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Plumbing Sort of: What size tap for Black Steel Pipe?
Posted by: C(-)ris
Date: April 14, 2021 07:16PM
I know I should be able to figure this out, but I'm having a hard time because I'm not using the pipe for plumbing and don't want NPT threads. I have 2 sections of 6' 3/4" Schedule 40 black steel pipe. I need to tap threads on the inside of the pipe in either SAE or Metric to allow a large bolt to be threaded in.

I originally was just going to weld in a nut and then use that, but the resulting bolt wasn't going to be strong enough to support the weight on the bars.

I THINK the ID of the pipe is: 13/16 in in but I really am having a hard time verifying that. It should be consistent regardless of manufacture. I also can't seem to figure out how you determine what the most appropriate size you should tap the pipe. I can find the NPT tap for the inside of a 3/4" pipe but that is tapered threads and I don't want that.



C(-)ris
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Edited 2 time(s). Last edit at 04/14/2021 07:27PM by C(-)ris.
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Re: Plumbing Sort of: What size tap for Black Steel Pipe?
Posted by: lost in space
Date: April 14, 2021 07:21PM
tap drill calculator



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Re: Plumbing Sort of: What size tap for Black Steel Pipe?
Posted by: Filliam H. Muffman
Date: April 14, 2021 07:57PM
If lost in space's link is not appropriate, I would work backwards. Pick a standard bolt size that is strong enough (7/8"?), then find the matching tap.



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Re: Plumbing Sort of: What size tap for Black Steel Pipe?
Posted by: C(-)ris
Date: April 14, 2021 08:40PM
Quote
Filliam H. Muffman
If lost in space's link is not appropriate, I would work backwards. Pick a standard bolt size that is strong enough (7/8"?), then find the matching tap.

That matching idea led me to the solution (and your 7/8" guess was right). I looked for a tap that used a 13/16" drill bit (which would be the same as the ID). That was the 7/8" tap.



C(-)ris
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Edited 1 time(s). Last edit at 04/14/2021 08:40PM by C(-)ris.
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Re: Plumbing Sort of: What size tap for Black Steel Pipe?
Posted by: deckeda
Date: April 14, 2021 08:51PM
If this is standard 3/4” black pipe for oil/gas it is indeed supposed to be 3/4” inside diameter.

The challenge comes from the threaded end, which will of course be tapered. How do you tap threads into a tapered end?

But wait ... 1.06” outside diameter, and that 3/4” is only the end thread opening, not the inside of the pipe?? The plot thickens.

Forget all that. Why not skip the welded nut and just thread on an end cap? You can drill and tap that for whatever appropriately-sized marching bolt you need, yes?
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Re: Plumbing Sort of: What size tap for Black Steel Pipe?
Posted by: C(-)ris
Date: April 14, 2021 09:19PM
These pipes were originally 10' long so both threaded tapered ends are gone. I'm dealing with the center section of the pipe. I was able to pick the pipes up for $2 from Habitat for Humanity Restore. Beats paying $30 at Home depot.

I might need to find my digital caliper and go measure to get exact ID if it varies.



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Re: Plumbing Sort of: What size tap for Black Steel Pipe?
Posted by: MikeF
Date: April 15, 2021 12:06AM
Quote
deckeda
If this is standard 3/4” black pipe for oil/gas it is indeed supposed to be 3/4” inside diameter.
^^This

Why call it 3/4" pipe?
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Re: Plumbing Sort of: What size tap for Black Steel Pipe?
Posted by: JoeH
Date: April 15, 2021 01:03AM
Quote
MikeF
Quote
deckeda
If this is standard 3/4” black pipe for oil/gas it is indeed supposed to be 3/4” inside diameter.
^^This

Why call it 3/4" pipe?

Almost all pipe and tubing sizes are nominal. The exterior size is fixed as it has to match up with standard fittings, the interior size can be a bit different. For instance one source of steel pipe lists 3/4" pipe as 1.05" exterior diameter with wall thickness of 0.113". That leaves an interior diameter of 0.824".
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Re: Plumbing Sort of: What size tap for Black Steel Pipe?
Posted by: C(-)ris
Date: April 15, 2021 08:55AM
Quote
JoeH
Quote
MikeF
Quote
deckeda
If this is standard 3/4” black pipe for oil/gas it is indeed supposed to be 3/4” inside diameter.
^^This

Why call it 3/4" pipe?

Almost all pipe and tubing sizes are nominal. The exterior size is fixed as it has to match up with standard fittings, the interior size can be a bit different. For instance one source of steel pipe lists 3/4" pipe as 1.05" exterior diameter with wall thickness of 0.113". That leaves an interior diameter of 0.824".

The wall thickness and NPT threads on the ends are the only part that is required to be consistent. All Schedule 40 black pipe lists the same wall thickness and the same threading. I also observed that the ID and OD seems to vary a bit between manufacturers.



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Re: Plumbing Sort of: What size tap for Black Steel Pipe?
Posted by: Buzz
Date: April 15, 2021 12:24PM
deckeda's cap n' tap premise sounds extremely promising....
How complete is your tap n' die set?
==
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Re: Plumbing Sort of: What size tap for Black Steel Pipe?
Posted by: NewtonMP2100
Date: April 15, 2021 04:39PM
....the black pipes.......tend to be........larger........



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Re: Plumbing Sort of: What size tap for Black Steel Pipe?
Posted by: andypie48
Date: April 16, 2021 02:23PM
I have 32 years as a journeyman maintenance machinist in a steel mill. You want to do this exactly right.

Fine threads are stronger than coarse threads. You want to drill out or turn the inside of the pipe to exactly the right size.

For coarse threads it should be just a few thousandths larger, like .010-.015" because pipe steel is junky and doesn't thread good. Use the chart.

When you drill, leave plenty of extra drilled space at the bottom.

It is important to use thread cutting fluid when tapping so the steel doesn't tear.

Try it out on a piece of scrap first to make sure everything is right.
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