[bobcowan]

Grade 8 bolts are very strong in tension, but can be brittle, and will break when exposed to lateral tension. A grade 8 should only be use in double shear, or when a locating dowel is used.

I would start by switching to a grade 5 bolt, which is more forgiving in single shear applications.

A split lock washer only works when torque has been lost, and the ends are allowed to split apart. If you're using rubber or poly bushings, a split lock washer is a good choice. Some people make the mistake of putting another flat washer under the split lock, but that defeats the purpose. The nut will still rotate and come loose. The split lock washer needs to dig into the nut and the base material.

It's hard to imagine that a nylock nut would come loose, but it does happen upon rare occasion. If you have solid bushings, I would suggest an internal star washer and a nylock nut. If it still comes loose, use two nylock nuts.