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NPT pipe plug size versus hole size...?

Discussion in 'Technical Q&A' started by fastradio, Jul 26, 2009.

  1. fastradio

    fastradio F1 Rookie
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    Apr 26, 2006
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    David Feinberg
    Why must this be so confusing?

    I have several non-tapped holes in an intake manifold that I'd like to plug using threaded NPT plugs.

    The hole size is 6mm (.23622").

    Despite reading the various charts online, do I need an 1/8" NPT plug, or a 1/16" NPT plug. I don't quite understand the relationship between the 1/8" and 1/16" nomimal sizes of the plug versus what size hole they'll actually fit.

    Can anyone shed some light on this subject?


    Thanks,
    David
     
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  3. 2NA

    2NA F1 World Champ
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    Tim Keseluk
    You should be able to tap that hole for 1/16" pipe.

    Here's a link to a page with Tap Drill sizes that include 1/16". The size you mention looks perfect already.

    http://academic.evergreen.edu/projects/biophysics/technotes/fabric/pipe.htm

    The tricky part might be finding a tap. I have a very extensive assortment of threading tools (including a set purchased from Ferrari in the 60's) but no 1/16" pipe.

    I need to get one. :D
     
  4. fastradio

    fastradio F1 Rookie
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    Thanks Tim for the clarification...
    FYI: McMaster Carr has the 1/16" NPT 27 tap PN: 2525A171

    David
     
  5. finnerty

    finnerty F1 World Champ

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    #4 finnerty, Jul 27, 2009
    Last edited: Jul 27, 2009
    DAVID ---

    Why do you want to use NPT plugs over straight-threaded plugs? You can get the same degree of sealing performance with straight threads if you use a proper thread sealant.

    The reason I question the NPT approach is this ---

    I assume the manifold is a cast Aluminum piece?

    When tapping an internal NPT thread in soft / brittle materials, it is preferred to drill a tapered pilot hole before tapping. This is very difficult to do accurately by hand --- milling equipment and depth monitoring methods are generally used to accomplish this.

    Attempting to force a tapered NPT plug tap into a straight-walled pilot hole can crack the piece --- if it is sufficiently thin and / or composed of weaker materials (e.g., cast Al).

    BE CAREFUL :) :)
     
  6. 2NA

    2NA F1 World Champ
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    In more than 30 years of drilling and tapping holes in all types of materials, I've never seen a tapered tap drill bit. A decent chamfer usually gets the tap started. Use a good lubricant and be careful and you're good to go.

    I've broken a few taps (usually in stainless or hard steel) over the years but I doubt there's much chance of breakage in an aluminum manifold.

    A straight-threaded plug like you recommend would need an o-ring or metal sealing washer against a precisely machined surface to seal properly. I don't think this was what David was looking for.
     
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  8. finnerty

    finnerty F1 World Champ

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  9. finnerty

    finnerty F1 World Champ

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    Since he's talking about an intake manifold, I'm assuming the goal is only to seal air at low pressure (positive and negative atmospheric)?

    Take a look at any intake manifold --- nearly every piece of plumbing coming off it is attached with a straight thread fitting (if it uses threads) and a sealant.

    Yes, using an NPT plug is the "better way" to go --- it's just not always required.
     
  10. 2NA

    2NA F1 World Champ
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    Tapered end-mills. Not really a drill bit and overkill for this application. controlling the drilling depth is pretty critical. Go a little deep and you're oversize.

    I'll stick with conventional tap-drills.

    Since you sourced McMaster Carr, here's one of their famous diagrams showing the difference between tapered and straight pipe threads. Straight requires a seal (and a flat surface perpendicular to the hole to seal against).

    http://www.mcmaster.com/ctlg/DisplCtlgPage.aspx?ReqTyp=CATALOG&CtlgPgNbr=2&RelatedCtlgPgs=3,2&CtlgEdition=115&ScreenWidth=1280&McMMainWidth=1062&sesnextrep=807257435329948

    Without a proper seal the plug is merely a bolt in a threaded hole. When tightened, there is a clear path for leakage around the "back" side of the thread. Using some type of sealant might prevent leakage in a low pressure application but I've seen it fail (oil sump studs are a typical example).

    I've looked at "any intake manifold" (I've got a few from different cars) typically they have straight thread fittings that use a sealing washer (typically copper or fiber or rubber o-ring) no sealant required, or they use NPT or BSPT tapered pipe fittings with "dope" or teflon tape on the threads.

    David was looking for drill size information and I think he's got what he needs. It doesn't sound like a big engineering project, just a few holes need filling.
     
  11. fastradio

    fastradio F1 Rookie
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    I had considered a straight-threaded plug, but felt a bit more comfortable using a tapered plug, in that I'll end up with both a mechanical and chemical bond. The "boss" surround the holes I'm going to plug area considerable in diameter, so I have little fear (although always very cautious) that I'll be able to correctly taper and tap these holes.

    Thanks for your thoughful input.

    Best,
    David
     
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