I just reread my Fulton Class 1 hitch instructions. It does say to use a 3/8" grade 5 bolt. Class 1 is 2000 total trailer weight and/or 200 max tongue weight. I will be 20% under both restrictions.
I had purchased two standard 1/2" bolts from HD, since the hole in the coupler needed only to enlarge the hole about 1/16" to accept the 1/2" bolt. I thought the 1/2" bolts would work well enough (3/8" grade 5 being recommended). I really didn't think the standard 1/2" would break with my towing 1500-1600 lbs with 160 tongue weight. So, I did some internet research.
I looked at the information on
this site about grade 5 vs grade 8. The chart on this site shows that a grade 5 bolt has a tensile strength of over 22,000 lbs per sq. inch and shear strength of over 18,000 lbs per square inch. I then found a chart with specs on
this site that states that a standard bolt is a grade 2, and a 1/2" grade 2 has a tensile strength of over 19,000 lbs per sq. inch and shear strength of nearly 15,000 lbs per square inch. Comparing that to the recommended 3/8" grade 5 which has tensile strength of over 17,000 lbs per sq. inch and shear strength of over just over 14,000 lbs per square inch, my standard 1/2" grade 2 bolt is stronger than the recommended 3/8" grade 5 bolt, reaffirming that I can safely use my 1/2" bolts.
The first site also discusses in detail comparing grade 5 to grade 8 and their argument that a grade 8 is superior makes sense to me.