How do I determine the proper wrench opening or socket size to tighten a specific size of bolt?


Socket sizes and wrench openings for tightening nuts and bolts are determined by the dimension across the flats of either the nut or bolt head. According to the Machinery’s Handbook, 26th edition, “Wrenches are marked with the ‘nominal size of wrench’ which is equal to the basic or maximum width across flats of the corresponding nut”. Although bolts are typically tightened by turning the nut, this would also hold true for bolt heads as well. The wrenches are designed to have enough clearance to fit over the maximum across the flats dimension.

Follow these links for “Across the Flats” dimensions for standard sized nuts and bolts:

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    The two nuts on my rider mower are 0.905″ across the flats and 1.105″ across the flats.
    Does that mean that I need a 29/32″ wrench and a 1 7/64″ wrench for these nuts?

    @Chuck- You could probably get by with 15/16″ wrench and a 1-1/8″ wrench. If those do not work, and you can’t use an adjustable, it is also possible that you have metric nuts.

    Trying to determine if a 7/16″ socket will fit into a 7/8″ opening in unistrut. Cannot find a website that tells me this information.
    Would you have this information?

    The first link in Gary’s post: Hex Nut Dimensions. Takes you do a different page, but when you click to show drawing, on the banner, it displays a Hex Bolt, not a Hex Nut. Also the Dimensions on the Hex Bolt page match the Dimensions on the Hex Nut page. Are they the same Dimensions per ASME B18.2.1 1996 for both the bolt and nut?

    Also why do these tables not go to 4″ in diameter? They stop at 2 1/2″.

    @James- Good catch. We recently revamped our website, and must have made an error here. We will fix it right away. The across the flats and across the corners dimensions are the same for both hex bolt heads and hex nuts, but of course the chart for nuts wouldn’t list body diameter. Finished hex nuts only have defined dimensions up to 1-1/2″ per ASME B18.2.2, most standards move to heavy hex above that.

    Nice post there Gary, it is also essential to keep in mind the dimension of your bolt so that your work will be done in an efficient manner. Thanks again for this post, it’s really helpful

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