The failures in 2007 occurred as I was removing the bolts in order to swap out the soft springs for the X Power ones and were single overload stress failures - I believe the bolts had been overtorqued when fitted.
I then lower the car off the stands and roll it forward and back to get the suspension to settle,.
Then raise the car and torque to the final figures.
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I suggest you get a set of vernier calipers and measure the tolerances on the bolt shoulder and the diameter of the bush.
I would suggest that there may well be wear in the damper bushes that lead to the problem.
The reason for the failure was that the muppet that fitted the dampers specified the wrong bushes with centres a whole mm greater than the nominal bolt diameter of 12mm - way outside the acceptable required minimum clearance of 0.006mm and maximum clearance of 0.035mm!
Hence the bolts were undergoing a hammer action from the damper leading to HCF.
My initial failures in 2006 were with bolts rated 10.9, springs softer than stock, and SPAX dampers.