heating and expansion of 2 different metals...sometimes if the stud isnt as you said correctly torqued it will work its self loose with just a nut holding it down. but if you over torque the stud tight as hell to start like on an aluminum block then as on a honda torque to say 70-73 ft pounds more on top of that as required for aftermarket head studs for honda, it might just pull slap out the threads...cast iron blocks dont have this problem.