Hein,

How much history do you have with the car? Owned it since new? Has the cylinder head ever been off the engine before?
If this is the first time the head is being removed and the head bolts now in there were originally placed there by the folks in the factory then your torque estimate makes sense to me. I do recall being quite worried when I loosened the first head bolt. It made a snapping noise and I thought the bolt or the 10mm tool had sheared. The good news is it was the sound the bolt made when it loosened after being in the head for 12 years and 260k miles.

If it would help you, I can attempt an experiment with the original head bolts which I still have. I'll have to think about how to do this but I will attempt to lock the bolt in a large vise and go to increasing levels of torque until the bolt fails if I can make it fail before the 10mm XZN tool fails. The trick will be estimating the torque to failure since I do not have a direct way of measuring it.
Ken

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2014 E250 Bluetec 4-Matic
2014 GL350 Bluetec 4-Matic
1996 Chevy Tahoe 4 x 4
2009 ML350 (sold)
2005 SLK350 (sold)
1999 E300 Turbodiesel (sold)
1998 ML320 (sold)
1991 300D 2.5 Turbodiesel (sold)