Galvanized Nuts Tapped Oversize
Question: Can I use black (plain finish) or zinc plated nuts on galvanized bolts?
Answer: No. Hot-dip galvanized nuts need to be used with hot-dip galvanized bolts since galvanized nuts are tapped oversize to accommodate for the extra 2 – 6 mils (.002 – .006) of zinc that is added to the threads of the bolt during the galvanizing process. A common misconception is that bolts that are going to be hot-dip galvanized are threaded undersize since the galvanizing will increase the size of the threads. This is not correct. Bolts are threaded to the same tolerances, regardless of whether they will be galvanized or remain plain finish. Instead, it is the nuts that are tapped oversize to allow for the zinc on the threads of the bolt. Zinc plated nuts will also not fit a galvanized bolt since they are not tapped oversize. Follow the link to our ASTM A563 Hot-Dip Galvanizing summary page for overtapping allowances based on diameter. Since galvanized nuts are tapped oversize, they would be too loose to be used effectively on black or zinc plated bolts.
