Well, the jist is correct but.... the only common bolts I know of are grade
2 this bolt has no markings and will bend prior to stretching or shearing.
Grade 5 with 3 marks on the head. a medium strength bolt that works well for
most applications and a grade 8 with 6 marks on the head with good
shear/stretch strength.
2 marks close together on the head I believe is 18-8 stainless steel. 316
stainless has 316 stamped on the head there are other stainless alloys with
a whole bunch of different markings..
Grade 2 can be used for holding on fenders or headlights. Grade 5 should be
used where you might need better shear strength. Grade 8 is an over used
strength bolt used for "head bolts" or in differentials ( just an example
where shear strength is needed. and yes there are nuts marked that fit these
bolts not to mention washers).
Never use grade 8 bolts on something intended to shear under heavy load,
like the PTO shaft on a bush hog.
There are many different alloys used to make bolts and all have different
applications. The threads on the bolts i.e. NC or SAE also have different
torque values.
As far as aircraft go, yes most fasteners are grade 8 but a lot are
stainless steel (316). There are many grade 5 bolts holding things together,
a whole different subject that deserves it's own time.
when the grade goes up so does the price. a grade 2 bolt is sufficient to
hold a bridge plate on but not strong enough to hold U-joints in place.
I have been wrong before so I will read and heed other responses.
Regards
Emmett
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