Richard J Kinch wrote:

What grade are the standard bolts, and what does "grade" mean?

There is no "standard". Perhaps you mean "no mark", which could mean an
SAE Grade 1 bolt (33 Kpsi proof strength) or could be just an ungraded
cheap import that is much weaker. The grade is all about the tensile
strength of the material, not the hardness (although harder usually
means stronger). Proof strengths run from SAE grade 1 at 33 Kpsi, 2 at
55, 4 at 65, 5 at 85, 7 at 105, and 8 at 120...

Nice answer :-) Looks like we add about 20K psi per grade.

To figure the actual strength of a bolt in pounds, you multiply the
cross sectional area of the minor diameter in inches times the proof
strength of the material in psi. So a grade 8 1/2-13 UNC bolt, having a
minor diameter of 0.407 inches, has pi*(0.407/2)^2 * 120,000 ~=
15,000 lbs proof strength; grade 1 would be 4300 lbs.

Hmmm. I woulda thunk the bolt would fail when the threads slip, eg when
the force exceeds the area of the ring between minor and major diameters
times the tensile strength. So if that bolt had a 0.427" major diameter,
it might fail at Pi((0.427/2)^2-(0.407/2)^2)120K = 1572 pounds, but
maybe that isn't true, if the nut is thick enough.

Nick