Bill Ball

I saw a car today that reinforced the fact that you should see SOME movement in the flywheel - i.e., some crank endplay - and that NO movement at all is a bad sign.

This 90 S4 had symptoms consistent with Thrust Bearing Failure (TBF) - car would stall once warm and then attempts to restart acted like the battery was low - the starter would turn the motor over slowly.

So, we decided to look at the flexplate and check the crank endplay. Someone had been in there before as the rear bolts on the lower bellhousing cover were not present - this allows you to drop the cover w/o unfastening the exhaust from the headers. Great, that makes things easy.

The flexplate had some bowing in it, but when we released the TT shaft clamp, it only moved a mm or 2. That's common and might be reassuring, as often the neglected cars can accumulate 10-15mm of preload which is very hazardous to the thrust bearing. Some bowing remained in the flexplate - not such a good sign (means the preload was probably greater and longstanding at some point in the past). Then we tried to move the flywheel/crank back and forth - NOTHING. We pried harder with a bigger prybar - NOTHING. The we REALLY pried harder and CLUNK! - massive movement. So, we pried in the other direction - NOTHING. So we pried REALLY hard in the other direction and CLUNK! - massive movement again. We mounted a dial gauge. And it took a lot of hard prying, but then CLUNK! --- 1.65mm (0.065"). The factory wear limit is 0.4mm. Most cars are under 0.2mm. Anything approaching 1mm is probably a death sentence. 1.65mm is certain death.

You might give up with no movement and think things are fine, but there is always SOME crank endplay. Factory is about 0.1 to 0.2mm, with the wear limit at 0.4mm as I noted before. The crank should slide easily and there should be some clunk. If there is no movement at all, I would be worried, as I was in this case. It made me suspect the bearing is partially seized to the crank, probably spun. There is some debris welded to the crank journal which prevents the crank from sliding easily in the bearing.

This motor is junk. There might be a small chance the block is not cracked, but I don't think that's more than 0.01%.

Leon Speed

GregBBRD

I've seen the same thing that Bill Ball saw today, several times. The crankshaft and bearing gets so damaged that the crank simply won't budge without a lot of force.

Your (Billtool's) complete lack of endplay (unless you are simply missing the movement) might be cause for concern. The low mileage, however, makes me think you are simply "missing" the movement. I've never seen an engine with this little mileage have a destroyed thrust bearing.

AO

Ouch! How did the owner take the news?

Dave928S

Here's what happens if the rear pinch bolt isn't replaced and torqued at regular intervals (on a manual).

The driveshaft slides forward in the torque tube, and the pinch bolt rides up on the splines. Pointless replacing the front pinch bolt and torquing it, or fitting an additional or alternative clamp, if you don't make sure the shaft is in the correct position within the tube so that pinch bolt grooves align, and the rear is locked in position.

My crank end float was within spec, but the way mine was I believe could have eventually led to TBF. It's a manual, but the shaft was jammed forward on the bolt groove at the rear, was riding up the edges of the groove at the front, and the clutch stub shaft was rammed into the pilot bearing (which had failed) in the end of the crank. That's not the same as constant forward flex pressure from a deformed flex plate, but the thrust bearing was still loaded.

The front connection and flex plate is what everyone concentrates on, but the rear is just as important IMHO, as is the correct position of the shaft within the torque tube, so that the bolt grooves line up with the midline of the bolt at the rear, and the front isn't pushed forward. The bolts on mine were stretched and wouldn't hold torque, which explains the movement, and is why I think they should be routinely replaced.

I know this on a manual 82 ... but the principles are still the same.

jeff spahn

Greg- You put the new Tranny in Ron Perry's car. Should I be checking the rear bolt now or is it good for a few more years before I need to. I looked and can't see where it is. Can someone post a pic for the rear bolt location?

GregBBRD

I'd certainly check it, whenever I had the chance. The whole torque tube shaft clamping thing isn't as tough as it could have been...

You need to pull down/move the two heat shields over the rear mufflers to see the hole in the back.

jeff spahn

Okay thanks. I appreciate the high level of work you did on this car for so many years. I am striving to keep it as you did for Ron. Spent the last week cleaning the engine with simple green and a tooth brush so when my intake refresh (cam covers too) is done it will continue to look like a SoCal car, clean and shiny all over and underneath.

Don Carter

Bill,

I checked my flexplate for the first time a few weeks ago. Very little bow in the plate, and very little movement when the tension was released. I also could not get the flywheel to move much at all, fore and aft, but I think there was slight movement, just so small it was hard to measure.

TBF is more common on 87+ cars, as I remember, so I wouldn't be too worried.

Bill Ball

The movement should be slight but definite. Usually it's just 0.1 to 0.2mm and it can't be seen but it can be heard (clunk). You need a dial gauge to measure it. I measure mine about once a year. It has been rock stable at 0.2mm. Worth checking at least once just per chance you are at the wear limit and can catch it before the engine is turned to junk. TBF is rare in cars up to 85 unless a real mistake has been made because they have a shim system on the front end of the TT shaft that prevents migration through the clamp and loading up of the flexplate. Your car should have the groove in the shaft for this system, but probably does not have the shims (the factory stopped installing them after 85). You could order up the parts and install them. Otherwise, you do need to keep checking and relieving the flexplate preload and do at least one endplay check.

anonymousagain

also to note regarding Bill Ball's explanation - the initial relief from loosening the clamp would have allowed the engine to turnover and start just fine.

If incorrectly thought to be okay (and we hadn't pursued the zero end-play abnormality), it could have been a real nightmare if unknowingly driven for some distance until it just fused to the block and stopped running...in the middle of nowhere.

Thank goodness for Bill's persistence to adhere to the procedure and make sure end-play was recognized, since it CAN'T be zero. Sure, verification of TBF sucks, but at least that verification didn't come out on the road in the middle of nowhere.

At this point, this may become an experiment - pull the engine, repair the damaged area and weld up the crack in the block, then put the engine back in. Lots of labor time, but minimal cost. My Dad's Top Alcohol friends do it all the time when they toast a block during a 1/4mile pass, so we may cave to their recommendations and try it.

blown 87

Greg and a few others that have built more 928 engines than I have will confirm or deny this, but from what I have seen is that there is a variance in the thrust bearings and the cranks.

Mine which was in spec showed Copper on the thrust bearing, while pauls at over .020" end play only showed slightly more.

Mine is on the tight side with new thrust bearings in it, but after almost 30,000 miles I think it is ok.