dprantl

Just goes to show that even Porsche really did sometimes get it *wrong*.

Dan
'91 928GT S/C 475hp/460lb.ft

PorKen

A while back I was thinking about cutting off the factory clamp and replacing it with a 'heavy double split collar'.

Glenn M

The washers and snap ring are a pain in the A$$ to get on. I did it on my 88. I have the Constantine clamp for the widebody when I put the 89 LSD trans in. I also want to rebuild the torque tube with his bearing while I'm in there. That way when I add more power down the road I will be ready!

Glenn

Bill Ball

The trick when I did this was to use a SMALL snap-ring tool. I had an expensive, heavy-duty tool, and it was very cumbersome to get it up in that small area at the end of the shaft. The el cheapo set I got from Kragen's worked much better and it only took a minute to get the snap ring on.

Mrmerlin

The interesing part of all of this is than nothing has been mentioned about checking the rear pinch bolt..
So to do this job with the highest possibility for success I strongly recommend that the rear pinch bolt be removed and checked for damage.
What i have found is that the rear bolts are usually loose compared to the front bolts, as such the rear bolts will usually get shoved into the driveshaft splines and begin wearing on the shank.
This indicates movment of the shaft/coupler, If this is happening then your front clamp even though its tight isnt going to stop the shaft from movment .
So check the crank end play, remove both of the bolts center the rear coupler/driveshaft connection ( this can be done by using the front coupler and tighten it ,then you can tap it forward or back with a mallet and piece of wood) This centers the drive shaft cut out in the rear, install a good bolt with some blue loctite torque to 66ft/lbs then fasten the front after prying back the crank and setting the flex plate .
To see if the rear bolts loose use the torque wrench set to 58 ft/lbs to undo the bolts you will probably find that the rear is much easier to unfasten.

Bill Ball

I just did my periodic front flexplate pre-load release. I do this every 3-6 months. I invariably find about 2mm of forward pre-load on the flexplate no matter how often I check it. I check the crank end-play about once a year. It hasn't changed since I started measuring it, measuring about 0.2mm (limit 0.4mm).

Here are pictures:
1. Just before loosening the clamp bolt.
2. Just after loosening the bolt.
3. Just after pushing the flywheel toward the rear and re-tightening the bolt.

Sorry no scale, but I'd say it moved a little less than 2mm.

Tails

I thought I would throw this back into the ring.

Some flexplate clamps have been reported as having over 3mm of migration movement by what ever reason causes the flexplate clamp to move forward towards the flywheel.

However, in my opinion the migration of the flexplate clamp will reach a a maximum movement when the counter force applied by the flexplate, due to its distortion, overcomes the friction resistance between the splines on the flex plate clamp and the drive shaft. Once this stage is reached I believe the clamp will oscillate axially on the drive shaft, but will maintain its relative migrated position. This oscillation will actually cause wear between the splines resulting in less friction and easier migration of the flexplate clamp once reset to the zero position. (I always clamped witht the crankshaft pushed to the rear of the car).

Any migration will of course apply a forward axial force on the collar of the crankshaft thrust bearing. In some instances it will cause failure of the thrust bearing and in others cases, even with a movement of over 3 mm, the thrust bearing bump clearance have remained within specification tolerances after may years of operation. If you require an indepth knowledge regarding why the bearing can fail you will need to study the art of "Tribology".

As stated above, in this thread, Porsche recommended an increase of the collar clamp Allen headed set bolt by 10%, to try and prevent migration, but this did not solve the migration problem.

As a precaution it has been recommended that at each yearly service or at more frequent intervals the flexplate clamp be inspected for forward migration and if discovered then the clamp bolt should be released, flexplate checked for 'no load position' and reclamped to 10% in excess of specified torque.

There are two main current methods used to prevent flexplate clamp migration:
Constantine's Clamp; and
Locktite.

The choice is up to each individual.

I personally applied locktite and the flexplate clamp on my car has maintained its position for well over two year with a crankshaft axial clearance of 0.008", however, I do like Constantine's Clamp, a great piece of engineering design and it is very effective, in my opinion.

If I ever have to remove the clamp I will apply heat to the clamp to release the locktite.

Tails 1990 928 S4 Auto

Bill Ball

Tails:

Agree.

The earlier TT shaft with shims is another option. We installed it on a 90 that had snapped the shaft at the neckdown point.

sty3x

I have a 87 S4 How can I check?

Black Sea RD

Porsche stopped using the circlip, bearing, washers for a reason. From what we ascertained from the research done into the TBF problem, Porsche field techs were having a problem setting the correct distances between the flexplate and flywheel with this method and customers were having TBF issues. At some point in MY 1984 Porsche stopped using these parts. Porsche should have re-designed the clamp at that point but did not. Why would Porsche, which prides itself as an engineering company first and then a car builder, did not change the front clamp design at that moment escapes us. That's why we designed the 928 Super Clamp to fill this oversight.

Another Porsche blunder in our opinion was when they changed the constant diameter 25mm drive shaft to the necked down 28mm one which has shearing issues.

I know I'll get flamed as before but we really advise against using loctite since re-positioning the drive shaft in the front flexplate clamp is difficult after using it. Heat and hammering is needed to disengage the two. Not good for the engine internals, TT bearings, or transmission internals. There was also an instance when a loctited drive shaft moved during driving and could not be re-positioned easily afterward in the front flexplate clamp. YMMV.

Cheers,
Constantine

Bill Ball

To check for excessive wear of the thrust bearing, you check crank endplay. Here's how I did it. Also, see Tony Harkin's method that does not require a dial gauge, linked at the beggining of my description.

https://rennlist.com/forums/showthre...hlight=endplay

Nightfly

Link for somewhere to purchase your clamp, Constantine?

Black Sea RD

Just as an historical note about the 10% over the 59 ft lb torque value of the pinch bolts.

This did not come as an official Porsche recommendation. This came from, supposedly, Porsche factory techs who were called by a mechanics shop on behalf of one of their clients having problems with his 928S4 and front flexplate migration. They were advised to 1.) replace the front flexplate with a new one 2.) to use a new pinch bolt and 3.) to clamp the new pinch bolt to a 10% higher ft lb rating.

As with any story passed down through others it changed. Now the story usually only includes the higher torque rating with using a new pinch bolt sometimes mentioned.

More important to replace and no longer remembered is the front flexplate with a new one. Why? The way the front flexplate clamp is made allows it to stretch out over time. Even with a new pinch bolt and the higher torque value the old flexplate clamp will not clamp as well as a new one. They are just not made for repeated clamping and un-clamping.

Cheers,
Constantine

Black Sea RD

I know this smacks of self promotion but why not just use the 928 Super Clamp? It was engineered just for this purpose, works very well, can be re-used without loss of clamping force and will probably outlast the 928 it's put in.

Always surprises me a bit that people would want to scrimp on buying the new clamp but will pop for a new interior, new sound system, wheels, exhaust systems, etc. which cost way more than the clamp.

Okay, stepping off my soap box now. Getting my flame suit ready...

Cheers,
Constantine

Black Sea RD

Both 928 Specialists and 928sRus carry it. New fresh shipment just in.

Cheers,
Constantine