Coated Nut Undercutting or Tapped Oversize
Nuts and other tapped fasteners that are Hot Dip Spun Galvanised, Sherardised
or have another heavy deposits will require re-tapping to clear coating material
from the threads in the nuts.
Nuts can also be tapped over size before coating to allow for heavy coating deposits.
Because hot-dip galvanizing is a coating of corrosion-inhibiting, highly abrasion-resistant zinc on bare steel, the original steel becomes slightly thicker. When talking about tapped holes and fasteners, the increased thickness is important.
In consideration of the added thickness, holes need to be overtapped (enlarged) in order to accommodate the increased size of the fastener to be inserted. While chasing or retapping the nuts after galvanizing results in an uncoated female thread, the zinc coating on the engaged male thread will protect both components from corrosion. For economy, nuts are usually galvanized as blanks and the threads tapped oversize after galvanizing.
Tapping of all holes after galvanizing is recommended to eliminate double-tapping costs and the possibility of cross-threading. If, after galvanizing, the hole is still not large enough, it can be chased or retapped. Chasing or retapping will not necessitate regalvanizing. Because of the zinc coating’s cathodic properties, the coating on the mating fastener will protect chased or retapped nuts.