Spark Plug Standard Thread Form - Page 2
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  1. #21
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    SPARKPLUG 7/8"-18 UNS-2B DIFFERENCE
    TYPE THREAD TYPE THREAD
    PER MACHINERY'S PER MACHINERY'S
    BOOK 1949 BOOK 1996
    PITCH DIA. HIGH .8384" .8449" PLUS .0065"
    PITCH DIA. LOW .8343" .8389" PLUS .0046"
    MINOR DIA. HIGH .8068" .828" PLUS .0212
    MINOR DIA. LOW NONE .815" ?
    MAJOR DIA. HIGH .8750" NONE ?
    MAJOR DIA. LOW .8668" 0.875 PLUS .0082"
    One thing that we all must remember is that old Cars that used these sparkplugs are not of high combustion pressures and the taps using Unified threads are likely made on the low limit of the pitch diameter tolerance. One must also remember that the Minor Diameter is not cut by the tap but by the drilling of the hole previous to tapping. The Major and Minor diameters are not a functioning portion of the tapped hole but are clearance. Only the angled faces of the thread are truly functioning parts of the threads.
    .8449” - .8343” = .0106” a lot of cast iron to remove by hand!
    To cut these threads at the high limit of the tolerance from the low limit of the old threads would take a lot of torque.

    M.L. Anderson

  2. #22
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    Please remove the two posts under my name as I am tryign to get a correct with the chart having lines to seperate the figures as to help prevent confusion. Yours, M. L. Anderson

  3. #23
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    SPARKPLUG 7/8"-18 UNS-2B DIFFERENCE
    TYPE THREAD TYPE THREAD
    PER MACHINERY'S PER MACHINERY’S
    BOOK 1949 BOOK 1996
    PITCH DIA. HIGH .8384" .8449" PLUS .0065"
    PITCH DIA. LOW .8343" .8389" PLUS .0046"
    MINOR DIA. HIGH .8068" .828" PLUS .0212
    MINOR DIA. LOW NONE .815" ?
    MAJOR DIA. HIGH .8750" NONE ?
    MAJOR DIA. LOW .8668" 0.875 PLUS .0082"

    This would be much easier if your program accepted Works Charts!
    Yours, M. L. Anderson

  4. #24
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    According to my knowledge there are some following standards:
    M18 x 1.5
    M14 x 1.25
    M12 x 1.25 and M10 x 1.0

  5. #25
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    Jun 2010
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    M14 x 1.25 is the euro std spark plug thread . Also this is one of the standard fine metric threads.


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