To enhance corrosion protection, hot-dip galvanized fasteners are recommended for structural assemblies. Importantly, using galvanized bolts for assembling hot-dip galvanized steel structures also eliminates complications related to contacting dissimilar metals.
Best practice suggests that studs and bolted assemblies be sent to the galvanizer disassembled.
On threaded assemblies with diameters greater than 1.5” inches (1.8 cm), it is often more practical, if design strength allows, to have the male thread cut 0.031” (0.8 mm) undersize before galvanizing so a standard tap can be used on the nut.
Low carbon and low silicon bars are recommended as the fastener material to be galvanized, since high carbon and silicon levels cause heavier, rougher galvanized coatings.
Sharp manufacturing tools should be used to minimize the possibility of ragged and torn threads opening up during the cleaning phase of the galvanizing process.
Standard-sized threads are cut on the bolt, while standard-sized nuts are re-tapped oversize after galvanizing to allow for the dimensional increase in the threads from the zinc coating.
Overtapping Guidelines for Nuts and Interior Threads
|Nominal Nut Size (inches)|
|For metric overtapping allowances, see ASTM A 563M, section 7.|