is offline  
Old 02-23-2019, 03:09 AM
  #26  
mike77
User
 
Join Date: May 2012
Location: Edinburgh
Posts: 598
Default

If there was 4.5 mm deflection and you removed the front pinch bolt doesn't that mean the flex plate is now locked 4.5 mm further down the shaft? Surely if you keep releasing tension on the plate at the front pinch bolt then you are allowing the driveshaft to migrate forward?
mike77 is offline  
Old 02-23-2019, 03:24 AM
  #27  
FredR
Addict
Rennlist Member
 
FredR's Avatar
 
Join Date: Jun 2005
Location: Oman
Posts: 5,814
Default

Originally Posted by mike77 View Post
If there was 4.5 mm deflection and you removed the front pinch bolt doesn't that mean the flex plate is now locked 4.5 mm further down the shaft? Surely if you keep releasing tension on the plate at the front pinch bolt then you are allowing the driveshaft to migrate forward?
The purpose of releasing the front pinch bolt is to reset the flexplate to the correction [no deflection] position. That it should never have slipped in the first place is the root issue and the cumulative effect of what is presumed to be incremental small steps of slippage is disastrous. It is quite common to see the front clamps slip by 3mm. Engines destroyed by TBF seemingly slip further but what the point of "no return" is remains to be seen but on the [limited] examples I have seen the flexplate was plastically deformed. .

The rear pinch bolt is seemingly much less of an issue but if that comes loose the rear position can also migrate with equally disastrous effects thus why both need to be checked and prudent to replace every 5 to 10 years or whatever interval the owner feels comfortable with.
FredR is offline  
Old 02-23-2019, 04:09 AM
  #28  
mike77
User
 
Join Date: May 2012
Location: Edinburgh
Posts: 598
Default

Surely if there is tension on the flex plate it means that the pinch bolt hasn't slipped? If it was slipping the flex plate wouldn't hold tension?

Not trying to create any confusion here. I did the same as what is described above and after further discussion here think i may have made a mistake. Due to a house move my car has been off the road for about a year. With spring coming im getting round to getting it back on the road and this thread reminded me.

I followed this write up when I did mine.

Web results
Porsche 928 Tech: Check Your A/T Flex Plate - Rennlist
PDFhttps://members.rennlist.com › 928TechF...

In the write up after the procedure there was 3.3 mm less spline and the shaft is now locked 3.3 mm further forward in relation to the flexplate. Doesn't that make the situation worse? Wouldn't it be better to release the tension at the rear?

Last edited by mike77; 02-23-2019 at 05:31 AM.
mike77 is offline  
Old 02-23-2019, 05:57 AM
  #29  
daveo90s4
User
 
Join Date: Feb 2006
Location: Australia
Posts: 598
Default

Mike, in the ideal world the central shaft arrangement is not under tension or under compression. Ie the flex plate is nice and straight, the rear bolt is done up nice and tight and THEN the front clamp is done up (typically with the crank at the rearmost limit of its minimal travel). Ok, that’s the ideal.

Then in what happens (we think). The car is driven. The central shaft experiences torque, and like just about anything twisting, it shortens. The shortening central shaft, locked at the rear by the pinch bolt that runs through the shaft, is under tension. That tension is relieved by the (shortening) central shaft pulling out of the front flex plate clamp.

Now reduce the torque effect on the shaft, ie eliminate the force that is trying to twist it shorter. The twisting stops. The shaft reverts to normal ‘longer-than-as-twisted’ length. But because by now the shaft, under tension, has pulled (say) 3mm out of the front flex plate clamp, on returning to ‘normal’ length it pushes the flex plate that same migrated distance forward. In this state it is under compression. That 3mm or so compression can then only be relieved by either a) releasing the front clamp and returning to normal (neither tension or compression) or b) the front of the crankshaft grinding out the rear of the crank thrust bearing.

Lots of options for better clamping the central shaft to the front flex plate to eliminate the thrust bearing causing compression phase. A sliding coupling between front flex plate and rear would eliminate both tension (pulling out phase) and compression (thrust bearing death phase).

Cheers

DaveO
daveo90s4 is offline  
Old 02-23-2019, 07:55 AM
  #30  
FredR
Addict
Rennlist Member
 
FredR's Avatar
 
Join Date: Jun 2005
Location: Oman
Posts: 5,814
Default

Originally Posted by mike77 View Post
Surely if there is tension on the flex plate it means that the pinch bolt hasn't slipped? If it was slipping the flex plate wouldn't hold tension?

Not trying to create any confusion here. I did the same as what is described above and after further discussion here think i may have made a mistake. Due to a house move my car has been off the road for about a year. With spring coming im getting round to getting it back on the road and this thread reminded me.

I followed this write up when I did mine.

Web results
Porsche 928 Tech: Check Your A/T Flex Plate - Rennlist
PDFhttps://members.rennlist.com › 928TechF...

In the write up after the procedure there was 3.3 mm less spline and the shaft is now locked 3.3 mm further forward in relation to the flexplate. Doesn't that make the situation worse? Wouldn't it be better to release the tension at the rear?
The rear clamp is located with the bolt running though a detente in the shaft- it can only be fitted in one position thus is not adjustable. The front clamp is simply tightened when there is no pre-load. When it slips the residual clamping force does not allow the shaft to slip back to the original position. The increments of slippage accumulate and for some examples the result is TBF- game set and match for the engine!
FredR is offline  

Thread Tools
Search this Thread

Contact Us - About Us - Advertising - Cookie Policy - Privacy Statement - Terms of Service

© 2019 MH Sub I, LLC dba Internet Brands

We are a participant in the Amazon Services LLC Associates Program, an affiliate advertising program designed to provide a means for us to earn fees by linking to Amazon.com and affiliated sites.
 
  • Ask a Question
    Get answers from community experts
Question Title:
Description:
Your question will be posted in: