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      10-23-2020, 03:06 PM   #1
jturboawd
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Crank Hub Theory & Fix

Peers,

I am a subject matter expert on connections, seals, etc, often dealing deeply with materials, fasteners, etc. I have a theory for the crank hub that i would like to float by the team. I have already installed a splinelock and CBB but this could help others if the hypothesis is true.

As we know the sprockets are held in place with preload through the crank hub bolt. This bolt is preloaded seemingly to yield. Preloaded fasteners tend to loosen over time through several different phenomenon. I have seen and proven this in other systems using strain gauges overtime in dynamic conditions, even when preloaded to extreme conditions. Sometimes preload is reduced 1-5% without radial movement through the relaxation or movement of strain at the thread interface (training, etc,).

It seems like if you check the preload of this stud against expectations that many cars will be preloaded less than optimal. Its hard to do this because OEM does torque + movement, so you cant check the preload post make up. I estimate 550 or so ft-lbf to make up.

To check this we could check the break out torque of a failed oem unit vs a non failed unit to see if there is a trend. We do not have that though.

From a engineering perspective, if i could not afford or justify a fix I would do the following:

1. Lock the flywheel using OEM tool
2. Remove belts and balancer
3. Get 3/4" cheater, (3-5') and socket
4. Mark position of stud vs crank hub
5. Test fit CBB and determine if you need to rotate to fit and how much you can rotate and still fit
6. Carefully apply torque while monitorying any movement
7. Apply 550 +/- ft-lbf using best guess methods (mannually calculated, use weight, use pull bar, etc)
8. Install CBB


I am of the opinion that you can loose enough preload without the fastener turning to cause issues. If you do the above you can confirm you still have preload and you can have the CBB for emotional support. If the above is correct this could be a maintenance thing that is done every X miles. I would expect the bolt to move 10-30 degrees the first time, then 0-10 after that.

Just my 2 cents.
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      10-23-2020, 03:57 PM   #2
jpy1980
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From what I gathered, the bolt loosening is only one way it could fail. The more catastrophic failure would be if the friction washers fail
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      10-23-2020, 04:48 PM   #3
Nisse Järnet
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DONT USE the flywheel tool to lock the crank when loosen or tighten the crank bolt! It's not meant for that, you need to use a crank hub lock/hold tool.
But yes, that test would be interesting! I might do some torque testing on my OEM crank hub.
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      10-23-2020, 04:50 PM   #4
Nisse Järnet
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Quote:
Originally Posted by jpy1980 View Post
From what I gathered, the bolt loosening is only one way it could fail. The more catastrophic failure would be if the friction washers fail
How would the friction washers fail if the bolt is still properly torqued?
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      10-23-2020, 04:53 PM   #5
nbennettksu
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Quote:
Originally Posted by Nisse Järnet View Post
How would the friction washers fail if the bolt is still properly torqued?
Not sure, but mine spun so bad the washer overlapped its self to a small degree. I had a pic, can't seem to find it.
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      10-23-2020, 05:13 PM   #6
Nisse Järnet
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Quote:
Originally Posted by nbennettksu View Post
Not sure, but mine spun so bad the washer overlapped its self to a small degree. I had a pic, can't seem to find it.
Are you sure the bolt was properly torqued when this occurred? I mean if the bolt has started to come loose or weren't properly torqued the washers will break
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      10-23-2020, 05:14 PM   #7
Nisse Järnet
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Btw, M50 and S55 seems to both use M18 bolts (Haven't checked length) and the M50 torque spec is 410Nm if I'm not mistaken.

Edit: or does S55 use M16? I've found different numbers...haven't taken my crank bolt our yet to check myself.

Last edited by Nisse Järnet; 10-24-2020 at 07:36 AM..
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      10-24-2020, 09:55 AM   #8
nbennettksu
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Quote:
Originally Posted by Nisse Järnet View Post
Quote:
Originally Posted by nbennettksu View Post
Not sure, but mine spun so bad the washer overlapped its self to a small degree. I had a pic, can't seem to find it.
Are you sure the bolt was properly torqued when this occurred? I mean if the bolt has started to come loose or weren't properly torqued the washers will break
It was oem and failed. Sure could have I suppose. My insane 2-pin hub has handled worse abuse with no issues.
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      10-24-2020, 04:17 PM   #9
Chris@VargasTurboTech
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Quote:
Originally Posted by jturboawd View Post
I am of the opinion that you can loose enough preload without the fastener turning to cause issues.
It's significantly less common but we have seen some CBC only spins, which would be explained either by overwhelming OEM hub capability with torque or losing preload.

Re-torquing TTY bolts is hairy territory and not a good day if she lets go. BMW says "no" to doing that, which is generally accepted industry standard operation. Anecdotally there have been plenty of people who have given the bolts a little extra love JIC and been ok. I wouldn't recommend re-torquing TTY bolts.

Prior to CBC/hub when doing initial R&D we found fairly loose crank bolts post-spin, and infact had some vibration issues on an early N54 engine config with a non-oem crank damper that clued us in that vibes could be a contributing cause of loosening bolts. Once CBC is installed, which is a fastener retention device, the mechanism of loosening can only be done with additional yielding of the bolt or damage to the stacked components.

With a known good hub and known good torque we were able to overwhelm the OEM friction capability doing N54 testing a couple of years back, but it wasn't the sort of experiment that we were interested in getting a lot of data points for -we wanted to fix it as it was causing us to sit out of races. Hence spline lock hub and CBC working together as the complete solution.

Chris
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      10-24-2020, 08:02 PM   #10
Nisse Järnet
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Quote:
Originally Posted by Nisse Järnet View Post
Btw, M50 and S55 seems to both use M18 bolts (Haven't checked length) and the M50 torque spec is 410Nm if I'm not mistaken.

Edit: or does S55 use M16? I've found different numbers...haven't taken my crank bolt our yet to check myself.
Seems to be M18.
Also breakaway torque for the OEM S55 bolt should be >600 Nm according to BMW.
I'll try to measure breakaway when loosening mine.
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      10-28-2020, 07:55 AM   #11
jturboawd
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The OEM specifies to use flywheel tool, its what i used and it worked great. Use TD pin , check timing, set flywheel tool, remove TD pin, work on hub.

I think a lot of people will need to move the bolt slightly when installing CBC, or atleast i did.
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      10-28-2020, 06:20 PM   #12
Nisse Järnet
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Quote:
Originally Posted by jturboawd View Post
The OEM specifies to use flywheel tool, its what i used and it worked great. Use TD pin , check timing, set flywheel tool, remove TD pin, work on hub.

I think a lot of people will need to move the bolt slightly when installing CBC, or atleast i did.
Aah I thought you meant the pin in the flywheel, but I assume you used the correct tool that locks the starter gear?
I usually lock at the front hub, to not put unnecessary load on the crank, but both ways should work really.
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      10-30-2020, 05:46 PM   #13
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My theory is that there's too much torque after tuning it. The main bolt comes loose after a few hard pulls. N then u got to retime the engine.
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