So I have a buddy who was trying to tighten down the bolts on his differential and each one would tighten to the specified 30ftlbs of torque. However, upon going back over the bolts they would continue to tighten without the torque wrench giving the sign that they were properly torqued. He tried 2 different wrenches and then came and got mine yet the bolts will not stop torquing down. One of the bolts eventually twisted and broke... wtf is going on here?
Nothing mysterious there.
When a screw fails, it's because it was overtightened (for the material.)
Torque specifications are generated according to anticipated loading, load-sharing between fasteners, and the like.
Your differential cover only need 30 lb-ft per screw because it's a low-pressure seal for a viscous fluid, you don't need to clamp it down stupid.
Being that you're using 5/16"-18 screws, you obviously don't need a lot of clamping force (if you did, they'd have used bigger screws. The cylinder head uses 1/2"-13 screws, for instance.)
The fact that there was "some turn left" after reaching specified torque isn't surprising to me. I'd have to look it up, but the maximum recommended
torque for a 5/16"-18 screw (SAE5 or SAE8) should be rather higher than 30 lb-ft.
This doesn't mean that you really need
to get stupid with torquing the screws - but, the fact that you've fractured an (OEM, I presume?) screw tells me you did. (If you went and bought a replacement, all bets are off! I was replacing an oil pump - 5/16"-18 screws again - and had screws fracture before reaching specified torque! Why? Chinese crap. Running into stuff like that is what makes me such a picky bastid...)
When given a torque spec, you torque to that spec and then stop
. There's no reason to "see if you can turn it a bit more" - doing so will invariably stuff something
up, and you end up making more work than you need.
Just as you did here.