Grade 8 bolts have 5 raised "stars" on the heads. They should be strong enough for most L/S engineering. Try checking if some other fastner has failed and is shifting the load to the bolt that is actually failing.
different bolts work good in differernt applications, sometimes different grades are more brittle than others thats why some lower grades might be a little more flexible instead of just snapping which usually takes a whole sh*t load of pressure
I am with Rookiemower, it depends on the application.
If you have something that takes alot of abuse and shock, you may want a grade 5, a grade 8 will be stronger, but will be more inclined to shear. Look at the bolts on plow rigs...grade 5. Look at the bolts used to mount big strong trailer hitches...grade 5. I don't think there will be much need for anything beside a 5 in a lawn type application.
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