even the White collar technicians (or rather rather engineers) that designed these engines in Germany call them stretch bolts = Dehnschrauben.

On the other hand it is good practice to replace headbolts and similar bolts like on rods once removed. Stretch bolts have always been used by BMW e.g. on flywheels of the very early 4 cylinder 1500cc engine in the 60s. Not knowing that I sheared off all bolts on a fly wheel.

Since bolts come very cheap, compared to the labor and potential damage if they are not replaced...

Thanks again for the info, Ekkehart

You've raised a good point, but I think it needs more emphasis. One MUST MUST MUST use new head bolts whenever a "stretch bolt" is involved. NO EXCEPTIONS.

Here is the reason: Once the bolt is stretched it will NOT return all of the way to its original position after the load is removed (i.e., after it is screwed out of the head), even if the bolt was not subjected to torque or strain beyond the yield point. Because of this "residual deformation," it is absolutely guarenteed that if the old bolt is reused and and torqued per specification, it will be stretched beyond the yield point and will quickly fail (e.g., on the M70, instead of 22 foot pounds plus 120 degrees, you'll be torquing to 22 foot pounds plus 120 degrees PLUS the residual deformation).

On non-stretch applications using new head bolts was just good practice because of possible fatigue in the old bolts. With stretch bolts, use of new bolts is absolutely required. Once stretched, the bolts CANNOT be loosened and then restretched.