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Thread: 1/8 npt?

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  1. #1
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    1/8 npt?

    I was eyeballing a project that I am going to have to retap something 1/8 NPT, grabbed my tap and die set and it dawned on me how small 1/8" actually is... when I measure a 1/8 fitting, it's actually more like 25/64 on the outside of the threads. Even measuring the inner diameter of a fitting, it's 1/4"? Perhaps this is a question, but where does the 1/8" come into play? On the same subject, what size tap would I use to tap a hole for a 1/8 fitting to screw into?

  2. #2
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    The size is loosely based off of the inside diameter of the pipe when schedule 40 pipe is used I believe. The id will vary depending on the wall thickness of the material actually being threaded.

    Check this link for drill size chart:

    http://www.engineeringtoolbox.com/np...ads-d_750.html

  3. #3
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    Huh. Chart has "R" as the tap size. I realized after that there is a specific tap for this after googling it. Not sure about the 1/8 designation though as I have measured multiple fittings and none of them measure anywhere close to 1/8" ID.

  4. #4
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    The "R" is a lettered drill size. It's slightly larger than 21/64" and slightly smaller than 11/32" Here is a drill size chart that breaks all that down.

    http://www.gearhob.com/eng/design/drill_eng.htm

    Pipe threads are tapered threads so that they form a seal when tightened together, unlike bolt threads. You cannot use a bolt thread tap here as you've discovered.

    In regards to the size... with the larger NPT sizes there is a closer correlation to the inside diameter size of the pipe. It gets screwy with the smaller sizes for some reason. In other words... for 1/8" NPT it won't correspond to any physical measurement being an actual 1/8"

  5. #5
    Quote Originally Posted by Levi View Post
    The "R" is a lettered drill size. It's slightly larger than 21/64" and slightly smaller than 11/32" Here is a drill size chart that breaks all that down.

    http://www.gearhob.com/eng/design/drill_eng.htm

    Pipe threads are tapered threads so that they form a seal when tightened together, unlike bolt threads. You cannot use a bolt thread tap here as you've discovered.

    In regards to the size... with the larger NPT sizes there is a closer correlation to the inside diameter size of the pipe. It gets screwy with the smaller sizes for some reason. In other words... for 1/8" NPT it won't correspond to any physical measurement being an actual 1/8"

    I just read up on this stuff yesterday about thread pitch gauges etc peaks and valleys and there are like 3 measurements then based on those three you refer to a chart then you get thread pitch type

  6. #6
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    Jun 2013
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    if its a macro line fitting 3/8th x 20 is what i use and it i have no issues

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