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 ...
Quote:
Originally Posted by DougPEX5
If these bolts are stretched during the torquing process, will the nut still easily run the complete length of the bolt?
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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).
Quote:
Originally Posted by DougPEX5
If they do, then they stretched back and are ok to reuse?
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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).
Quote:
Originally Posted by DougPEX5
If they stretch and stay longer, the nut would not easily run the length of the bolt, then I assume they are irreversibly damaged.
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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 ...