Thread tolerance for galvanised tapped holes

[bugbus]

I see a lot of sources saying that the 6g/6AZ tolerance class should be specified for holes that are intended to be galvanised after thread tapping.

However, reading BS ISO 965-5:1998, for example, they say:

Internal screw threads with tolerance class 6AZ are primarily intended to mate with external screw threads centrifuged after hot-dip galvanizing.

I presume this centrifuging process is something that is done to hot-dip galvanised nuts to avoid zinc pooling between the threads.

When we are dealing with fabricated structures with tapped holes, how is this zinc pooling avoided?

Can anyone share any insight here? Is it enough just to specify the 6g/6AZ tolerance?

I understand that it is also common to specify a more typical tolerance class such as 6g/6H, and to re-tap the holes after they are galvanised, but I believe this is not best practice as the galvanising in the hole is basically destroyed.

 

[dauwerda]

When we are dealing with fabricated structures with tapped holes, how is this zinc pooling avoided?

It's not, the threads have to be re-tapped after galvanizing.


Male threads are the same size whether galvanized or not and they are not supposed to be chased after galvanizing.
For the female threads to fit, they must be tapped oversized to allow for the zinc on the male threads. After galvanizing it is standard practice for female threads to be re-tapped to clear out all the excess zinc. The female threads are protected from corrosion by the male threads of the fastener through cathodic protection.

Here are a few pertinent articles addressing this:

https://galvanizeit.org/hot-dip-gal...nuts or tapped,clearance for the coated bolt.

https://www.portlandbolt.com/techni...arts are removed from,a trouble-free nut fit.

https://www.portlandbolt.com/technical/faqs/galvanized-nuts-tapped-oversize/