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Thread: 1/8 npt?

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

  7. #7
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    Quote Originally Posted by Deviant View Post
    if its a macro line fitting 3/8th x 20 is what i use and it i have no issues
    Those fit, but it puts the seal and all of the stress on just a few turns of threading. Pipe threads are tapered, so a straight thread doesn't match up quite right and doesn't have as much sealing surface.

    Whenever you deal with pipe thread, always carry around the "NPT" with the size. It is like saying "coarse" or "fine" with the standard straight sizes. The 1/8" NPT taps are not much more expensive within a particular brand.

  8. #8
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    Yes, with NPT (National Pipe Thread), both male and female threads are tapered so they tighten as they screw together and there is lots of contact between the threads for good sealing and mechanical strength. On running threads, you get a good fit if the threads are a nice tight tolerance but that rarely happens on generic threaded parts. Any dings can make them hard to screw together. They also never get tight unless they bottom out or tighten against a stationary surface. A tapered thread screwing into a running thread will tighten, but you only get a good seal on a few of the threads that start to get shallow towards the back of the threading. It puts a lot of stress on those threads.
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  9. #9
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    I got a 1/8 npt (national pipe thread) tap at my local hardware store. You just got to insist on a pipe thread tap, as has been said they're tapered. And iirc it's an 18 thread. Use lots of oil even re-threading and don't try to go too deep. I've tried to tap further to make a micro fitting fit closer and ended up with massive leaks.
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  10. #10
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    Quote Originally Posted by Runamok View Post
    I got a 1/8 npt (national pipe thread) tap at my local hardware store. You just got to insist on a pipe thread tap, as has been said they're tapered. And iirc it's an 18 thread. Use lots of oil even re-threading and don't try to go too deep. I've tried to tap further to make a micro fitting fit closer and ended up with massive leaks.
    It's nice to have the fitting you want to use in hand when you tap. You can thread it in and see where it starts to seat, and some fittings are a little fatter than others. If you are side tapping something round (like an ASA or round body), it may help to drill the hole a 64th (or more) over size to get the tap started correctly, but that depends on your drill bit, tap, and part.

    This is from a toy of mine (<-link). This fill nipple was wider at the tip than most of my brass fittings, and getting the tap started on that double hill and seated "just enough" was several passes.


  11. #11
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    Quote Originally Posted by Runamok View Post
    I got a 1/8 npt (national pipe thread) tap at my local hardware store. You just got to insist on a pipe thread tap, as has been said they're tapered. And iirc it's an 18 thread. Use lots of oil even re-threading and don't try to go too deep. I've tried to tap further to make a micro fitting fit closer and ended up with massive leaks.
    It's NOT an "18 thread". It's a 27 thread per inch. 1/8"x27 tpi NPT
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