these places such as ARP have very,very,very highly skilled engineers and top of the line equipment for measuring and testing such things to come up with their torque values.when you think you can do something at home or in your shop such as torquing a ARP stud down past what they recommend just because it doesn't go "snap" right off,then your fooling yourself that you know better than those highly payed,best in their field collage grad engineers with their measurement equipment that we probably can't even spell let alone use lol! follow their torque values.don't go beyond and don't let someones rusted out threaded bolt holes scare ya guys from torquing to what they recommend either.this is simply a case of an old engine block that's seen better days.rusted threads will eventually fail.......stuff happens.but hey,look on the bright side,better in the shop than while boosting 10-15 lbs 50-100 miles from home.