[dleccord]

why do you have to apply a certain amount of torque to a bolt/nut and then turn it 90 degrees? why not just give the final torque reading that includes the 90 degrees?

[Speed_Addict]

Stretch Bolts - these fasteners are designed to stretch some at their final torque setting. The initial torque value gets you to the torque value that is the starting point for the part of the tightening process where you actually stretch the bolt by turning it an additional 90 degrees (typically 90 or whatever is specified). This is a one time process and thus the need to replace thees types of bolts in most use case situations - they are generally one time use, otherwise you stretch them again and they are beyond design spec and could shear / break.

[Volasko]

^^
Great answer.
/thread

[Digitalize]

AvE explains it here too, embed seems to pick a random time but around 9:30 onwards he explains it -

Basically because they know how far the nut moves in that space it's actually far more accurate than torque rating, which can vary depending on friction etc.

Originally Posted by Digitalize

Basically because they know how far the nut moves in that space it's actually far more accurate than torque rating, which can vary depending on friction etc.

This right here ^^^

Originally Posted by Speed_Addict

Stretch Bolts - these fasteners are designed to stretch some at their final torque setting. The initial torque value gets you to the torque value that is the starting point for the part of the tightening process where you actually stretch the bolt by turning it an additional 90 degrees (typically 90 or whatever is specified). This is a one time process and thus the need to replace thees types of bolts in most use case situations - they are generally one time use, otherwise you stretch them again and they are beyond design spec and could shear / break.

Your first reference is to torque-to-yield (TTY) bolts. Your second reference is to torque-to-tighten (TTT); which is not a bolt but a procedure used for tightening fasteners. TTY fasteners are often tightened using the TTT method, but both are obviously two completely different things.

Pretty neat article on that here if anyone's interested:
http://www.boltscience.com/pages/a-c...tightening.pdf

[dleccord]

Are there incidents where BMW snapped their suspension bits due to not replacing nuts and bolts?

Originally Posted by dleccord

Are there incidents where BMW snapped their suspension bits due to not replacing nuts and bolts?

You won't find a lot of aluminum fasteners holding the suspension pieces together. That's where I'd be most worried of breaking bolts while trying to recycle them.

Any of the other TTT bolts I'd be weary of using factory recommended tightening procedures on more than a few times, for sure.

[feuer]

A lot of bolts are CAD plated that resemble aluminum and often people mistake them for aluminum bolts.

[jfandl]

Originally Posted by dleccord

Are there incidents where BMW snapped their suspension bits due to not replacing nuts and bolts?

I haven't snapped any after many years of service but when I reinstall the stretch bolts into something for example like the stiffening plate, it's designed to be torqued at 56Nm. I torqued it to that plus 15*. There was no way I was going past that as I'm sure 90* would have surely snapped the bolt head or something. I'd rather use some locktite if necessary because getting a snapped bolt out is one of the most painful processes on a car. Next time I pull off the plate, I'll probably just keep it at 56Nm without any angle of rotation. I think what's more important is they are all torqued the same as opposed to tightening them without a torque wrench.

[Simbl]

I have not found any TYY bolts on any of the chassis. Many of the bolts have a round thread profile which deforms to act as a locking feature. Every bolt has some sort of locking feature with BMW, flanged hex, lock nut, oval nuts. The bolts marked blue in the engine are all TTY.