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-   -   Failed torque converter bearings or why do we have these flex plate issues??? (https://rennlist.com/forums/928-forum/228734-failed-torque-converter-bearings-or-why-do-we-have-these-flex-plate-issues.html)

Schocki 09-29-2005 10:08 PM

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Update:
I worked on the car all day today and everyhting is back in the car except the exhaust. I'm waiting for the O2 Sensor from 928 Specialist which should be here tomorrow.
Everything went bak in the car with no problems. The flexplate was adjusted to factory sepc. and glued with Loctite 290 on the shaft. I also put some white markings on the shaft for easier checking in the future. As you see in the pictures, I used an old washer and bolt to have a closer visual mark on the shaft itself. Because I want to know if the shaft actually moves or stretches over time.

Borland,
I used a 19 mm wrench and a 3/4 inch breaker bar extension to remove the upper bellhousing bolts. It was no problem with this setup, but with the wrench only it is impossible!

Steen Jensen 09-23-2006 10:43 PM

Can anyone tell me what distance "A" is in frame 7? I don't think it is too critical as there is lots of room on the splines on either end. Anyone differ with that asessment? Also what is the 110% torque value on the pinch bolts?

Steve Cattaneo 09-24-2006 12:05 AM


Originally Posted by Schocki
Update:
I worked on the car all day today and everyhting is back in the car except the exhaust. I'm waiting for the O2 Sensor from 928 Specialist which should be here tomorrow.
Everything went bak in the car with no problems. The flexplate was adjusted to factory sepc. and glued with Loctite 290 on the shaft. I also put some white markings on the shaft for easier checking in the future. As you see in the pictures, I used an old washer and bolt to have a closer visual mark on the shaft itself. Because I want to know if the shaft actually moves or stretches over time.

Borland,
I used a 19 mm wrench and a 3/4 inch breaker bar extension to remove the upper bellhousing bolts. It was no problem with this setup, but with the wrench only it is impossible!

That front shaft looks like its set too deep, you said you changed the torque converter bearings, did you install and tighten the rear pinch collar and shaft first, before you tighten the front one.

Schocki 09-24-2006 12:05 AM

Answer for "A" is 0.4mm

Look at the pinch bolt value and give it an additional 10%. This was aproved by the factory and made in an attempt to control the movement of the flexplate.

Schocki 09-24-2006 12:11 AM


Originally Posted by Steve Cattaneo
That front shaft looks like its set too deep, you said you changed the torque converter bearings, did you install and tighten the rear pinch collar and shaft first, before you tighten the front one.


???

Steve Cattaneo 09-24-2006 12:48 AM

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marton 09-24-2006 02:13 AM

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I think the only real fix would have been if Porsche would have
adopted a clutch like flexplate assembly.

I thought the early models had a circlip and never moved but Porsche went away from this simple solution.
Why would they invent a more complex and costly solution?

Marton

Bill Ball 09-24-2006 02:32 AM


Originally Posted by Schocki
???

I think what Steve is indicating is that there is almost no spline showing on the front of the TT shaft. Usually 3/4 inch or so shows behind the clamp.

Schocki 09-24-2006 02:48 AM

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That's how it was with the old transaxle too. Everything was adjusted to spec. It is a year now after my repairs and everything is working well. The flexplate has not move since then (loctite 289) and sits exactly at the same spot. The car drives great and is my daily driver.

Below is a picture from Pirtle's homepage, mine is not a lot different...

marton 09-24-2006 03:14 AM

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the front clamp on the flex plate (and maybe rear clamp too) slips
(or more precisely, the tube slips out of the clamp).
When the tube lengthens again, the clamp is pushed forward.

Could be, but why would the clamp slip in one direction and yet hold in the other?
Also this sort of ratcheting effect would in theory continue until the centre of the flex plate is pressed against the flwheel and can not physically move any further; does this happen?

Marton

Black Sea RD 09-24-2006 08:10 AM

We have studied this problem and concluded the forward pressure found on the forward flexplates comes from the driveshaft twisting under load, shortening it which pulls it out a bit from the front flexplates. The front flexplates do not have a groove to hold the driveshaft as the rear trans coupler does. Porsche stopped using a circlip, shim and bearing design at the very front of the driveshaft sometime in 1984 which located the flexplate to the driveshaft at the front, sort of like the grooved area of the rear driveshaft and bolt design.

In 1985 and onward the HP/TQ of the cars were increased which further compounded the driveshaft twist problem. Shortly thereafter Porsche increased the diameter of the driveshafts from 25mm to 28mm to handle this increase better. This is from a Porsche engineer.

