Bolt torque discussion

Most fasteners on engines are spec'd to a torque well below the plastic deformation range (where the metal starts to "stretch" farther than it can spring back). Very high stress areas such as heads and connecting rods need all the clamping load they can get so that's why you see TTY bolts more often in those apps. It should be verified with ARP but I don't think the vast majority of their fasteners are meant to be loaded to the point of yielding; their specialty is making extremely strong fasteners that don't need to be stretched to get the most clamping force; since the metal itself is stronger the bolt/nut can be torqued farther before the metal starts to stretch. OEMs don't do this because it's more expensive than making smaller bolts that are stretched to their absolute max tension and for that I personally replace any fasteners on newer engines that have any indication they might have been stretched (like if the instructions call for set torque plus a 1/4 turn, or if the spec'd torque seems high for a fastener of that size). Also important to keep in mind for rod bolts as mentioned above, those are under very high reciprocating (back-and-forth) loads and usually get torqued to where they start stretching so after a set number of torque-downs the bolts will become weaker.

I can try to dig up my old notes from a Machine Design class I took, we actually learned how to calculate the clamping force put on two parts held together by a bolt or stud depending on thread type, fastener size etc. If you know how much clamping force you need to hold 2 parts together there is a way to calculate the minimum bolt size needed based on the material (Grade 5, 8 etc.).

The most interesting thing I learned was how a bolt or stud actually works... when the parts are clamped together, it removes all external forces meaning the bolts themselves only ever "feel" the stress from being torqued down; for example the pressure of combustion trying to push up on a cylinder head won't put any more force on the head bolts/studs than they are already under UNLESS they aren't torqued far enough or something happens that allows some of that clamping force to go away (failed gaskets, heat cycling). When you look at an engine with a blown head gasket where a head fastener failed, it's usually the threads in the block not the bolt/stud itself that failed. The threads are the weakest part of any fastener.

Hope all that rambling helped at least a little bit LOL