Re: Grade of bolts for trailer
Grade 5 for everything, the only grade 8 fastener on a boat trailer is the wheel nuts/bolts. You could use grade 2 to hold reflectors/lights and other less critical/light items.
Heavy hot dip grade 5 galvanized last the longest but are hard to find everywhere, zinc coated is what you will find in most hardware supplies and work well but will eventually rust due to the thin coating.
A good inexpensive combination is a zinc coated grade 5 bolt with a hot dip galvanized washer/lockwasher underneath for extra anodic protection near the fastener...its cheap and lasts.
Plain stainless steel 18-8 fasteners are only slightly stronger than a grade 2 bolt and the expensive 316SS bolts are not as strong as a grade 5 fastener, it would be an expensive upgrade to go stainless and I wouldn't use them on suspension or critical tongue hardware...their charpy (notch impact) ratings are not good enough for those uses unless you go significantly larger in diameter to reduce the risk.
If I had to choose one bolt, it would be heavy galvanized grade 5 for everything including the nuts, washers and lockwashers. Stay away from nyloc nuts for trailers because once they corrode internally from crevice corrosion the plastic loses its grip and they come loose. If you want long term durability a drop of blue loctite (and a lockwasher) will seal the nut and is still removable later.
