Fair enough.

- Let's say BMW says nothing about these bolts, the best way to approach this problem is: is there any scientific evidence to support replacing the bolts every time? If not, re-use the bolts.

- To me, these bolts are no different than the wheel lugs.

- This has been a boring thread to be honest...re-use the bolts and move on...

I think you just made it longer and still boring.

So, when I click on the link on RealOEM for 31101096987, I get a price of $3.53, but when I look at it on FCP, ECS, Turner, etc.., I get prices between $15-20. Would that be for all 6 bolts? None of the listings show it as 6 bolts, but based on the price, I'm hoping it is.

BTW, my 3.0i has build date of 9/06, and I have the aluminum plate. I believe that's the last month they made E53's.

6 bolts is wishful thinking. Those bolts are not specific to the stiffening plate, so there is no reason for it to be 6 rather than 1.

If you're really going to pay that much for BMW bolts, to ease the pain, first look up prices on the 77mm-long versions - PN 33306783637. (over $100 list price for one M10x77 bolt with a large captive washer). After seeing that, $20 each will seem like a deal.

Or you could buy non-BMW M10x55 Class 10.9 bolts for a fair price. Or re-use like most others do, and torque carefully.

My questions are:

If these bolts are stretched during the torquing process, will the nut still easily run the complete length of the bolt?

If they do, then they stretched back and are ok to reuse?

If they stretch and stay longer, the nut would not easily run the length of the bolt, then I assume they are irreversibly damaged.

Makes sense to my engineering mind, but I only had a couple materials courses, and focused on electricity.

Maybe if I take off the belly plate I’ll try. Don’t hold your breath, I have enough on my to-do list.


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My post on this in another thread (#712 of the stiffening plate bolt threads):
https://xoutpost.com/1109289-post66.html
has some of the simplified concepts of yield stress, etc. as they relate to this issue. If you can follow that stuff, that's a good place to start.

Then with that as background, to try to answer your questions ...

Depends on "easily." The deformation will be different on the bolt threads where they were contacting the nut. I've noticed on my own bolts that some seem to be deformed visually and by not threading back easily, and some seem like they did not deform at all.

Going back to the ".9" in "Class 10.9," that means the yield stress (where plastic deformation begins) divided by the ultimate tensile stress (where it snaps) is 90%. Everything has tolerances and safety factors, but if you really are in the yield region of a Class 10.9 bolt, you don't have much further to go (in terms of stress, although it can strain more since the yield curve has flattened off in that region) before it breaks. Given all that, I can see that they would not design it to yield "much," and maybe not actually at all. So I bet it is possible to torque it to spec and take it off and find that nothing actually yielded (proven by the nut finger threading the length of the bolt).

I'd say that yes, your thinking that easy threading over the full length means no plastic deformation actually occurred is a safe assumption. And in this case they can be re-used, perhaps even with the original torque+full-angle spec (but I'd go with a smaller angle).

They are irreversibly deformed. But depending on just how deformed you're talking, I would not worry about it - and would re-install them as described in the other post.

I know this is a controversial topic so I try to be careful and write only things that are supported by basic theory. As others have said, unless we know exactly what the engineering reasons (if any) led to the TTY spec, it is at least a little dangerous to not follow the spec ...

Except in the case of the bolts themselves. Those (unless anyone has any information otherwise) are not special TTY bolts, or special or magic in any way. They are just M10x55 Class 10.9 captive washer bolts. I expect BMW uses them in non-TTY applications too.

You should be able to get exact non-BMW equivalents of those without any fear at all. Class 10.9 specifies the relevant material properties. You can match the coating if you like, in case you're one of the lucky few that does not have an oil-coated stiffening plate. And a loose washer would probably work just as well as the captive one.

One more thing - the nuts deform too, if the bolts do. So ...

I wouldn't lose sleep on this, looking at mine they have been re-used several times and I even re-used them last time when I checked the under belly for leaks (PO disconnected the oil sensor before selling it to me... no lights OK..)

looking at it, if all 6 bolts say snapped out then what will happen next? the X5 will collapsed underneath because the stiffening plate is gone? what's the possibility of them all snapping together at the same time and causing an accident?

I've re-used nuts when the service manual says to discard them, I just put loc- tite on them and so far so good.

If it's part of the engine, transmission, differential, transfer case I would definitely follow the torque specs and use new nuts/bolts.

Is the reason BMW recommends not to re-use them because they will come loose?

I don't know why they do what they do. But I do know ...

If the torque procedure does actually cause plastic deformation ("yield"), then that procedure (e.g., removal and reinstallation) cannot be performed indefinitely. Each time you'd re-install with a little more plastic deformation, you'd be moving closer and closer to the ultimate tensile stress ==> fracture.

So if re-using, and yield may have occurred previously, you can't blindly do the torque + angle installation procedure. You need to be a little smarter and more careful. Knowing that might be beyond the ability of many professionals and DIYers, that could be why BMW decided it's pretty easy to just say re-use them.

BTW, other stuff that might be safety related, and is easy for BMW to say (from the owner's manual):

"For your own safety, use genuine parts and accessories approved by BMW."

"Installation and operation of non-BMW approved accessories such as alarms, radios, amplifiers, radar detectors, wheels, suspension components, brake dust shields, telephones (including operation of any portable cellular phone from within the vehicle without using an externally mounted antenna) or transceiver equipment (for instance, CBs, walkie-talkie, ham radio or similar accessories) may cause extensive damage to the vehicle, compromise its safety, interfere with the vehicle's electrical system or affect the validity of the BMW Limited Warranty. See your BMW center for additional information."

"For your own safety: use only brake pads that BMW has released for your particular vehicle model. BMW cannot evaluate nonapproved brake pads to determine if they are suited for use, and therefore cannot ensure the operating safety of the vehicle if they are installed."

Similar thing on BMW-only wheels.

Snow chains only on the 17" wheels.

"In order to ensure the safety and reliability of the brake system, have the brake fluid changed every two years by a BMW center"

I'm sure that's all fair advice, and for some people, they'd be better off following blindly rather than thinking. But it also shows that BMW will say things in the name of safety that may be overly conservative for people who know a thing or two.