different grades of steel bolts
Grade 8 bolts have 6 evenly spaced radial lines on the head. Grade G is roughly equivalent to Grade 8. Grade G nuts are used with Grade 8 bolts. Alloy Steel. Alloy steel bolts are made from a high strength steel alloy and are further heat treated. Alloy steel bolts are typically not plated, resulting in a dull black finish. Alloy steel bolts are extremely strong but very brittle.
Grade 2 is a standard hardware grade steel. This is the most common grade of steel fastener and is the least expensive. Except a possible manufacturer's mark, Grade 2 bolts have no head marking.