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http://www.galvanizeit.org/design-and-fabrication/design-considerations/fasteners-bolts-and-nails/threaded-assemblies

Threaded Assemblies

Best design practice for corrosion protection suggests to avoid dissimilar metals in contactwhenever

possible. Zinc, used to coat hot-dip galvanized fasteners, sacrifices itself to protect nearly every other

metal, diminishing its longevity. Therefore, to enhance corrosion protection, hot-dip galvanized

fasteners are recommended for galvanized structural assemblies.

Best practice also recommends studs and bolted assemblies be sent to the galvanizer disassembled.

as!ing to prevent galvanizing threads on pipe or fittings is very difficult. The recommended

practice is to clean and tap after galvanizing. "nchoring devices #such as threaded rods and anchor

bolts$ sometimes are specified to be galvanized in the threaded areas only or in the areas to be

e%posed above ground. This can be more e%pensive than galvanizing the complete unit because of the

additional handling re&uired. 'omplete galvanizing can be specified for items to be anchored in

concrete.

http://galvanizeit.org/design-and-fabrication/design-considerations/dissimilar-metals-in-contact/http://galvanizeit.org/design-and-fabrication/design-considerations/dissimilar-metals-in-contact/

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icrograph of a Bolt

anufacturers of threaded parts recognize special procedures must be followed in their plants when

certain items are to be galvanized. (ollowing are some e%amples:

)ow carbon bars are recommended since high carbon or high silicon cause a heavier, rougher

galvanized coating on the threads.

*ot-formed heading or bending re&uires cleaning at the manufacturing plant to remove scale

before threading. +therwise, over-pic!ling of threads will result during scale removal.

harp manufacturing tools are mandatory. agged and torn threads open up in the pic!ling

and galvanizing processes. orn tools also increase bolt diameters. (re&uent chec!ing is necessary

on long runs.

tandard sized threads are cut on the bolt, while standard sized nuts are retapped oversize

after galvanizing.

The table below shows the recommended overtapping for nuts and interior threads as detailed in

"T "01, Specification for Carbon and Alloy Steel Nuts. +n threads over 2. inches #13 mm$ it is

often more practical, if design strength allows, to have the male thread cut 4.4125 #4.3 mm$ undersize

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before galvanizing so a standard tap can be used on the nut.

Overtapping Guidelines for Nuts and

Interior Threads

Nominal Nut Size (inches) and Pitch iametrical Allo!ance (inches)

4.64-64 4.420

4.126-23 4.427

4.17-20 4.427

4.817-28 4.423

4.44-21 4.423

4.06-26 4.464

4.06-22 4.464

4.74-24 4.466

4.37-9 4.466

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Nominal Nut Size (inches) and Pitch iametrical Allo!ance (inches)

2.444-3 4.468

2.26-3 4.468

2.26-7 4.468

2.64-3 4.468

2.64-7 4.468

2.17-3 4.467

2.17-0 4.467

2.44-3 4.467

2.44-0 4.467

2.74- 4.44

6.444-8. 4.44

6.64-8. 4.44

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Nominal Nut Size (inches) and Pitch iametrical Allo!ance (inches)

6.44-8. 4.44

6.74-8 4.44

1.444-8 4.44

1.64-8 4.44

1.44-8 4.44

1.74-8 4.44

1.74-8 4.44

8.444-8 4.44

(or metric overtapping allowances, see "T " 01, section 7.