A really thrilling page about Bolts
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There are 3 common grades of bolts: 4.6, 8.8 and 12.9. This grading is usually stamped in the head of the bolt.
A 4.6 grade bolt is also known as a commercial grade, the ones we usually use are the 8.8, known as structural grade and the 12.9 are high tensile bolts.
The first number is the ultimate tensile strength of the bolt in Megapascals (Mpa) and the second the point at which the bolt will permanently stretch (the percentage of the ultimate load at which this will happen).
For example an 8.8 graded bolt will have a tensile strength of 800Mpa and will permanently stretch at 80% of it's ultimate load.
Note: When buying bolts be aware that "bolts" are only partially threaded; "set screws" are fully threaded.