From: CAPTEARLG@aol.com Sent: Tuesday, December 05, 2000 1:30 PM To: 928 Subject: [928] CS Thrust bearing failure I have some possible good news for automatic owners that are worried about crankshaft thrust bearing failure. One of the cars that I have been monitoring for drive shaft movement in the front hub of the thrust plate has stopped moving. We did an experiment on Gary Knox's '88 automatic (car "C" in my previous email) that seems to have stopped the movement of the shaft. Here is what we did: Removed the pinch (clamp) bolt in the front driveshaft hub. This relieved the thrust plate tension and allowed the shaft to attain the proper position. Poured Loctite 290 (penetrating formula) into the three slots in the hub. The intent was to Loctite the shaft to the hub without disassembly. Disassembly requires removal of the torque tube. You should "prime" parts before assembly, but it is pretty time consuming to R&R the TT. Installed a new bolt P/N 931.421.240.00. using Loctite 242 before assembly, torqued to 65 ft.lb. Porsche had recommended to Fred Rourke's mechanics, replacing the bolt and tightening to 110% of recommended torque. The standard torque is 59 ft.lb. The Loctite was our idea. I doubt that the Loctite on the bolt was necessary, but it can't hurt. Let the car sit for 24 hours. Loctite says that a full cure takes 24 hours. The car now has about 2000 miles on it since we did this, including two days of drivers education with NO hub movement. You can say that this is not a long term test, which is true. But we checked this car previously and found that the hub had moved .016 in about 2,000 miles and one DE. The first check we did, there was .105" movement (40,000 miles). So it is very encouraging. This is not an expensive procedure. About $6.00 for the bolt and $6.00 for the Loctite. It takes about an hour or two depending on experience. Check the crankshaft end play while you are in there. We painted the shaft splines white where they enter the clamping hub. After reassembly we could look through the hole in the bottom of the bell housing cover and see the painted splines. We think that we can do a quick spot check for shaft movement without any disassembly at every oil change by looking in the hole. Maybe a small diameter fiber optic light would make the job easier but the white paint helps a lot. I have included my first email on this, as well as the procedure. Car "D" was done on 11/11 using Loctite on the splines and was added to the original email. Unfortunately we used red paint, so it may be harder to check hub movement without disassembly. Here is the first email sent on 9-24-00. I have been assisting 3 friends with automatic transmissions in trying to determine if crankshaft thrust bearing wear is excessive on their cars. For simplicity, I have named the cars A,B and C. Here are the findings. "Hub movement" means: with a dial indicator on the center of the thrust plate hub, loosen the hub clamping screw and record movement of the hub. This shows the preload on the crankshaft. Presumably, no preload is desirable. Car Year Mileage Hub movement CS End play "A" '89 85K .120" .040" .009" "B" '89 114K .100" .002" "C" '88 44K .105" .016 .007" "D" '89 58K .134" .005" (added 12-05-00) We also checked the difference in preload caused by a cold car and a hot car using car "C". After making the initial check and finding .105" hub movement, we tightened the hub clamping bolt with no preload. The car was then driven ~2000 miles including one DE. We then checked the car cold and found .016" preload. Hub migration? We reclamped the hub and drove the car ~30 miles and rechecked the preload hot and found .003" preload. It appears that the shaft/torque tube do not change enough in length from cold to hot to affect the preload. Or was the .003" change caused by migration? Car A was also rechecked after 2,000 miles and the hub moved .040". It also appears that the hub does migrate forward enough to put significant pressure on the thrust bearing, although the wear on cars A and C seems acceptable. To try to stop migration on car "C", we used Loctite 290 penetrating formula on the hub splines and 242 on the threads of the clamping bolt, although the bolt does not appear to loosen. We will check periodically to see if this stops migration. ( the results are above) On cars "A and "B" we used 271 on the threads. All bolts were replaced and torqued to 110% as per advise from Porsche. One thing that we don't understand is: why does car "B" only have .002" crankshaft end play? On car "A"and "C" the crankshaft "clunks" back and forth when we pry them. Car "B" does not appear to move, but the dial indicator shows .002" movement. The specification is .00236" (.06mm) minimum. It is hard to believe that a car with over 114,000 miles has less than the minimum axial play. (Update: 12-05-00 an oil analysis showed no problems.) Is this a symptom of the dreaded thrust bearing failure? Is there another way to check for thrust bearing failure without tearing the engine down? Earl Gillstrom '88 5 speed (.007 CS end play) 99,000 miles Here is the procedure: Drop the exhaust at the manifolds. Remove the bell housing bottom cover. Install a dial indicator on the rear side of the forward Drive Shaft Hub. Loosen The hub clamping screw and watch for hub movement. On the three automatics that I have done this on, the hub moved back between .100"and .120". This preload puts pressure on the rear face of the crankshaft thrust bearing. If the hub does not move more than a few thousandths of an inch when you loosen the pinch bolt then all is OK. (Update 12-05 -00, Car""D" had .134" movement.) After the pinch bolt is loose then put the dial indicator on the flywheel, (not the flexplate). Pry the flywheel back and forth to check the crankshaft end play. The spec for later engines is .0024" to .0076". Wear limit is .016". If your end play is more than .016", then you are experiencing "Thrust Bearing Failure". If you are installing a TT, clean the shaft and hub splines with Loctite primer before assembly. Tighten the forward hub pinch bolt last. Pour Loctite 290 (penetrating formula) into the three slots in the hub. It doesn't matter if hot or cold or the position of the crankshaft. Install a new bolt P/N 931.421.240.00. using Loctite 242 before assembly, torque to 65 ft.lb. The standard torque is 59 ft.lb. I doubt that the Loctite on the bolt is necessary, but it can't hurt. Paint the shaft splines white where it enters the hub for future verification. Reassemble cover and exhaust. Let the car sit for 24 hours. Loctite says that a full cure takes 24 hours. Look through the hole in the bottom of the bell housing every oil change to check for hub migration. The pinch bolt is 10 mm with an 8 mm "Allen" head. Hardness is 12.9. Part number is 931,421,240,00. I will try to assemble a list of results if you try this experiment. Let me know how you make out. Earl Gillstrom '88 5 speed (.007 CS end play) 99,000 miles From: Fred Rourke [rourkefg@omantel.net.om] Sent: Wednesday, December 06, 2000 9:46 AM To: 928 Subject: [928] Re: CS Thrust bearing failure Earl, Thanks for the note. Initial checks on my S4 showed that the pinch bolt technique was working however I have booked a slot for tomorrow morning and we are going to re-check the situation. Painting the shaft is essential to easy monitoring and white is probably the best colour [we used gold because it was there]. Interesting the vibration period that I experienced at 3050 rpm has attenuated significantly since we the modification I advised you about. Before that, re-setting the bolt seemed to smooth things up but then the vibration would return as did the tension from the hub. I am more convinced than ever that the cause could simply be that the pinch bolt creeps with age and at some critical point it starts to let slip. There is no service inspection/replacement of this bolt or its tension and eventually disaster strikes for want of a 10 minute$6 job. Porsche need a good arse kicking for not owning up to this, if that is what the cause is. I still do not know what causes the drive shaft to snap although that is more forgivable [but not by much]. Maybe it is a flaw but not connected to the thrust problem. I'll let you know what I find tomorrow. Keep up the good work. Fred R 1990 S4 auto