chaadster

I've never heard of Rennstat, nor have I heard of Tim Pogg. Are you sure it's not Tim Potts (of Eurotec)? FWIW, I'd feel better if it was Potts, as he's an experienced Porsche tech and racer (moreso with 911s, but he knows the 928, too) and has a good team. His services will cost you, though, as Andrew said.

camboinc

Here is an update:
finally checked the tension on the flex plate... Sproing! Bad news was that the torque tube is shot (the front bearing slid out and was buzzing a groove into the flywheel) and there was major crank endplay (well over 10mm).

Oddly enough, when I ran the car, most of the knocking noise has disappeared. That is still with the bad torque tube in there.

I trailered the car to Jack Riffle and his fine mechanic Chris just South and West of Cleveland. Chris will be installing a freshened motor in my car from his 87 project.

Can anyone tell me the best place to find a rebuilt torque tube? 928 Specialists perhaps?

I would assume that by the big crank endplay that the motor needs to come out and the block checked. Any ideas on what to look for in determining how salvageable this engine is?

killav

Do you mean 1mm? 10mm is almost 4/10 of an inch which seems almost impossible. In any event, 1mm is still way to much endplay. I think .4mm is the max. You might be into the block already with that much endplay. You'll definitely need to pull this and look.

Tony

is that 10mm the distance the flexplate moved..or the actaul crank endplay.

if it is truely the crank, its toast, Im amazed the thing ran at all! thats 1cm!!
With that type of gross wear you could check it with out a dial gauge! UGLY!

camboinc

my mechanic actually checked the end play for me and he said that the crank moved that much. like you said... WOW! surprised the car still ran so well or at all. in any event, when this motor comes out, ill take a closer look at the block and crank for myself. hope it can be salvaged (read- sold for money to pay for my other engine!)

Bill Ball

I think some things may have gotten lost in translation. Maximum endplay tolerance is 0.4mm. The thrust bearing walls are only 2mm thick. Anything over 2mm endplay and the car will not run as the crank would be hitting the block hard. It might if less than that (between 0.4 mm and 2). Probably the flexplate moved 10mm. It is quite normal for it to move 2-3mm. 10mm would be a bad sign - excessive preload that would wear the thrust bearing, but 10mm crank endplay is unheard of.

Also, the TT bearing can't wear a groove in the flywheel. The bellhousing and the flexplate are in the way. The bellhousing has ears that block the end of the TT so the bearing cannot move beyond the end of the TT. Bearings do migrate, but not completely out of the TT.

Anyway, let's presume the crank endplay is more the 0.4mm and the TT is shot.

camboinc

thanks for the clarification Bill. So... just the flex plate moved big, not the crank. Since the car runs (and sounds like it runs pretty well), should I just replace the TT and see what happens? I have another guy helping me out at the moment. I will have him pull the TT, get a new one from 928 specialists and then start the drill all over. Does the fact that the flex plate moved that much indicate anything (like TBF) or is it just the crank endplay movement being more than .4mm ?

daveo90s4

Camboinc, I suggest (from experience on my 1990 S4) as follows:
1. With the collar that clamps the central shaft to the flex plate nice and loose, use a decent sized bar to gently pry the flywheel fore and aft. This movement is the crankshaft end float. A bit difficult to measure accurately, but can be done.
2. If crankshaft endfloat is within specifications, proceed with new / replacement torque tube and central shaft, as you propose. I'm sure you'll be aware that its important to do this job right the first time to avoid thrust bearing failure in future. Use a new bolt in the clamp. General concensus is to tighten the bolt 10% tighter than factory specs. 65 ft / lbs by memory, but you should check with others.
3. If crankshaft endfloat is not within specifications then that would mean thrust bearing failure and engine out and maybe bearing replacement or if the crank has eaten into the block, then engine replacement.

If it is thrust bearing failure, it seems somewhat coincidental for this to have happened right on the day you bought the car - maybe the gearbox had been out previously and reinstalled with pre-load on the central shaft?

I hope you find the end float is within specs.

David
1990 S4

camboinc

thanks much David. will do.

Tony

http://members.rennlist.com/v1uhoh/cranksha.htm

quick and dirty run down on how i checked mine.

Bill Ball

So, it looks as if you haven't had crank float or endplay done yet. Although 10mm of flexdplate movement is a bad sign, the party is not over until the endplay is determined. Anything over 0.4mm is out of spec, but anything less than 1mm and the engine may be salvaged with a rebuild that replaces the worn thrust bearing. This requires engine removal.

So, see if the endplay is more than 0.4mm and we'll take it from there. Don't fuss with the TT until this is measured. It will change how you do the TT.