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  #1  
Old 07-24-2023, 02:30 PM
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N55 E70 Spun Crank Hub

So I have an E70 with the N55. New to me. On the way to the Airport, on an on ramp shifting from 3rd to 4th (I think) felt a thud, got the engine reduced power warning and coasted to the shoulder. Then engine shut off on it's own 20 - 30 seconds later. Towed home, investigated, all symptoms of a catastrophically spun crank hub. Inspecting for piston valve contact to see if it had spun so hard as to give me bent valves I found this





My borescope's not super great but to me that looks like clear evidence of valve to piston contact. I also don't have much experience with this kind of failure, so I wanted to ask those who might have seen similar things before if I'm correct.
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  #2  
Old 07-25-2023, 09:56 AM
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Leak down/compression test to determine if there's valve damage.

If not, set timing and count lucky stars. You should have definitive errors related to over retarded cams both intake and exhaust if the crank slipped. (even if no errors, on setting timing it will show). It is crazy this motor isn't keyed at the crank, but the incidence rate is not high makes me wonder if people are reusing crank bolts and being surprised later. Do you know the engine's full history?
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Old 07-25-2023, 05:16 PM
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I did indeed have all kinds of cam shaft position sensor warnings both exaust and intake. Having charged the battery on my borescope and continued inspecting, every other cylinder Iíve looked at has marks on both the intake and exaust side.

At this point Iíve got a pretty good idea of the history of the engine. Looks like it ingested the serpentine belt about 10K miles ago. It was serviced by a pretty reputable Euro centric Indy shop here in Charlotte. I initially thought I was safe because a good shop had dealt with the aftermath of that. But now Iíve extrapolated (based on CARFAX and some things the guy selling it to me said) that the engine ate enough belt to jump time so they had to loosen the crank hub to re time it. Clearly they didnít replace the hub bolt and or didnít torque it to Spec. I guess this serves as a cautionary tale.

That being said any BMW enthusiasts Iíve mentioned this to have been super surprised that this happens to a N55 engine. They say itís super common on S55ís tuned to over 500bhp.

Guess Iím shopping for a new/used engine.
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Old 07-25-2023, 05:36 PM
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N55 E70 Spun Crank Hub

Untill you confirm bent valves no reason to give up on the motor. Even then not a reason.

I've helpee bring an m62tu back from the dead where timing chain broke while running and took out 8 valves. (Annoyingly not same head).

The marks on the cylinder can just be carbon and close calls.

If you look at a new piston you'll see thru have dents for the valves to get very very close.

Get a compression test and start there
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Old 07-25-2023, 06:16 PM
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Good to hear and I wonít totally write the engine off yet, especially with the price of comparable used N55ís.

Iím a little stuck because I wasnít looking for a project car when I bought it (unlike with my E53), and Iím in and out of town for work till mid September. That being said, I can think of how I might be able to sort a leak down test, but canít think of a way to get a proper compression test done with out taking the time and trouble of re timing it. Which from what I understand is a pretty huge job, even just to get the crank hub tightened again. Luckily I got a good enough deal on the vehicle that I wonít take a total bath if a new engine is indeed in order.

Just trying to best manage my time and money investment in the vehicle
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Old 07-26-2023, 12:45 AM
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Having just replaced my valve cover I can say the job of getting in there wasn't so bad. Loosen the ac line on the right wheel arch so you can lift them about an inch it makes removing the VC at least 50x easier.

I made a tool from angle iron in the past to counter torque the crank bolt and used a pulley system to pull the bolt to the torque and angle.
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