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Tap and Die question

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  • Tap and Die question

    I need to re thread a bolt on my jointer fence. It's a 1/2" - 14. I went and bought a set at harbor freight which had the requisite die size but when I got it home it's way too large. I must be missing something on how they designate sizing. Below is a pic of the nut and the die side by side. I want to order the correct one but am worried I'll get the wrong size again.
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    Last edited by jussi; 03-21-2021, 07:20 PM.
    I reject your reality and substitute my own.

  • #2
    I am afraid you bought a tapered pipe threading die. Its a 1/2"-14 NPT (NPT is stamped on the die). O.D. of 1/2" pipe is 0.84 inches. Non tapered is NPS.

    For nuts and bolts they will be either of the UNC or UNF series, coarse or fine thread. Both will have a bolt major diameter (outside the threads) of 0.50 inches.
    The UNF has a 20 thread per inch pitch and the UNC has a 13 thread per inch pitch.

    You will need a 1/2-13 UNC or a 1/2-20 UNF threading die.

    You will need to determine which you have. Bring the nut to the store, they will have thread checkers in the nuts and bolts dept or simply try against parts for sale.
    Another way to check your nut for thread size is to put a 1/4-20 UNC most common 1/4" bolt into it and see if the thread pitch matches.If it does you have a 20 TPI UNF, if not then you have a 13 TPI UNC. That is assuming you have correctly identified it as 1/2".

    the UNC series is much more commonly found on stuff.

    Here are my dies.

    Click image for larger version  Name:	P3210310.JPG Views:	0 Size:	107.1 KB ID:	843442

    If you just need a single die, it might be cheaper to go to amazon than to buy a whole mixed kit. 7 bucks shipped if you have Prime.
    Last edited by LCHIEN; 03-21-2021, 08:22 PM.
    Loring in Katy, TX USA
    If your only tool is a hammer, you tend to treat all problems as if they were nails.
    BT3 FAQ - https://www.sawdustzone.org/forum/di...sked-questions


    • #3
      The nut in your pics looks like it fits a 1/2 inch wrench... but bolt and nut thread sizes are based on the bolt diameter, not the wrench size of the nut.
      It's usually easier to find the appropriate sizes by using the bolt. I start with the little thread pitch tool on the bolt threads. Once I know the thread pitch, that pitch will reduce the number of possible dies in the kit to just 2 or 3 in a typical kit. If none of those work, that indicates the bolt/nut are a less-common size + thread pitch combination. You'll probably have to purchase that size tap or die individually rather than finding it in typical sets.

      "NF" and "NC" are abbreviations for "fine" and "coarse" threads; the number after them is threads per inch.

      Pipe threading stuff is measured differently. Depending on the pipe material it'll be the outside diameter of the pipe or the inside diameter. Tap & dies intended for pipes (i.e. ones labeled "NPT" for "National Pipe Taper") will be sized based on pipe diameters. Proper NPT threads will have a slight taper to them to make a leak-proof joint. "NPS" is "National Pipe Straight" for non-tapered pipe connections for a stronger mechanical joint (no taper to reduce actual contact surface area) but a sealant must be used. "BSP" is "British Standard Pipe" and is used outside the US; "BSPT" is the same thing.

      Many common tap & die kits include one or two smaller pipe tap & die sizes with several regular bolt/nut size & thread pitch combinations. Harbor Freight sells (or at least used to sell...) a kit that was nothing but NPT taps & dies.

      Last edited by mpc; 03-21-2021, 07:47 PM.


      • #4
        MPC makes an excellent point. 1/2 inch bolts have a 1/2" shank diameter. The hex head of a 1/2" bolt is going to be 3/4 or 7/8" across the flats.a 3/4 or 7/8" wrench.
        A 5/16 inch bolt is going to be 1/2". wrench flats but the diameter across the threads will be a little under or right at 5/16"

        I was thinking perhaps a 1/2" bolt on a jointer is rather larger than I would expect to see anywhere. 1/2" bolt is really big!
        Are you sure you have sized the bolt correctly?
        Again, if you have the matching nut you can check the bolt size and pitch at the hardware store very easily... they usually have a bolt checker hanging from the display.
        Last edited by LCHIEN; 03-21-2021, 08:19 PM.
        Loring in Katy, TX USA
        If your only tool is a hammer, you tend to treat all problems as if they were nails.
        BT3 FAQ - https://www.sawdustzone.org/forum/di...sked-questions


        • #5
          Thanks guys. I didn’t realize there was a difference between pipe and nut/bolt treads. And I think the bolt might actually be metric
          Last edited by jussi; 03-22-2021, 12:19 AM.
          I reject your reality and substitute my own.


          • LCHIEN
            LCHIEN commented
            Editing a comment
            They'll have metric bolt and nut thread checkers at the hardware store, too. Usually metric threads tend toward the finer pitch than ASNSI/US threads. and the pitch is in distance, not threads/distance.

        • #6
          Normally if it takes a 1/2” wrench or 13 mm it’s an 8mm bolt with 1.125 thread pitch. (Standard Thread) Or if it is a fine thread it’s 8mm bolt with 1.0 thread pitch. These metric fasteners are easily mistaken for fractional bolts, 5/16” x 18 (UNC) or 5/16” x 24 (UNF). Boy I wish we (USA) had gone metric like they started teaching us in school in the 1960s.