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I’m in a position where I need to order high volumes of nuts and bolts for a project.

I read through a South African National Standard, SANS 2001-CS1, in which it states that when I use hot dip galvanized fasteners the nut should be of a higher class than the associated bolt.

Could you please specify whether this is accurate and what the reason will be for this?

Your assistance will be greatly appreciated.

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I'd like to preface this with the statement that I am entirely unfamiliar with any South African standards, so take my educated guess for what it is.

I was able to find what I think is the correct version of SANS 2001-CS1:2005 online. I'm assuming your confusion stems from the seemingly contradictory nature of Section 4.5.1.1 with Section 4.5.1.6, which are copied below:

4.5.1.1 Bolt/nut Combinations

The combination of bolts and nuts shall not be less than the following:

a) class 4.8 bolts or screws with class 4 nuts in accordance with SANS 1700-5-2; or

b) class 8.8 bolts with class 8 nuts in accordance with SANS 1700-5-2.

Any bolt assembly which seizes when being tightened shall be replaced.


4.5.1.6 Galvanized nuts

Nuts that are to be hot-dip galvanized shall be of a higher class than the associated bolt or screw. Nuts shall be checked after being galvanized for free running on the bolt and shall be re-tapped to the permissible deviations if necessary to ensure a satisfactory tightening performance.

My guess is one of two things:

  1. either the hardness of the nut is expected to decrease after the hot-dip galvanization process (which dips the nut in molten zinc, thereby tempering it a little), thus "downgrading" it, or

  2. galvanized nuts are sold to a different class system, so you have to make sure your nut class exceeds the bolt class (I don't know how things work in South Africa, so this is a wild guess).

Section 4.5.1.1 shows that a "matching" nut is used with the corresponding bolt. You want a nut that's stronger than the bolt, such that if the bolt fails (fractures), it's very apparent that the failure has occurred. If a thread fails, it's much less noticeable, and hence more dangerous since that bolt assembly cannot be counted on. See this PDF for more information on the topic.

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