Friction between the threads of the nut and bolt, along with the nut grinding on the flat washer is most likely your problem. While galvanized fasteners come with “waxed” nuts (barely better than nothing at all), and black bolts come with left over quenching fluid (water soluble oil, yes it evaporates). You are fighting friction so use a stick wax on the washer face of the nut and the threads of your bolt, this will cut down the friction of the assembly pieces allowing you to achieve bolt tension before bolts twist in two. Here are some popular waxes we have used and where you can get them:
- Fastenal - #0343078 - Hougen Slick Stik
- Fastenal - #0603954 - CRC TruTap
- Fastenal - #0343079 - 1.68 Ounce Hougen Mini Slick Stick
- Grainger - 4KK73 - Tapmatic
- Grainger - 1MKN6 - 2.2 Ounce Accu Lube