Measure black iron pipe

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Raditz84

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Hey everyone. I am doing a project in plumbing school and was having troubles. I need to thread and build a design using 1/2 " threaded black iron pipe. For example I have a piece of 1/2" threaded pipe. I need to thread on a 1/2" 90 on both ends and at a length of 12" to both centres. Calculating the thread engagement and with 4-3 hand turns and 4-3 wrench turns I need to keep three threads exposed. How long should I cut my pipe and how long should my threads be made?

Hope this made sense and someone can give me an answer.
 

DonL

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Welcome to Terry's Forum.

That must be a trick question.

Why does it need to be Black Iron ?

Are you working with gas ?


Good Luck on your test.
 

Raditz84

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No, sorry. I am actually building something and need to use black iron. Its a first year plumbing/pipe fitting

So to clarify. I need to have a 1/2" black iron pipe that is 12" center to center. I need to thread the pipe myself and have two 90's of both ends. I am having trouble getting the pipe to thread on and stay at 12". I cut the pipe at 10 5/8" with the math of the thread engagement. However it always comes up short of 12". Any idea what I am doing wrong. My teacher wants 3-4 hand turns, 3-4 wrench turns and a remaining 2-3 threads
 

Raditz84

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So for say a 2" pipe it would be 2" a side? How come with the math of my fittings book it came out to 10 5/8 with there measurement of thread engagement? obviously it doesn't work because I did it this way but seems weird that there measurements aren't working out.

Also is there a trick to how many threads I should be putting onto the ends of my pipe?
 

Reach4

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You tried 10 inches and came up short by X. Next time cut it 10+X inches long.
 

Raditz84

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Ya but the number of threads comes into play as well though...
 

Asktom

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The number of turns and all that come into play in a book, but in the real world the depth of the threads on the pipe and in the fitting will vary. 1/4" is as fine a measurement as you are going to need threading pipe.

I should have said pipe size of the FITTING. Center to center of a 1/2" 90 and a 3/4" X 1/2" X 3/4" would require deducting 1/2" + 3/4" = 1 1/4".
 

Raditz84

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Ok thanks. So since its a 1/2 90 on either end I would deduct 1/2" + 1/2". Wanting the centre to centre to be 12 the cut of the pipe would be 11". Ill give it a shot tomorrow and let you guys now how I do. Thanks so much
 

Nukeman

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Another approach would be this:

- end to center of a 1/2" iron elbow is about 1 1/8" from what I've seen
- NPT pitch for 1/2" pipe is 14 threads/in

12" - 2 x 1 1/8" = 9 3/4"

Each end is inserted 6-8 turns (3-4 hand + 3-4 wrench). Let's say 7 turns. So, 1/2" per side (7/14).

9 3/4" + 1/2" + 1/2" = 10.75"

So, you would cut the pipe 10.75" long, and have ~10 threads (about 3/4") on each end. The 10 threads accounts for the 6-8 threads covered by the elbow and 3 threads left exposed.

In theory, there is about 1/8" of slop in either way in the length depending if you did 6 turns on each side or 8 (instead of 7). If you did 10 5/8" with 6 turns on each side, it should have been close (at least in theory!).

The rules of thumb posted are going to be a lot easier to remember and faster to implement. This is just what I come up with based on the problem given.
 

Raditz84

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Yes I did do 10 5/8 and 10 3/4 as well and It was very close. within an 1/8" when I did 10 3/4. However doesnt the depth of the threads have anything to do with where the 90's will end up?
 

hj

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I am not sure WHAT you are doing, but a "rule of thumb" is just an average. YOUR fittings will have to be tightened as much as necessary to not leak, and that depends on how the pipe is threaded and how the fitting is tapped. If it has to make a precise dimension, then you just use more or less torque, or a bigger wrench.
 
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If you want to really accurate about it use the table below to figure it out. In reality the pipe length will be determined by how the threads on the pipe and fittings are cut. NPT threads are tapered and the interference will occur at lengths.


npt.jpg
 

Asktom

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The marker I use makes a mark of at least 1/4" wide and there is one on each end of the cut. Calculations to within 1/8" go out the window at that point. Yet, it works.
 

Tom Sawyer

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These days you can't count on things to be the same. The only way to know what the make in allowance is for sure is to thread the elbow on, count the visible threads and then take the joint apart again.
 

Jadnashua

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There are standards and then there's reality...especially if you're dealing with pipe and fittings from China. If you're cutting your own threads, it depends somewhat on how sharp or worn the die is. On the pipe, it will depend on how round it actually is and how close to the specification it is on overall diameter. Then, depending on whether you're using tape or pipe dope for the seal, you can adjust the diameter of the pipe a little with the tape by putting another wrap around it and affect when it will seal and how many threads you use up. IOW, it's not a precise medium. ANd, especially if you want each fitting to line up, where on the diameter the die actually starts could get things tight at different points with precisely a certain number of threads so you'd have the right spacing, but then the fittings don't point in the required direction.
 

DonL

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No wonder PVC is used most now a days.

The China and US fittings do not fit the same.
 
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