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  #211  
Old 04-08-2019, 06:28 PM
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https://www.youtube.com/watch?v=fvzdehnJA9k

I'm trying to stay out or at least limited here, but here are some general and specific comments. Not trying to give any answers, just information for those interested.

There is a ton of bad info on the internet. It's a challenge to sort through some of it.

Stress-strain of steel (and other similar materials) is a very important and pretty well studied and understood science. Important for it to be well understood for building bridges, pressure vessels, pipelines, etc.

That science can be extended and applied to help understanding of the specifics of threaded fasteners - nuts vs. bolts, yield, proof load, torsional vs. axial stress, etc. That's where things can get a little complicated and it may seem to be purely empirical and more black magic than it really is. Thinking back to the basic materials science of stress-strain helps bridge the gap.

For example, "yield" means some of the steel plastically deforms, which is a permanent change in shape, and strengthens the material. From that point on, it can still function elastically, including returning to the previously reached deformation point repeatedly in an elastic state, with the same elastic modulus but a now higher limit before yield will occur. But pushing beyond that point has its limits, and will eventually lead to fracture.


Those were the general comments. Now for some specific stuff:

Be careful if torquing with a non-flanged nut and bolt. The BMW ones are both flanged. Due to the increased area and torquing radius vs. non-flanged fasteners, and the fact that about 50% (depends, of course on surface prep, smoothness, etc.) of tightening torque is used to overcome that surface friction, using non-flanged fasteners will provide more stretch (and I use that term without distinguishing between plastic and elastic deformation) than will flanged ones.

Similar to how using really well lubed surfaces and threads might result in excessive tightening.

So that could mean if you're using non-flanged fasteners, even on the initial installation you are over torquing the 56 Nm stage, possibly causing plastic deformation where it would otherwise not occur.


Different thread pitch
If you try using SAE fasteners or a fine thread, the bolt stretch you get from applying a 90* angle will be different. The torque will be different too, but that is less clear.


Variable nut-thread loading
That is a basic fact of threaded fasteners. It happens regardless of any plastic deformation.

Fasteners are generally designed to one one hand have the nut be more compliant than the bolt to allow for better load distribution, trying to reduce this effect.

And on the other hand, the nut is always designed to be stronger than the bolt. The reason for that is that a nut would fail by having the threads progressively strip out, vs. the bolt would just fracture. You need to worry about the first nut thread stripping out, because if it starts to strip it will pass that load down the line and all the threads will strip out. Vs. the bolt fracture would most likely happen during assembly which is not such a big problem, vs. having a nut progressively strip out would happen literally "down the road" when it would be more of a problem.

For similar reasons, you should never use a weaker nut than bolt if there is any concern for nut failure. So using a 10.9 nut on a 12.9 bolt is not something to do casually. Doing that, you will be ensuring that the nut will fail before the bolt, which is the opposite of what is safer. Here's a pretty good reference for that and other things: Frequently Asked Questions on Bolting Matters

On the other hand, my M54 engine uses Class 12.9 head bolts, which do yield along their whole 110 mm length, threaded into an aluminum engine block ... or during a typical head gasket repair threaded into a ~15-thread long steel insert, which is probably not Class 12.9. So if it's designed carefully, it can be made to work.



When terms like plastic deformation, yield, proof, ultimate, etc. are used, they sometimes refer to total system failure (like "ultimate tensile strength" tells you when the bolt will snap [system failure] when increasing load is applied).

And sometimes they refer to localized material effects - where for example, the first thread in a nut may plastically deform very slightly - and it's not a problem at all, but it did deform. BTW, by the time the nut threads noticeably plastically deform, the bolt will have deformed a lot more.

True "stretch bolts" or TTY bolts - by which I mean bolts that are made specially for a TTY application, rather than regular bolts that are used in a TTA/TTY(?) application (as we have here) - are typically designed with relatively oversized heads and threaded sections, but with a long, thinner, uniform portion that is designed to yield, and to have all the yield there, and none anywhere else. Those are easier to analyze.


On figuring out whether your previously used nuts and bolts have deformed enough to significantly affect reinstallation by requiring more torque to overcome the bad fit caused the deformation ...

One pretty simple test is to see if you can finger-thread the nut onto the bolt. If you can do that, your torque wrench will probably not be fighting too hard to overcome that, and the measured tightening torque will have the same clamping effect as when they were initially used.
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  #212  
Old 04-08-2019, 08:58 PM
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great stuff, especially how of course the angle changes directly with respect to the thread pitch when using TTA; the specs are only for one exact bolt. Also with the details about lubricate torque; my 'torqueometer' shows both the dry and lubricated torque.

re: hand thread nut; of course with these nuts you can't hand-thread them because they are oval in the center self-locking nuts, so you won't be able to tell that way. I was concerned that the squashed nuts would affect the torque numbers but i guess once the nut gets started it's much easier to turn because i got no reading on the torque adapter from the nut before snugging tight.

