I know regular bolts have a series of lines to show the grade of the bolt, but how do you decipher the grade of a metric bolt?

The bolt I have says 8.8, and U .
I want a grade 8 bolt, to use on a hitch application, help!
Thanks!
Matt
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1978 F250 4x4 - 351M
Don't crush them, restore them!

 

The number is it's grade specification. The letter, I would guess, is it's finish (i.e. corrosion protection). I am not completely sure, but I think the 8.8 is comparable to a grade 5 bolt. They are common. I always specify 10.9 when I need one with exceptional strength.

Again, I'm not sure, but I think the first number refers to tensile strength in MPa per mm^2. The second number is yield strength. An 8.8 would have a yield strength of 80% of tensile, and 10.9 would be 90%.

I am no expert, but am 99.999% sure that the higher the number the stronger the bolt. If you want to be safe, use the highest number you can get your hands on. If you really need to be sure, consult a reputable fastener supplier. In my area (Detroit), I always trust CJ Fastener.