What are the overtapping allowances for mechanically galvanized nuts?

The overtapping allowances for mechanically galvanized nuts are the same as overtapping allowances for nuts that are hot-dip galvanized.

Mechanically galvanized nuts that adhere to ASTM B695 are “cold-galvanized,” which implies that they are coated with zinc at room temperature. Neither electricity (electroplate galvanizing) nor heat (hot-dip galvanizing) are applied in a mechanically galvanized process. Hot-dip galvanized nuts are tapped oversize after the galvanizing process because of the tendency for zinc to build up in the internal threads. Due to the nature of the mechanical galvanizing process, zinc does not build up on the internal threads of the nut. Hence, mechanically galvanized nuts are tapped oversize prior to coating. In both cases, the coating thickness on the exterior of the nut remains the same. However, hot-dip galvanized nuts are tapped oversize after the galvanizing process and mechanically galvanize nuts are tapped oversize before the galvanizing process.

The overtapping allowances are specified in the table below:

Nominal Nut Size, in. and Pitch Diametral Allowance, in. Pitch Diameter
Min Max
0.250 - 20 0.016 0.2335 0.2384
0.312 - 18 0.017 0.2934 0.2987
0.375 - 16 0.017 0.3514 0.3571
0.437 - 14 0.018 0.4091 0.4152
0.500 - 13 0.018 0.4680 0.4745
0.0562 - 12 0.020 0.5284 0.5352
0.625 - 11 0.020 0.5860 0.5932
0.750 - 10 0.020 0.7050 0.7127
0.875 - 9 0.022 0.8248 0.8330
1.000 - 8 0.024 0.9428 0.9516
1.125 - 8 0.024 1.0678 1.0768
1.125 - 7 0.024 1.0562 1.0656
1.250 - 8 0.024 1.1928 1.2020
1.250 - 7 0.024 1.1812 1.1908
1.375 - 8 0.027 1.3208 1.3301
1.375 - 6 0.027 1.2937 1.3041
1.500 - 8 0.027 1.4458 1.4553
1.500 - 6 0.027 1.4187 1.4292
1.750 - 5 0.050 1.6701 1.6817
2.000 - 4.5 0.050 1.9057 1.9181
2.250 - 4.5 0.050 2.1557 2.1683
2.500 - 4 0.050 2.3876 2.4011
2.750 - 4 0.050 2.6376 2.6513
3.000 - 4 0.050 2.8876 2.9015
3.250 - 4 0.050 3.1376 3.1517
3.500 - 4 0.050 3.3876 3.4019
3.750 - 4 0.050 3.6376 3.6521
4.000 - 4 0.050 3.8876 3.9023
Inch Fastener Standards. 7th ed. Cleveland: Industrial Fasteners Institute, 2003. B-173. For Nuts: Hot Dipped Galvanized per Specification A153, C1.C

Although Portland Bolt does not typically provide mechanically galvanized fasteners, we do have an in-house hot-dip galvanizing line for all your galvanized threaded fastener needs. Hot-dip galvanizing is not limited to some of the size restrictions associated with mechanical galvanizing. Please note also that mixing galvanized finishes is not recommended. Contact us for information on ordering your galvanized products.

Written

6 comments

    This chart is all over the internet, but what I cannot seem to find on the internet is the translation of this table to a type of tap.

    For example, does a standard tap have an H3 thread limit and not work with hot dip galvanized screws/bolts? So therefore a higher H number is needed.

    So what H number is recommended, H7 or H11, or some other number?

    And can a higher H number tap be used in a previously tapped lower H number tapped hole?

    @PB- H limit taps are not typically large enough for use with galvanized bolts. For example, the largest H limit tap that I’ve seen is H11, which is only 0.005″ oversize. The recommended oversizing for nuts (and holes) for use with a 3/4-10 galvanized bolt is 0.020″. A 3/4-10 +0.020″ tap can usually be made within a few days, but may require a special order. Yes, a larger tap can be run through a hole that had been previously tapped.

    I”m looking for a locknut to use with a 1/4″ galvanized eyebolt. Do you have any such locknut or recomendations? The eyebolt is HDG.

    Thanks
    Regina

    @Regina- That isn’t an item that we can supply. Your local fastener distributor should be able to assist you with that. There are several types of locknuts to choose from, they should be able to let you know the availability and pricing.

    Hi,

    Could you please advise which one is better between hot-dipped and mechanically galvanizing for ASTM A325 bolt ?

    Also, could you please advise if the vedio of overtapping is available.

    Thanks,
    Bingwei

    @Bingwei- Hot dip galvanizing is a thicker and more even coating, so from a corrosion standpoint, it is better. However there may be applications where mechanical galvanizing may be desirable. We do not have an over-tapping video available at this time.

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