Bill, I'm sure you will be deluged with answers, but just in case not; here's
the straight skinny. Common hardware bolts are made with generic steels of
various metallurgical quality (some good - some not so good). These are
generally refered to as plain bolts or grade 1 bolts. There are several
different grades of bolts with gradually increasing quality of steel. The
grade 3 are tougher and have a higher yield strength (where the bolt itself
will rip apart), than the grade 1. Conversely the Grade 5 is of better
(stronger) steel than the Grades 3 and much better than Grade 1. Grade 8 is
one of the highest commonly encountered ratings of steel bolts. Grade 8 will
withstand the most rigerous test without yeilding. It (from memory) probably
has at least a 10 time stronger yield strength than a Grade 1 or common bolt.
Grade 8 is the minimum allowed in aircraft components.
Of course, it should be noted that a grade 8 bolt needs a grade 8 nut.
Putting a grade 1,3,5 nut on a grade 8 bolt downgrades the fastener to the
lowest denomination. The softer nut will rip off of the grade 8 bolt at the
grade 1,3,5 level of the nut.
These bolts, at least in the hex head variety are normally maked with a
series of dash marks around the edge of the head. Grade 1 are not marked.
If I remember right the Grade 3 has one slash, Grade 5 has two slashes and
Grade 8 have 3 slash marks on the top of the head. I'm sure someone will
correct me if I'm wrong.
Hope this helps. Tom Campbell - Birmingham, Alabama
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