The driveshaft acts just like a torsion bar in a torsion bar suspension set up found in 911s, 944s and other Porsches and cars. The concept behind this suspension is that the torsion bar twists and acts like a spring. These bars are not as long as our driveshaft but still twist. When a steel bar twists it shortens. The shortening effect is finite and when the parameters are reached the driveshaft is no longer pulled from the front flexplates.

The theory of the torque converter (TC) bearings wearing out and causing the forward push is not plausible. There would have to be constant forward pressure being exerted to the driveshaft from the rear. The design of the TC housing would not allow this just as Shocki found when he tested this theory using a press on the TC housing. If this theory were true then the Loctite, circlip/washer/bearing, and our clamp would not work. But they do.

As far as using loctite there are drawbacks. Once Loctite is used it is near impossible to release the driveshaft from the front coupler and people have had to unbolt the front clamp from the flexplates to remove the torque tube from the car. There was also an instance when the driveshaft did move after loctite was introduced and the owner could not release the forward pressure he found at the front flexplates.

We have also conducted tests on the OE Porcshe clamp and found that it loses clamping force when its temperature increases. Its design also does not lend itself to much re-use and using a new clamp bolt and torquing it to a higher torque setting had no effect in our tests.

The clamp we designed was tested over two years in the 1986.5 track car. The movement was stopped. The clamp was released and re-clamped a few times to test the clamps ability to be re-used which it did well. Its clamping ability was tested against the OE clamps and our clamp held fast past 4500 PSI, even after being heated in an oven to 200 degress F. The OE clamps were found to let the driveshaft move at about 1700-2100 PSI cold and about 1400 PSI when heated to the same temperature. This is important since the front flexplate lives near the engine and catalytic converters which generate a lot of heat.

Why did Porsche stop using their original design at the front flexplate area to locate the driveshaft to the flexplates? From our gathered information from a few different sources we have concluded this set up was difficult to install correctly by Porsche techs in the field and sometimes caused TBF to happen to customer cars. It is our opinion that Porsche should have re-designed the front clamp when they took these parts away.

I posted our findings in a more detailed fashion a while ago and this post can be found by doing a search using my name and TBF as the search parameters if anyone is interested.

Cheers,
Constantine

marton 09-24-2006 09:33 AM

Hi Constantine,

Your theory is that under certain conditions the drive shaft shortens. Then when the bar returns to its "normal" length and so flexes the flex plate..
OK, but after this happens for the first time then next time when the bar shortens again and lengthens - why does it repeat the cycle to cause greater flexing when it lengthens again. It should be simply taking the flex out of the plate when it shortens after the first time and be putting the same flex back as the first timke when it lengthens again.

Marton

Black Sea RD 09-24-2006 10:01 AM

Hi Marton,

The OE front clamp allows movement of the driveshaft until the driveshaft and flexplate reach an equilibrium. The OE clamp allows the driveshaft to be pulled out since it was intially designed to work with the circlip, bearing and washers. Porsche never re-designed the clamp.

A stronger clamp at the front allows the flexplates to work as intended, that being to flex to compensate for the driveshaft shortening.

Hope that helps,
Constantine



Originally Posted by marton
Hi Constantine,

Your theory is that under certain conditions the drive shaft shortens. Then when the bar returns to its "normal" length and so flexes the flex plate..
OK, but after this happens for the first time then next time when the bar shortens again and lengthens - why does it repeat the cycle to cause greater flexing when it lengthens again. It should be simply taking the flex out of the plate when it shortens after the first time and be putting the same flex back as the first timke when it lengthens again.

Marton


heinrich 09-24-2006 11:06 AM

Constantine is correct, I have said this same thing for years.

Marton, yes when the shaft shortens and releases pressure again, it is only while torque is being applied. Undewr low-torque the shaft is long and untwisted, which maintains pressure on the flex plate and therefore crank shaft.

That Constantine is correct becomes luminously clear when one looks at a broken torque shaft. I have owned TWO TBF engines.

Steen Jensen 09-24-2006 11:09 AM

Steve...I havn't installed the transaxle and TT yet. I lost the notes I wrote with the measurements of how far the driveshaft sticks out at either end. The notch in the transaxle end that is viewed through the hole in the tt gives me a rough idea. Just line the notch up with the hole.

Schocki...thanks for the info , it will address my problem above. Having said that , it looks to me that as long as you can get at the pinch bolt to tighten it that any variance in length at the transaxle end can be dealt with at the engine flexplate end. Would you agree?


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