mj1pate

I've read the many threads of TBF possible causes. Since it seems to create damage late in an engine's life, mustn't some dynamic be occurring as the car ages to make the engine more susceptible later in its operational life? I've never heard of TBF happening with any frequency with 928s as younger cars. Any thoughts as to why aging cars are more susceptible?

Bill Ball

If you're talking about thrust bearing failure, it takes time for the approximately 2mm thick thrust bearing face to wear down enough, usually a mm or more, for the bad stuff to start happening. In my car, there has been no measureable wear (less that 0.001 inch) in the 5 or 6 years (approximately 100K miles) since I started measuring crank endplay.

Cosmo Kramer

Is there anything that gives a warning that this is happening without releasing pressure at the flexplate? This is on my to do list on my car, but no metal in the oil and it turns over nice and fast when it is hot. I am wondering if the clamp is disturbed it will cause creep in the future, where if it was left alone it would be fine.

If the side pressure was tight and the thrust bearing was tight against the crank would it not turn over slow or is that only when the internal flywheels starts hitting the block?

Ben

From what i've found out, its already too late when you start finding metal in the oil, and have problems with cranking and poor idling/stalling.

Check that flexplate....

marton

About "I am wondering if the clamp is disturbed it will cause creep in the future, where if it was left alone it would be fine."

I suggest you look at the clamp; if the flex plate is flat then leave the clamp alone - if the flex plate is bowed then you might consider relieving the pressure?
If you do not look at the clamp but wait for obvious symptoms then it may be too late!

Marton

Bill Ball

By the time you have symptoms (car stalls on hot idle and is hard to crank over and restart when hot), it's way too late. Metal in the oil could be from other bearings. Anybody with an 85+ automatic needs to check the flexplate now if it hasn't been done before. If it is bowed at all when checked with a straightedge, the clamp needs to be loosened to allow the flexplate to move. If the flexplate stays bowed it should be replaced. I would always do a baseline crank endplay check.

Cosmo Kramer

I am going to get this done. I friend of mine has a hoist I can borrow to make it easier.

My plan was to get it on the hoist, release the pressure, check endplay and install Roger's new clamp when it is ready.

It is too bad Porsche couldn't have designed a shaft that floats and doesn't put pressure on the flexplate.

H2

This Thrust Bearing stuff makes it scary to buy an automatic. If you're bidding, you really haven't much time to have it checked. If you're buying at a distance, I'm not sure how many on this list would let some mechanic do the check. It would take a real motivated seller to sell an automatic to me...but I like them and may get one if the prices keep dropping....and I somehow can be assured that the TB is sound.

H2

SteveG

It does add another complication to the ebay mix or any purchase And even if you arrange for a PPI, the routine (?) $150 PPI is not going to include the time to check this. When I had the GTS FS, of course all I got was low ball offers, but they didn't know a flex plate from a dinner plate. I guess if somebody showed me a receipt for either of the after-market clamps, it would make a difference, but you still have to take their word.

86'928S MeteorGrey

Any 85 or later auto needs to be checked either with your own hands and eyes, or by a shop that knows what to do. You shouldn't have to explain the proceedure to the mechanic. He should tell you the process for checking the thrust bearing play. Unless the car is cheap enough that buying and installing a used engine will not put you upside down in the endevor, a full PPI (about $300) is essential IMHO...

worf928

As this group has investigated TBF, many potential causes have surfaced, but TBF happens, most commonly it would seem, as the direct result of a torque tube R&R wherein the WSM procedure is not followed. Didn't, PAG issue a dealer bulletin (that has been posted on one of the TBF threads) that informed their network that they would no longer cover TBF under warranty or goodwill specifically because dealers were not following the WSM procedure for torque tube installation?

So, aging cars are more susceptible to TBF because they are more likely to have had a torque tube replacement.

IMO, the tell-tale sign of a necessary end-play check is documentation of a recent torque-tube R&R. If you are looking at a used auto 928 with documentation of a torque tube replacement in the last few thousand miles consummation of the purchase should be dependent on a end-play check.

Black Sea RD

1. TBF can happen to any year 928, automatic or 5 speed. Most commonly with autos.

2. TBF can be caused by improper installation of an engine, clutch, TT or trans.

3. Forward thrust (toward the engine) seen at the front flexplate of autos is bad.

4. Every 928 should have a thorough inspection before purchase, hence the name "PPI." During the PPI the engine's thrust bearing clearance should be checked as well as the oil filter for any signs of engine particles.

HTH,
Constantine

H2

Wait...

This is the first time I've heard of TBF on a 5-speed. I can see this with a 5-speed that's had the trans, TT replaced, due to improper installation....but on an untouched 5-speed? Has anyone experienced this?

H2

Black Sea RD

Hi H2,

I didn't say "untouched."

The possibility of finding an untouched 928 anymore is very slim...

Cheers,
Constantine

worf928

IIRC, someone, in some thread, reported that use of super-strong after-market clutches did kill thrust bearings. The theory is sound, but it was not a direct report (as in "my TB failed....")

Any 5-speed owner can accelerate wear of their thrust bearing by always starting the car with the clutch pedal all the way to the floor. This technique for wearing away the thrust bearing on 5-speeds is more effective if the car's sat for enough weeks that there's little (or no) oil film on the bearing. Disengaging the clutch pulls back on the flywheel and therefore backwards on the crank which then puts pressure on the thrust bearing. Best to have oil circulating when that happens.