Referencing Chips comment about using harder bots. He is absolutely correct.. Many trailers I've seen, mine included, have multiple washers at the mounting point to move the strut and spring arms out away from the trailer. Use caution.

Metric bolts have a different hardness grading system. A metric bolt marked 8.8 is the same hardness as an SAE grade 5. If you want to upgrade the hardness of a metric bolt, use a 10.9 or even 12.9. Frank's comment about using a large diameter bolt would be a good idea as well.