I'm a little confused by the 'stronger nut' vs. Fastenal quote above saying that the nut is 'softer' I believe that means that the metal of the nut is a lower ksi than the bolt, but because the nut 'melds' into the bolt it holds stronger or something, i would love to have some clarification on the seemingly contradictory things.
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  #213  
Old 04-09-2019, 10:54 AM
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Thanks guys. Keep the info coming. Definitely going to measure my bolts when I drop my plate to do the front diff fluid.

Quote:
Originally Posted by bcredliner View Post
I just want to know what is the right thing to do.
Buy new bolts/nuts.
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  #214  
Old 04-09-2019, 11:19 AM
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On a 17 year old X I would absolutely start with new bolts. I would not buy official BMW bolts I will be getting black oxide 12.9 and washers. If they fit I will likely get the T nuts and epoxy them in place to convert my e53 to act like the new e70 that has threaded inserts in the axle support.

Once replacing the bolts I'll be simply set I'm opting for the forever 12.9 bolts.

If I got the 10.9 bolts I would torque to spec and keep track of the use count with a center punch and dents in the plate, replace after five uses or when torque drops below about 97 N·m before 90°

(That number might change higher or lower once I test with new bolts).

I linked above to a 15 pack of decent bolts for $17.

My determination is that the high force on the spec of the design is mostly for the well being of the CAR in a crash. The bolts can be at 1/3 of design spec and handle anything you can possibly subject them to by driving. That is why "everybody" gets away with treating the metal at if it's just for under belly protection and not an important part of the frame
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  #215  
Old 04-09-2019, 11:27 AM
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Cant believe this discussion is still going on had to unsubscribe as this has become as painful as an oil thread

Carry on
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  #216  
Old 04-09-2019, 11:33 AM
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Quote:
Originally Posted by andrewwynn View Post
My determination is that the high force on the spec of the design is mostly for the well being of the CAR in a crash. The bolts can be at 1/3 of design spec and handle anything you can possibly subject them to by driving. That is why "everybody" gets away with treating the metal at if it's just for under belly protection and not an important part of the frame
Would a subframe connector at the back of the "U" resolve that desire to twist? I have not checked clearances or anything down there but was just thinking out loud.

Interesting theory as well due to the many different variants of the plate. I think there are 3 or 4 total part numbers for the plates with i6's, v8's, early models, diesels, and late models all getting different versions. And to add to complication some (like the 4.8is) have steel plates while others have aluminum. The latter of which, with the large access holes cut in them, don't seem like they'd off much protection against twisting or deformation of the subframe.
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  #217  
Old 04-09-2019, 11:38 AM
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The plate will not do a lot to mitigate torsion of the frame since it's straight across the bottom. It's the diagonal connections that matter. It resists a parallelogram force that would push for example the left of the car forward and the right of the car backwards in addition to keeping the back of the subframe from moving toward and away from the center.

Many cars use a giant "X" made from stamped steel but BMW chose a thick alum plate to add in underbelly protect and airstream improvement as well.
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  #218  
Old 04-09-2019, 02:03 PM
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  #219  
Old 04-09-2019, 02:28 PM
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Quote:
Originally Posted by OptimusPriM5 View Post
Cant believe this discussion is still going on had to unsubscribe as this has become as painful as an oil thread

Carry on
Quote:
Originally Posted by StephenVA View Post
This is an automotive forum. Things are bound to go circular from time to time. Is what it is.
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2006 X5 4.8is Build 11/05 Maintenance/Build Log
Nav, DSP, Pano, Running Boards, OEM Tow Hitch, Cold Weather Pckg (Purchased 08/15 w/ 90,500 miles)

2010 X5 35d Build 02/10
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  #220  
Old 04-09-2019, 02:39 PM
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oldskewel and Andrewwynn
Thanks guys for the input and fact based insight on these bolts. It is always a welcome sight to read your posts. There are only a handful of regular posters who actually understand that sharing standards, insight, analysis, and "TESTING vs guessing" actually provide tips on keeping these X5's on the road.

Thanks!
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The Blue ones are always FASTER....

Current Garage:
2005 X5 4.8is
2002 M5 TiSilver
2003 525iT
1998 528i
Former Garage Stable Highlights
2004 325XiT Sport
1973 De Tomaso Pantera, L Model
1970 Dodge Challenger T/A 4 sp Alpine White
1970 Dodge Challenger T/A 4 sp GoManGo Green
1971 Dart Sport, “Dart Light” package
1969 Road Runner 383
1968 Ply Barracuda 340S FB Sea-foam Green
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