Just bought a 2003 Pt Cruiser Gt, 141k, totally stock. All the front engine mount bolts were broken flush with the block. The head had just been replaced not long ago. 9 times out of 10, if you have problems, it is where the last work was done. Working on cars is just not taking things apart and putting them back together how ever you want.
You have to have finess and a very good understanding how tight tight is. Tight on a 1/2" bolt wil snap a 5/16" bolt. Tight on a 5/16" bolt is not nearly tight enough for a 1/2" bolt. You need to understand what 15 ft/lbs feels like, what 20 ft/lbs feels like and so on. You might be able to get a bolt super tight on a steel block, but do the same on an aluminum head and you just stripped the head. That's why they make torque wrenchs. If you don't know what you're doing, be sure to use one.
I have a feeling the reason my bolts broke off were because they were not tight enough to start with. Everything has to work in unison. When one bolt breaks the same stress gets multiplied over the remaining bolts until they all fail. Too tight will cause them to break also. The harder the bolt, the more brittle it is. Where a softer bolt, like a
#2 , will bend like a liquorice stick, a harder bolt, like a#8, will snap like a candy cane. Thus, why proper torque is important. When you tighten a bolt you have to have a feel for the bolt. If it suddenly feels like it slips or gets easier, something failed. Either the bolt stretched or sheared, or the threads pulled. Your best bet is to try and back it out slowly and maybe you'll get lucky and get it out all in one piece. Leave it hoping it will hold is guaranteed failure wainting to happend at the worst possible time and causing catastraphic failure that will be 10 times more expensive to fix if you're still alive to fix it.