Chris, a good writeup on this information is generally in the first chapter
of any of the Haynes Automotice books - generally available at places like
Auto-Zone and probably most book stores.
The short version is that there is a wide range in the metallurgy of bolts
and fasteners. The common (garden variety) bolts you get in the hardware
store or off the rack at K Mart/Walmart, often come from numerous sources -
some of which MAY use questionalble metallurgy in them. These are common or
Grade 1 Bolts and nuts.
Steel is measured mechanically in two primary methods, tensile strength and
shear strength. The tensile strength is the amount of pressure, weight that
will stretch the bolt to a breaking point. This would be important in such
things as head bolts and the like. Shear Strength is the resistance to cross
pressure on the bolt, like gear bolts. Generally when the tensile strength
goes up, the shear strength goes up with it. Both are directly related to
the chemical make up of the metal in the bolt, for all fasteners - steel,
aluminum, brass, etc.
The common bolts are considered grade 1 and have no markings. The next most
commonly encountered bolts are the grade 5 bolts, which on US bolts have
there slash marks "/" around the head of the bolt. Grade 5 bolts are
probably 50% stronger than grade 1 fasteners. Grade 8 bolts are stronger
still and have five slashes around the head.
Grade 1 bolts are generally used for applications where the tensile strength
or the shear strength is not as important as it may be in other applications.
Typically, the fenders, and body parts are secured with Grade 1 bolts. The
more important bolts of the motor and transmission will be grade 5 bolts.
The really critical and high stress bolts for the heads, and gear
applications should be grade 8 bolts.
You need to be sure that you use the Grade 8 bolts with the Grade 8 Nuts.
Use of a lower grade nut with a high grade bolt will reduce the strength of
the bolt/nut assembly.
Hope this helps some and didn't just add to the confusion.
Tom Campbell - Birmingham, Alabama
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