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Hi ///ASN - a very interesting question, thank you.
Torque To Yield (TTY) is the (yield) point at which a material has moved from the elastic limit to the plastic zone and permanent deformation will occur according to Young's Modulus of Elasticity.
Once a material has exceeded the proportional limit then the material can no longer return to the elastic modulus.
A TTY would never be applied to a fastener where a gasket would be in place as the fastener cannot expand or contract according to the operating conditions.
In a suspension component, requiring angular force, the torque applied to the fastener takes it to the proportional limit and any further angular force required takes the material into the plastic zone. The 'stretched' fastener is permanently deformed and is NOT recommended to be used again.
BMW AG provides Normative reference GS 90003-2 which details tightening torque for metric thread and calculating procedures. This is the standard to be observed if no cross references are provided for tightening torque in the repair instructions.
Predominately BMW uses thirteen different surface treatment coatings.
(ZN) Yellow and (ZNS) Silver can easily be identified visually. The metric class is marked on the fastener cap eg.4.6, 4.8, 5.8, 8.8, 9.8, 10.9 and 12.9 Thirdly the thread pitch is identified as fine or coarse.
Now that you have identified the fastener then it can be cross referenced to GS 9003-2 and the correct torque settings can be applied without guestimating.
Attached is tightening torque extracted from BMW GS 90003-2.
Confucius said - I hear and I forget. I see and I remember. I do and I understand.
///ASN I hope this is beneficial.
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