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  #11  
Old 12-17-2016, 10:45 AM
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Quote:
Originally Posted by CapeX5 View Post
I have re used mine. And I think I have read most of the threads regarding this issue. In none I have read have I seen an explanation of what kind of engineered task that plate provides that necessitates the use of a TTY one time bolt! I don't want to start a whole pissing match on this, but would be very curious as to why. While I was working on my 01 I ran quite a while with the plate off and didnt' notice anything going on. Inquiring minds want to know....
+1 on wanting to know. I mean really the plate appears to more of protection from the underside of the engine if you were to go off road. It doesn't appear to be structural since is attached to the subframe.
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  #12  
Old 12-17-2016, 01:56 PM
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Originally Posted by X53Jay4.8is View Post
+1 on wanting to know. I mean really the plate appears to more of protection from the underside of the engine if you were to go off road. It doesn't appear to be structural since is attached to the subframe.
The plate IS structural in that it stops the sub-frame from twisting under torsional load. It works in a similar way to a diagonal brace on wall framing in a house. It doesn't need to be super thick or strong to stop diagonal distortion of the sub-frame but the mounting bolts DO need to hold it place very tightly to allow it to do its job.

This is why some folk get creaking from the plate when the bolts aren't tight - the plate is pulling sideways on opposite corners and the bolt-to-plate joint creaks and groans.
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  #13  
Old 12-17-2016, 03:31 PM
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Originally Posted by CapeX5 View Post
I have re used mine. And I think I have read most of the threads regarding this issue. In none I have read have I seen an explanation of what kind of engineered task that plate provides that necessitates the use of a TTY one time bolt! I don't want to start a whole pissing match on this, but would be very curious as to why. While I was working on my 01 I ran quite a while with the plate off and didnt' notice anything going on. Inquiring minds want to know....
Agreed. There are a few reasons they might have done it, but I have never seen a reliable explanation of the true reason(s). It is not as simple as that they needed it really tight and the bolt could not take it more than one time. No. If they needed it really tight, they could have used a bigger bolt that would not plastically deform, and could be reused without worry.

A smaller bolt, like this, has more compliance than a bigger bolt would have. Maybe they needed that. Head bolts are often TTY for this reason.

A 10.9 bolt that is already torqued past its yield stress is, by design, close to breaking. Maybe there is an intention for it to break away on a collision, as part of the crash design.

Making your customers pay $100 for bolts each time the pan is removed may make financial sense. And it may dissuade some DIY mechanics from getting into the car more than they maybe should.

Using M10 bolts may have saved them a couple of dollars over M12 bolts when initially built. And maybe they had a mandate to not spend a penny on lifetime beyond 100k miles.

So while I re-use mine, and think I know enough to let me do it safely, I'd feel much better if I knew the true reason they did it. I doubt we'll ever know for sure.
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  #14  
Old 12-17-2016, 05:22 PM
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Quote:
Originally Posted by CapeX5 View Post
I have re used mine. And I think I have read most of the threads regarding this issue. In none I have read have I seen an explanation of what kind of engineered task that plate provides that necessitates the use of a TTY one time bolt! I don't want to start a whole pissing match on this, but would be very curious as to why. While I was working on my 01 I ran quite a while with the plate off and didnt' notice anything going on. Inquiring minds want to know....
If you left the plate off, you will still need to reattach some of the bolts since these bolts are also used to secure the sway bar bushing to the sub-frame.
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Old 12-17-2016, 05:39 PM
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Originally Posted by wpoll View Post
The plate IS structural in that it stops the sub-frame from twisting under torsional load. It works in a similar way to a diagonal brace on wall framing in a house. It doesn't need to be super thick or strong to stop diagonal distortion of the sub-frame but the mounting bolts DO need to hold it place very tightly to allow it to do its job.

This is why some folk get creaking from the plate when the bolts aren't tight - the plate is pulling sideways on opposite corners and the bolt-to-plate joint creaks and groans.
Okay got it. When I think of structural I am under the impression that the without it the structure is going to fail during use. So without this stiffening plate will the structure of the X5 bend and twist thus making it unsafe to operate or is it just there for more added support?
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  #16  
Old 12-17-2016, 06:02 PM
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I have to remove this plate since I have to get to the oil pan to remove all the plastic bits from my failed timing chain guides. I'm going to reuse the bolts and just torque them down super tight, so I guess we'll see what happens. No way am I ever paying that much for some bolts.
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  #17  
Old 12-17-2016, 06:49 PM
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Originally Posted by X53Jay4.8is View Post
Okay got it. When I think of structural I am under the impression that the without it the structure is going to fail during use. So without this stiffening plate will the structure of the X5 bend and twist thus making it unsafe to operate or is it just there for more added support?
I'm guessing now but I would say that as the sub-frame is tubular steel with welded junctions, the reinforcing plate is to reduce the stress on those "corner welds" when the sub-frame is subjected to twisting (torsional) forces, like entering a driveway at an angle etc.

Welds on tubular steel like this are very strong when compressed or stretched but not when twisted; the steel tends to fracture on the tube body along the weld line (the weld is often stronger than the thin-walled steel tube).

Lean way back on the rear two legs of a welded steel chair for a bit and see what happens...

Now bolt a square piece of thin alloy sheet to the legs on one side (or both), using four bolts and see if you can lean back for longer. The thin sheet will prevent the "square" steel frame from becoming a parallelogram, breaking the welds and collapsing. the sheet doesn't have to be thick, as all the forces are in the plane of the sheet. Just like on the E53... Need good bolts though (able to withstand high sheering forces), also like on the E53.
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  #18  
Old 12-17-2016, 07:06 PM
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I think the main purpose of the plate was to act as a drip pan for any oil leaks from the oil pan, or transmission. Why else would they add a foam piece on top but to absorb all the oil. After all, when you are trying to sell an upmarket product, you don't want oil leaks on the marble tile floors in the garage.
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  #19  
Old 12-17-2016, 07:29 PM
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It would appear to this pair of eyes to be both structural as well as functional. Note the comments about tubular steel strength at welds for the structural functionality.

It also functions as a "skid Plate" for any off roading type activities where something could impact and damage the oil pan.

Would have been nice to have something going all the way back or another plate for the transmission pan protection

Yeah, I would reuse the bolts for this plate application, however, for a bolt such as the crank pulley bolt, I would strongly consider replace that one.

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  #20  
Old 12-17-2016, 07:38 PM
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Originally Posted by semcoinc View Post
Yeah, I would reuse the bolts for this plate application, however, for a bolt such as the crank pulley bolt, I would strongly consider replace that one.
Anything internal or related to the engine gets new bolts (if required) as a rule of thumb when I work on cars. Whereas secondary parts like the skid plate won't really cause a catastrophic failure if an old bolt breaks, so I don't feel wrong reusing it.
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