Ask the Expert

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.

Posted by Admin

Email: sarah@portlandbolt.com View other posts by Admin

Posted on Tuesday, July 3rd, 2007 by Admin. Filed under Galvanizing Bolts. Comments may be followed using the RSS 2.0 feed.

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4 Responses to “Galvanized Nuts Tapped Oversize”

1. Jonathan Waltner says:

   @Mike McGuire – I’m are glad you found this article useful. Thanks for the heads up!

   September 7, 2010 at 8:33 am
2. Mike McGuire says:

   No comment, just a thank you for your valuable Fastener Tech Tips. We used one on our NEW feature located on FASTENER NEWS NOW called 60 Seconds with…. We did the one on Hot-Dip Galvanized Nuts Tapped Oversize.

   I hope you approve and we can use others from time to time and you get some inquiries and/or orders.

   September 7, 2010 at 8:24 am
3. Erik Waelchli - IBD Connection, Inc says:

   You write: “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.”

   In Europe the old DIN standard 267 Part 10, and now the new ISO Standard 10684 calls for optional processes, i.e. oversized nuts or undersized screws before hot-dip galvaizing.

   Are American Standards such as ASTM A563 only addressing the oversized nuts and do not allow undersizing male threads?

   January 6, 2010 at 11:12 am
4. TWY says:

   Is the galvanized bolts, screws and nuts is better than Mild Steel Cadmium bolts, screws and nuts in terms of corrosion resistance ?

   Your advice is appreciated.

   Thanks.

   November 26, 2009 at 2:54 am