The correct bolt is a grade 8.. one common issue people (Myself included) have is using a bolt that is too short.
Bolts come in two types.. one has threads the entire length of the shaft, DO NOT USE THIS TYPE OF BOLT, the other the threads only run part way.
Make sure NO part of the threads are inside the shaft/channel, only the smooth SHANK should be in the working area.. This, of course, means you won't be able to tighten the nut (Nylock by the way) properly unless you washer up.
So washer up.
How much difference does using the right bolt make.
used to buy quarter inch with too much thread by the dozen 2x a year
Now buy long enough smooth shank types by the dozen.. Every few years, and I use them for other things as well (like when I sit down on the end of a picnic table in one park and the )@(#$)@$@ bench Teeter Totters.. Yup, same bolt.