Stiffening plate bolts

[garrett.fell]

So - Do you still over torque them? I think I read the spec was like 45# then another 90 degrees past that.


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[Omerta]

Originally Posted by garrett.fell

So - Do you still over torque them? I think I read the spec was like 45# then another 90 degrees past that.


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When the bolt is yielded, it is cold worked and will take more force to yield a second time but will reduce the energy it can absorb before breaking. Thus it will become more brittle and will be further prone to breaking.

They are class 10.9 bolts with a minimum yield strength of 940 MPa... M10x55 I suspect zinc plated so a yield torque of 67 lbs-ft..

See on the chart below - each time the bolt is reused its properties change. It is no longer of 10.9 strength. Bringing it to yield each subsequent time brings it closer to its ultimate strength.


That all being said - its location is not "critical" persay. The plate provides lateral stiffness and fasteners for the sway bar. If one of the fasteners fail and all other conditions are equal it will say on the road... but will behave differently in a collision.

[cn90]

Some bolts are tightened using "angle torque" but not necessarily TTY bolts.

Cylinder head bolts, when you compare old vs new, they are different, the old ones are longer from stretch.

The stiffening plate bolts, they are big and fat, very much like a wheel lug. They don't elongate with tightening. They will never break if you torque them properly.

[TiAgX5]

I think some of this was covered here.....

http://www.xoutpost.com/bmw-sav-foru...ternative.html

[lo_jack]

Could one not just use some M10x55 9.8 grade bolts and call it a day? Those are likely still cheaper than replacement non-stocked BMW tty bolts.

[Omerta]

Originally Posted by lo_jack

Could one not just use some M10x55 9.8 grade bolts and call it a day? Those are likely still cheaper than replacement non-stocked BMW tty bolts.

No, the supplied bolt grade is 10.9. Using a 9.8 grade bolt is 15% weaker...

12.9 would be the way to go in that direction. Won't have the TTY properties but has a higher tensile strength which will increase its high cycle fatigue life. New torque spec would be 62 lbs-ft for a 12.9 grade M10x1.5.

[David.X5]

Does anyone have documentation that those stiffening plate bolts are actually torque to yield and not simply torque + turn angle?

This link has a great article on bolts and tightening methods. They discuss that torque + turn angle (they call that TTT) does not have to take the bolt to yield. It is a better method than plain torque to eliminate the unevenness and uncertainty of fastener friction.

Threaded Fasteners Torque-to-Yield and Torque-to-Angle - Engine Builder Magazine

1) There is a method of tightening a fastener that is much more accurate than measuring resistance to turn. It’s called Torque Turn to Tighten (TTT), often referred to as angle turn. With this method, you use a relative low torque to run down and align the fastener, then rely solely on a measured turn to tighten the fastener to the desired level. What we’ve done has not affected the friction in our fastener, it has taken it out of the equation when it comes to tightening.


For instance, 90 degrees of turn is 90 degrees of turn; old bolt, new bolt, rough threads, new threads, it doesn’t matter. The amount of stretch will be extremely uniform from bolt-to-bolt across the joint. Load scatter is kept to a minimum.


2) ...TTT is a far superior method of tightening critical fasteners regardless of whether you tighten them to yield or not...

[Omerta]

The requirement to replace with new (via Bentley) is the indicator that they are TTY bolts.

You are correct the torque + torque angle is a more accurate method. It is probably why they do not just spec the 64 ft-lbs as the yield point for that type of fastener.

[cn90]

I have some practical use for TTT bolts.

- Oil change: when I tighten the drain bolt: snug ---> turn the 17-mm socket another 30 degrees (such as from 5:30 ---> 6:30 o'clock if you look directly at the bolt). Many years ago, I measured with a torque wrench and this amounts to about 25 ft*lb or so. I have done this 5:30 ---> 6:30 o'clock thingy for 25 years and all drain bolts still look good.

- Changing wheels (snow tires etc.): snug the bolt down, then another 45 degrees, such as from 1 ---> 3 o'clock. This amounts to about 80 ft*lb or so. I may be wrong, you may need to do your own experiment.

[X5only]

Guys, let me tell you from personal experience because of my steep drive way that is slanted left to right. If I don't replace the bolts, when I'm at the bottom of the drive way going up or down, the front suspension creaks. Problem always solved by replacing these bolts, therefore it's necessary to replace them. But I always do so when I'm finally done with any repairs that requires removing them. For example, I have new ones waiting till I'm done with all the oil leaks etc I'm dealing with.