Doug Hillary

Hi,
perhaps Owners should also consider Used Oil Analysis (UOAs) as another "tool" in anticipating/predicting TBF

This type of quite inexpensive "tool" has been used for nearly 60 years (by CAT since around 1960) by many engine manufacturers including Porsche, with great results

A UOA every four months or 2-3000miles depending on use seems logical as these wonderful vehicles age

Regards

Black Sea RD

A little clarification on a good question:

The way the Porsche WSM instructed to intially set the distance between the flywheel and the front flexplate using the circlip, bearing and washer setup would have had a slight tension of the flexplate pulling back a bit on the flywheel. Porsche wanted to be sure that absolutely no forward pressure was seen by the flywheel/crankshaft.

By pulling back on the flywheel (toward the transmission) before tightening our clamp allows it to float a bit since there is a slight tension from the belts at the front pulling the flywheel a bit forward again. This sets it in a more nuetral position. This is the default setting we came up with after taking into account the Porsche WSM instructions.

Cheers,
Constantine

Steve Cattaneo

+1000000000000

Especially with the early design shafts with the circlip.

shmark

You guys are enough to talk me out of looking for an auto. I knew there was a reason I liked 5 speeds.

Mark

Black Sea RD

There are 1000s of 928 autos running around that have not had any problems with TBF. TBF is a problem that should be looked out for and managed. 5 speed cars have also had TBF issues although at a much lower rate.

From what we learned during our research there was no information of a 928 engine suffering TBF in the reverse, ie. from the front of the engine back. We cannot rule this out positively and say it never happened since with complex mechanical items that are sometimes worked on by individuals with questionable mechanical skills, anything is possible.

Constantine

doktor-t

What was the right torque for the allen clampbolt?

/Tobias

AO

IIRC 65ftlbs

SeanR

Replace the bolt with a new one after you check the tension. Learned that yesterday. No, not the hard way.

Imo000

How about using 2 factory clamps on the pinch collar? Since the clamping force of a single clamp is almsot enough to keep the drive shaft in place, 2 will be more than enoght ot keep it from moving. There are people that have sucessfully used Locktite, so the required additional foce is not that much, one more clamp can easily provide that.

mj1pate

Jim, et al:
I checked my 86.5 crank end play after using a pry bar to force the crank as far forward and back as I could and the mic indicated that wear was well in tolerance. There was "zero" flex plate movement after loosening the pinch bolt (this surprised me) and the crank (with oil pan removed) looked normal.

Are some MY autos more susceptible to this than others?

Imo000

As a comparrison, my parts car, also an 86.5, had about 2mm flex in the plate but the tolerance of the crank play was in the middle of the spec.




On another note, there a lister here (local to me) that just recently (about a year ago) fixed his TBF S4. I beleive he machines shimms for it and used larger (early 32V) trust bearings. I think he put on at least 1500km on it since with no issues what so ever. His block was not cracket by the TBF but it was damaged quiet a bit.

erwalker

Similar question as above - at what point in production does the TBF become an issue? Is it 32 valve models only?

Glenn Evans

Page 39-48 of the workshop manual states that the required torque for the panhead bolts in both the front and rear couplings is 80 Nm (59 ftlb). I believe that there is a consensus to add 10 per cent to this in an effort to prevent the bolt loosening, making the recommended torque 65 ftlb.

Vilhuer

TBF can be found from first '78 MY automatic made if its TT or gearbox was mounted wrong but these cases are rare and fault of mechanic who put system together. Since '78-84 MY cars have lock ring and washer setup at front end of TT they do not suffer from TBF as long as washers are correctly installed.

'85-87 MY TT center shaft has machining at front end ready for those washers but they were left of at production line as there were difficulties at dealer to install them correctly during repairs.

'88-95 MY TT do not have place to put washers since center shaft was redesigned. These can use Constantines clamp to achieve same result as earlier TT's have with washers.

Jim bailey - 928 International

The earlier 4 speed torque tubes have a snap ring groove at the front and the workshop manual discusses using spacer washers of .5 mm , 1 mm, and 1.5 mm to make it IMPOSSIBLE for the clamping collar to slide forward ( shaft to pull back) around 1986 Porsche issued a technical bulletin stating that this was no longer necessary and STOPPED installing the ring and spacers at the factory or when replacing tubes at the dealership......the groove was deleted when the larger diameter shaft was introduced in about 1990. The clamp bolt torgue is 80 Nm it is a 12.9 material higher strength bolt.