bolts, shear strength, grade 8

my question is this. What do the markings on the head of a bolt mean, by markings i mean the lines and/or numbers

Answer
If a bolt has dashes it is a standard US thread. with numbers it is a metric. The more dashes the harder the bolt. Three would be a grade five and five would be a grade eight. Same with the metric the higher the number the harder. A 9.8 is equivelent to a grade 8 standard. If your switching bolts you can always go up but not down in grade. If it's for a specialty application be sure you get the correct type of strength. Some are tensile( pulling or stretch strength) while others are shear strength ( side to side).  
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