When I first started making these I included lock washers. The problem was people were installing them with not enough extra thread exposed on their u-bolts. I was seeing pictures of them installed with the threads not making it all the way through the nuts. 1/2" lock washers are fairly thick, I stopped including them so there would be maximum thread engagement through the nuts. I recommend cleaning off the threads of the u-bolts and putting a couple drops of loctite on before installation and I make it clear that the u-bolts must be long enough so the threads make it all the way through the nuts for the tie-down brackets.