HDG stands for hot-dip galvanized, a finish applied by immersing fasteners in molten zinc at about 450 C and spinning off the excess. The coating, often 50 microns or more, gives outdoor structures a long service life, but it is thick enough that nuts must be tapped oversize after galvanizing to fit the coated bolt. The highest-strength grades, such as A490 and Grade 8, are not galvanized because of embrittlement risk.
Bolts are HDG, so you need the galvanized overtapped nuts, a plain nut won't even start on that zinc.
Never run a die over a galvanized bolt to make a plain nut fit; you strip the zinc and the thread strength with it.
Also heard as
- hot-dip galvanized
- galv
- galvanized
Related terms
Overtapping
Tapping a nut oversize after galvanizing so it will fit over the thick zinc on a hot-dip galvanized bolt.
A325
The workhorse high-strength structural bolt: heavy hex, quenched and tempered, 120 ksi minimum tensile.
Zinc plating
An electroplated zinc layer, usually with a clear, blue or yellow passivate, the cheapest common fastener finish.
Hydrogen embrittlement
Delayed brittle cracking of hard steel fasteners caused by hydrogen absorbed from plating, cleaning or corrosion.