To snap the bolt, either you are a gorilla or the bolt was duff! Regardless of its useage the max torque of an eight mm bolt(13mm head) is about 31ft/lbs but I would suggest using 25 and loc-tite.You would be shocked at the max torque on smaller bolts unless they are hi-tensile(gr8 in colonies I think),even a .......ian can break a 6mm bolt with little effort.CLEAN the threads and the hole in the car.