Can I use a hot-dip galvanized nut on a plain finish bolt?
Hot-dip galvanized nuts cannot be used on a plain finish bolt. Hot-dip galvanizing adds between 2 – 6 mils (.002” – .006”) of zinc to the threads of the bolt and due to this increased thread thickness, hot-dip galvanized nuts are tapped oversize to accommodate and create a workable fit. These oversized internal threads on a galvanized nut will not engage properly with a plain finish bolt and the fit of the nut will be too loose to develop the engagement/strength needed.
ASTM 563 specifies the exact amount of oversizing required per each diameter of nut. Portland Bolt has previously answered the opposite question of using plain or zinc plated nuts on a hot-dip galvanized bolt. The answer is the same: only a hot-dip galvanized nut, that is tapped oversize, will work with a hot-dip galvanized bolt. A plain finish or zinc finish nut will not have the oversized threads that will allow for a correct fit, instead the nut will strip and gall a galvanized bolt to the point of not be able to assemble the nut correctly onto the threads, if at all.
In either situation, it is the nut that must be tapped oversize, or not, to accommodate. The bolt is threaded the same if the bolt is plain finish or hot-dip galvanized finish. Never chase or rethread a bolt to remove the added thickness that galvanizing adds. Chasing a bolt will remove the protective zinc coating and may even remove parts of the thread, damaging the corrosion resistance intended for the threads of a bolt, or even the bolt itself. The ASTM F2329 specification for hot-dip galvanizing specifically prohibits thread chasing.
Plain finish bolts must be used with plain finish nuts and hot-dip galvanized bolts must be used with hot-dip galvanized nuts that are tapped oversize. Being a hot-dip galvanizing company with our own specialized galvanizing line in our manufacturing facility, Portland Bolt has the expertise and the manufacturing capabilities to be a qualified and trusted supplier on your next job.