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I have a customer who wants to oversize the threads on nuts after I galvanize the blank (unthreaded) nuts, but he is having trouble finding oversized taps to cut the threads. Where can I find correctly oversized taps to cut threads on nuts or holes used with hot-dip galvanized bolts and why is oversizing necessary?

Hot-dip galvanizing is a corrosion resistant zinc coating with a measurable thickness. When threaded parts are galvanized, this added thickness can increase the diameter of the threads. If not addressed, the added thickness can affect the fit of the fastener system (nut or threaded hole and bolt). 

The overtapping process accommodates for the increased diameter of the galvanized coating on the bolt threads by cutting deeper threads into the nut or threaded hole than would be used with uncoated bolts. This allows for proper assembly of the fastener system.  Table 5 of ASTM A563, Standard Specification for Carbon and Alloy Steel Nuts, documents the recommended dimensions of these overtapped nuts or threaded holes in order to permit proper assembly of the fastener system (Table 1). 

Table 1

Nominal FemaleThread or Nut Size (in.)Diametral Allowance (in.)Pitch Diameter: Minimum Increase (in.)Pitch Diameter: Maximum Increase (in.)
0.2500.0160.23350.2384
0.5000.0180.46800.4745
0.7500.0200.70500.7127
1.0000.0240.94280.9516
1.2500.0241.18121.1908
1.5000.0271.41871.4292
1.7500.0501.67011.6817
2.0000.0501.90571.9181
2.5000.0502.38762.4011
2.7500.0502.63762.6513
3.0000.0502.88762.9015
3.2500.0503.13763.1517
3.5000.0503.38763.4019
3.7500.0503.63763.6521
4.0000.0503.88763.9023
Dr Galvboltmicro
Figure 1

Galvanizing a blank (unthreaded) nut or hole and later overtapping it to the recommended diameter in Table 5 of ASTM A563 is a common practice in the industry. The galvanized coating on the bolt threads will provide cathodic protection to the uncoated threads on the nut or threaded hole when the fastener system is assembled, as shown in Figure 1

Re-tapping an already threaded and galvanized nut is also an option, but it can damage the threads.

Tool companies supply oversized taps to be used with hot-dipped galvanized parts on a custom-built basis. Table 2 presents a list of such companies.

Table 2

CompanyPhoneWebsite
Bass Tool & Supply Inc.800-442-0042www.basstool.com
C.W. Rod Tool 281-449-0881www.cwrodtool.com
Drillco Cutting Tools586-246-1268www.drillcocuttingtools.com
Ronco Industrial Supply Company, Inc.630-238-8800www.goronco.com

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