The TSB says the upgraded bolt is supposed to be a grade 10 ( where it should have a 12.9 mark on the head ), but every one so far that anyone in captivity has ever seen , has had markings that looked a lot more like a grade 8 on the head (10.9 mark )
, now , this being a specialty manufacture bolt, it very well could be a 10 equivilant as a specialty alloy, but there have been suspecions for a while.

Either way, if it has had teh upgrade, My money is on improper torque.
High torque bolts have a nasty habit of breaking if not tightened just right, or if a lubricant ( locktite etc) is left out of the equation . Problem is Ive found very few people who actually know how to use a torque wrench.
most folks dont realise the things are calibrated within a certain temperature range and you cant store them in a freezing or 115 deg garage and put them straight into service and expect even remotely close torque figures, you have to put them inside climate control long enough to equilize temperature .

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-------Robert,'95 960, '93 940t, '90 240 wagon, '94 940SE, '84 240 diesel (she's sick) , '80 245 diesel, '86 740 GLE turbo diesel, '92 Ford F350 diesel dually