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"How do we tolerance and gauge galvanized fasteners?"

Current question:

February 12, 2021

Q. Oversizing tap for internal threads if using Hot-Dip Galvanized bolt?

I have an application where I need to tap 5/8"-11 threads into an ASTM A588 (50ksi) plate (very thick... 2.75" plate). The application will be for a structure close to the beach so corrosion resistance is a concern so want to use a high strength Grade 5 bolt/screw with HDG finish. Going to drill hole 0.531" (17/32) and tap and leave internal thread unfinished. Do I need use an oversized tap for this application or am I ok with standard tap given only the bolt is galvanized and not hole?

Harley Hollenbeck
- Merritt Island Florida

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Previous closely related Q&A's starting in:


Q. We would like to have the following clarification:

Component threads:

Male thread (Bolt) M6 x 1p - 6g Female thread (nut) M6 x 1p - 6H. These components are to be zinc plated (galvanised). After coating the bolt thread dimensions (namely pitch diameter, major and minor dia.) become slightly increased due to presence of coating, and for nut the dimensions become slightly under sized. Because of that, the components are not answering to 6G (bolt) and 6H (nut) thread gages.

Standards say that the internal threads shall be tapped over-size after galvanising and then oiled for corrosion protection. The excess material due to coating on external threads are allowed as long as they do not transgress the basic thread profile. In essence the bolt after plating should be freely assembled by hand with the nut.

Are the nuts (internal threads) being re-tapped by the manufacturers to remove the coating? What thread gages (tolerance class) should we use for checking the bolts and nuts after coating?

A N Balaji
Industrial valves - Coimbatore, Tamil Nadu, India


A. We undercut the male thread and then galvanise them. The nuts are tapped inside to remove zinc. You have suppliers in India who do this.

You have referred to a standard. Which standard is this?

We use SS fasteners for M10 and below.

GR Patel
ABB Ltd - Baroda, India

To minimize search efforts and to offer multiple viewpoints, we combined previously separate threads onto this page. Please forgive any resultant repetition, failures of chronological order, or what may look like readers disrespecting previous responses -- those other responses may not have been on the page at the time :-)

Treatment of Galvanized pipe after cutting threads, welding or drilling


Q. Is there an industry standard that addresses treatment (painting) of galvanized pipe after cutting threads or otherwise removing the galvanize material from portions of the pipe? NFPA 13 [affil. link to spec at Techstreet] -- table refers to ASTM A705.

I could not find a clear direction that addresses a treatment to be applied after the galvanized protection has been removed.

Kevin Hutchison
Engineering design Firm - Pittsburgh, Pennsylvania, USA

April 19, 2013

Q. Hi everybody ;
I'm having trouble with the grooved hot dipped galvanized pipes. Especially 4", 5" and 6" .
We open groove/thread to pipes (4", 5", 6") after the galvanizing. But always the galvanize is chipping and flaking.
Please help me.

Best Regards...

Mustafa Sag

Painting plus galvanizing on Anchor Bolts and Sag Rods

April 29, 2015

Q. Dear Sir,

Can Anchor Bolts or Sag Rods For Purlins and girts with both ends threaded be galvanized and painted over galvanizing, or only galvanized or only painted?

As my client need galvanized, and painted over galvanized sag rods and anchor bolts -- hence thickness of coating will be greater (Galv 85µ plus Painting 150µ) -- so what about the nut tapping? Will normal galvanized nut fit the threaded ends or not?

Please advice

Regards & Thanks

Abdul Qureshi
Estimator - jeddah, Saudia Arabia

April 2015

A. Hi Abdul. I did some quick binging and googling and saw that some manufacturers galvanize 80% of their sag rod product line, so it's accepted technology. Obviously, painting is accepted technology too.

Peeling paint is a very common complaint when things are galvanized then painted; although the problem can be overcome, I'd be wary of specifying galvanized plus painted sag rods and anchor bolts unless you can find them prefinished from multiple suppliers.

Although I have no expertise in metal building construction, paint is soft, no match for steel, so I don't think people worry much about interference from paint on husky fasteners like this, nor count on it offering any corrosion protection to the threads. Galvanizing is metal, so dimensional allowance for it must be made --generally by cutting the male threads deeper, and not cutting the female threads until after galvanizing or chasing the female threads. Good luck.


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Ted Mooney, P.E. RET
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