Spaw has it right: "a half turn before it breaks."

The safest method would be to find a service manual for the engine you're working on, but if you believe in the designers just about any manual for a similar engine in a near year will give nearly identical torque specs for the same bolt in the same use.

For the theoreticians:

The indentical specs for a given bolt are because a bolt tightened to less than "80% of yield" will almost always back itself out eventually, especially if no "retention method" such as locking nuts, sticky goop, or (per British specs) cotter pins in every nut, is used, which sets a minimum safe clamping force for a particular bolt.

In "high stress" areas, or where there's lots of "thermal cycling" or vibration, the common setting is intended to be around "112% to 120% of yield" so that the bolt is actually "stretched into place" and is "springy" against the load. Since you'll probably go the "extra half-turn" if you try for anything over 120% of yield, you'd break the bolt, so that's a "maximum in any case."

Since theoretically, the "high stress" bolt (a.la. Head Bolt) is actually stretched a bit each time it's installed, for some "fussy applications" it may be a good idea (or a legal requirement) to use a new bolt each time the thing is put back together. "Fussy mechanics" in less fussy applications (i.e. when the FAA isn't looking over your shoulder) may insist on at least a magnaflux inspection before a bolt is re-used. (A new bolt may be cheaper than the inspection.) Most autoshops go by the "use what's there" theory, with little trouble; but a "visual" to replace any bent or "obviously twisty" bolts would be worth considering.

The "yield point" for a bolt is for the longitudinal load (i.e. the "clamping force") and can be easily calculated from the area of the bolt cross section and the "yield stress" for the bolt type. Unfortunately there is no reliable way to calculate what torque must be used to get the intended/wanted clamping force, so most large shops use "torque tables" worked up from tests or plagiarized from a "possibly reliable source." These tables are the source for the torque tables in the service manuals.

THE BOTTOM LINE is that for a given bolt the torque spec is one or the other of two values, depending (sometimes) on the place where it's used.

Even the ambiguity between the "80%" and "112%" values isn't as ambiguous as it could be, since the higher stress levels are used (usually, at least, in cast iron block times) only with SAE Class 5 or Class 8 bolts, and the lower values will more likely be a Class 3 (but sometimes a Class 5). Going back to "theory" again, you should be able to tell the Class of the bolt from the markings on the head.

For "instructional purposes only," you can look at http://www.americanfastener.com/technical/grade_markings_steel.asp for samples of head markings. Auto engines in cast iron days would likely have used SAE J429 style markings, with the good ones quite a ways down the list. The ASTM specs are mostly identical to the SAE, and the same markings from either spec will generally mean "the same bolt."

To be "the same bolt," the bolt "diameter," "thread class and pitch," and "material spec" must ALL match. Safest is to look for the same bolt head markings.

Note that "all bets are off" once you get into Al blocks, since the "torques" used for the bolts in cast iron heads may strip the threads in an Al block long before "bolt clamping capacity" is reached. "Sticky goop" and other specialized "retention methods" are fairly frequently used with the bolts at lower tension/torque. Studs are perhaps more commonly used as well, since the stud half that screws into the Al block can be installed at low torque/zero tension, and is thus fully engaged in the threads before the nut goes on and applies the tension. (It should be noted that the "nut torque" may be somewhat different than the "bolt torque" to get the same "clamping force," so the actual engine spec is more critical for Al block work.

None of this, of course, is particularly useful to those who asked the questions; but perhaps the bit of theoretical stuff may be of some interest to a couple of our stranger members.

Just remember that even when there are "rules" there are also "creative designers" so don't expect anything to work all the time.

John