Overtapping Guidelines

Typical coating thickness on bolts can range from 1.8 to 3.5 mils (0.045 to 0.09 mm), which can make standard bolt and nut tolerances difficult to maintain for correct assembly. If bolts are hot-dip galvanized, then the nuts should be oversized to accommodate the 3.6 to 7.0 mils (0.09 to 0.18 mm) increase in bolt diameter after galvanizing (oversize tolerances are detailed in ASTM A563 Standard Specification for Carbon and Alloy Steel Nuts).

If the nuts or tapped holes in a steel article are hot-dip galvanized, they should be re-tapped or re-threaded after galvanizing to remove the zinc coating and provide clearance for the coated bolt. When the fastener system is assembled, the coating from the bolt will provide protection for the uncoated nut thread since zinc coatings cathodically protect uncoated steel. The re-tapping is done on the nut side so that no uncoated threads on the bolts will be available to weather without galvanized protection. If re-tapping the nuts, its important to note that all of the zinc coating must be removed from the inner threads. Standard practice for structural connections is to galvanize the nuts as blanks and then to tap the threads after galvanizing. Nuts and threaded holes are traditionally galvanized as blanks and threaded after galvanizing.  The hole should be drilled undersize before galvanizing so the threads can be cut after galvanizing. The holes and/or nuts threaded after galvanizing, although bare, will be protected by the zinc on the coated threads of the fastener. The corrosion protection for the hole and/or nut is provided by the bolts threads.

Clearance holes must be designed to account for zinc pickup. The table below shows the recommended overtapping for nuts and interior threads as detailed in ASTM A563. On threads over 1.5 inches (38 mm) 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.  For metric overtapping allowances, see ASTM A563M, Section 7.