I have a question about NPT (National Pipe Thread) tapered connectors that are used in fuel systems of several airplanes. Please take a look at this standard-wall one-side threaded nipple (3/4 pipe size, 2" long). 9110T34

My questions are:

1) How do I decipher the specified pipe size? This one is 3/4, but 3/4 is not any of the dimensions.

2) Why would the unthreaded side have the diameter 1.050"? I want to connect it to a pipe with a 1.0" diameter (yellow color, on the right) as shown in the picture below but I am concerned about mismatch when I use a threadless compression coupler.


  • $\begingroup$ Are you sure NPT fittings are used in your fuel system? The problem with NPT fittings is that they require PTFE tape or paste to create a seal, which can break down or enter parts of the fuel system if not properly applied. Many fuel systems employ AN type fittings . $\endgroup$
    – Ron Beyer
    Jun 9, 2017 at 2:22
  • $\begingroup$ @RonBeyer, you are absolutely correct about AN type fittings. I have to stick to NTP. Even if I can get to AN, I would still like to know why the unthreaded part of the nipple is 1.05". 1" would make more sense, right? $\endgroup$ Jun 9, 2017 at 2:34
  • 1
    $\begingroup$ If your question is about dimensions and measurements making sense, you must be from a metric country. Please accept that the old imperial system makes no sense at all. $\endgroup$
    – Koyovis
    Jun 9, 2017 at 3:13
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    $\begingroup$ @mins, that answers my question. Also, I see the problem with a "pipe" that has 1.0" diameter. Such pipes just don't exist. It is a structural tube. A pipe, such size 3/4 would have OD 1.05". This helped me find a mistake in one diagram. Thank you very much. $\endgroup$ Jun 9, 2017 at 7:53
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    $\begingroup$ @mins, could you please write an answer for part 1) and then provide explanation for part 2) from my comment above? I will then mark your answer as solved. $\endgroup$ Jun 9, 2017 at 7:55

1 Answer 1


Mechanical engineering uses a particular wording, I hope I'll not lose myself in the translation... Feel free to correct.

This fitting, standardized in ASME B1.20.1, has the particularity to have conical nut and screw:

enter image description here

The thread is tapered:

  • Larger diameter ($\small \rm E_2$) = 1.00178 in
  • Smaller diameter ($\small \rm E_0$) = 0.96768 in
  • Effective length $\small \rm L_2$ from $\small \rm E_0$ to $\small \rm E_2$ = 0.5457 in

1) How do I decipher the specified pipe size? This one is 3/4

Difference in diameter over $\small \rm L_2$ = 0.0341 in. As $\small \rm L_2$ = 0.5457 in, this is equivalent to 0.75 in per foot (12 in) or 3/4 in/ft.

This is where the size 3/4 comes from.

2) Why would the unthreaded side have the diameter 1.050 in? I want to connect it to a pipe with a 1.0 in

When fully engaged, the screw thread and the nut thread are fully adjusted. The extremity of the screw is at the reference plane (see figure), and the nominal diameter is $\small \rm E_0$

$\small \rm E_0$ is smaller than $\small \rm E_1$, the diameter at the extremity of the nut, both are smaller than $\small \rm D$, the external diameter of the screw.

For a threaded screw, the "diameter" is often the diameter of the unthreaded rod, this is also the case here $\small \rm D$ = 1.05 in.


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