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.76mm might just be bearing/crankshaft damage.
You might want to pull the pan and see if the center main has spun...that's what ruins the blocks.
And std/std 5.0 crankshafts are pretty common.
.76mm might just be bearing/crankshaft damage.
You might want to pull the pan and see if the center main has spun...that's what ruins the blocks.
And std/std 5.0 crankshafts are pretty common.
Thanks Greg.
This is unchartered territory for me. That could be very good news for the owner.
I have a couple of 5.0 crankshafts in my storage unit. Both are low mileage <50k.
The engine stalled once - just for me - so maybe just maybe the center bearing is intact.
The thrust bearing has turned about 30 dg. There are no replacement blocks to be had.
There are threads on RL that discuss the possibility of repairing the block. I have a top notch machine shop local to me.
If there is a way to save this engine that would be great.
If the center main only spun 30 degrees and stopped, an align bore might not even be required.
If the main spun a little bit and there is minor damage, the block can be align bored, back to standard.
If the bearing spun a bunch and ended up at 30 degrees, the block will likely have a crack in the center main and be trash.
Needs to come completely apart to know.
While I don't have a short block, I've got a couple of complete engines of this vintage, with good thrust bearing measurements. (One is in a car and runs...or ran when I got it.)
I guess I could remove the heads (which I don't really want to loose.)
Or maybe you ship me your heads back?
Of course, I find the engine had a timing belt failure in the past. What's left of the timing belt was found between the crank and the oil pump.
The head gaskets look almost new. I'd bet the heads were refurbished with the required number of replacement valves the event bent.
Now why the PO's current mechanic made no mention of this when he did the timing belt and waterpump or didn't even clean the remnants is complete mystery.
Last edited by Kevin in Atlanta; 10-04-2021 at 10:59 PM.
The last time I had it here I discovered it would start in park. I was adjusting the parking brake, started it with me in it as it headed towards my workbench.
After a bunch of looking around and quality time with the WSMs it turned out the rear harness wiring was for a manual transmission and I had to fix the wiring at the the CE panel and the boot.
Why was a different harness in there? Mysteries abound.
At the same time I released the flex plate and added some white-out. When I released it this time the difference was jaw dropping.
The owner's mechanic says he was Porsche factory trained.
The last time I had it here I discovered it would start in park. I was adjusting the parking brake, started it with me in it as it headed towards my workbench.
After a bunch of looking around and quality time with the WSMs it turned out the rear harness wiring was for a manual transmission and I had to fix the wiring at the the CE panel and the boot.
Why was a different harness in there? Mysteries abound.
At the same time I released the flex plate and added some white-out. When I released it this time the difference was jaw dropping.
The owner's mechanic says he was Porsche factory trained.
That follows- they were seemingly trained not to do anything about the slipping flex plate clamp - checking for slippage was "verboten" and to this day never introduced into their service schedules- that would be like admitting there was a problem! Yet back in 1999 when they could not stop my flex plate from slipping [becuase I knew what to look for] when I told the main agents I would not accept the car back until such time as they solved the problem- within 10 minutes of contacting Herr Ferdinand in Stuttgart, the main agents got a reply instructing to use new bolts and over torque by 10%. Not bad for a problem "that did not exist"! Needless to say their solution was complete BS and fared no better than the stock setup. I took the Loctite solution to them, supervised its implementation and it still holds to this day. Looking forwards to having a chance to install my Constantine clamp though!
That to this day we keep seeing examples of this problem is heartbreaking. Porsche really deserve to have a class action lawsuit taken out against them- somewhere where there is no statute of limitations!
That follows- they were seemingly trained not to do anything about the slipping flex plate clamp - checking for slippage was "verboten" and to this day never introduced into their service schedules- that would be like admitting there was a problem! Yet back in 1999 when they could not stop my flex plate from slipping [becuase I knew what to look for] when I told the main agents I would not accept the car back until such time as they solved the problem- within 10 minutes of contacting Herr Ferdinand in Stuttgart, the main agents got a reply instructing to use new bolts and over torque by 10%. Not bad for a problem "that did not exist"! Needless to say their solution was complete BS and fared no better than the stock setup. I took the Loctite solution to them, supervised its implementation and it still holds to this day. Looking forwards to having a chance to install my Constantine clamp though!
That to this day we keep seeing examples of this problem is heartbreaking. Porsche really deserve to have a class action lawsuit taken out against them- somewhere where there is no statute of limitations!
could you explain more as to what happens to the flex plate? I have done some research and as of yet not found a fully detailed answer
Auto equipped 928's drive shafts want to pull the crankshaft out of the engine. The harder you drive them the more they tug on the crank. There's a joke in there.
The effects show up in the flex plate. It bows toward the rear. When you release the flex clamp the flex plate returns to its original flat position.
It also relieves the pressure on the crankshaft.
Measuring crank end play is the best way to know the health of your thrust bearing.
Others will correct me or add more flesh to the answer
When the flex plate bows to the rear, there is of course significant pressure pulling it there. If the clamp on the shaft connecting the flex plate/crankshaft to the shaft in the torque tube 'slips' a little, then when the acceleration force is diminished (car coasting or engine stopping), the flex plate is still bowed a small amount and tries to push the crankshaft forward. When driving further at moderate speeds, that forward thrust is still there, and brings the throws of the crank closer to the thrust bearing. Multiply this 'experience' for the drive train by several hundred or several thousand times and now the crank throw is rubbing against (machining the surfaces) of the thrust bearing. Eventually, as Greg has stated the force against the bearing will cause it to spin - resulting in severe engine damage.
Only happens on automatics as Kevin said, and only on the late cars (I think '86.5 to '95, but might be earlier as well). There are a few ways owners and aftermarket suppliers have developed to increase the 'bond' between the flex plate and the torque tube shaft. For those of us who have owned automatic later model cars, it is essentially mandatory that we check the end play of the cranks shaft with the OE clamp pressure reduced to zero at reasonable intervals, then install a new clamping bolt, increase the torque to 10% above specification and or add additional 'bonding' approaches to avoid this type of engine failure
When the flex plate bows to the rear, there is of course significant pressure pulling it there. If the clamp on the shaft connecting the flex plate/crankshaft to the shaft in the torque tube 'slips' a little, then when the acceleration force is diminished (car coasting or engine stopping), the flex plate is still bowed a small amount and tries to push the crankshaft forward. When driving further at moderate speeds, that forward thrust is still there, and brings the throws of the crank closer to the thrust bearing. Multiply this 'experience' for the drive train by several hundred or several thousand times and now the crank throw is rubbing against (machining the surfaces) of the thrust bearing. Eventually, as Greg has stated the force against the bearing will cause it to spin - resulting in severe engine damage.
Only happens on automatics as Kevin said, and only on the late cars (I think '86.5 to '95, but might be earlier as well). There are a few ways owners and aftermarket suppliers have developed to increase the 'bond' between the flex plate and the torque tube shaft. For those of us who have owned automatic later model cars, it is essentially mandatory that we check the end play of the cranks shaft with the OE clamp pressure reduced to zero at reasonable intervals, then install a new clamping bolt, increase the torque to 10% above specification and or add additional 'bonding' approaches to avoid this type of engine failure
Ok great and thank you. I have mine on the lift now and will check for sure. It does appear to be fine on my car but this will be done moving forward. What bonding agent is everyone using? 638 cylindrical bonding loctite or ?
08-01-2021, 03:07 PM
ISO 1987-91 Short block
