Sharp vs flat tops on pipe threads

JHZR2

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I was replacing a shower arm which is old US-made Kohler, and the replacement does not have the same threads... Old: New: The new one came from Lowe's. There was nothing there that was claimed to specifically be NPT. This new one does say IPS, FWIW, which as I understand, can mean lots of different things... In doing searches on NPT, it seems like NPT is supposed to have a little bit of a flat on the peaks of the thread. So was the old part wrong/different? Is the new part for something else? The new part does have somewhat of a taper, though not necessarily the same taper as the old NPT. What's right thread-wise?
 

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Originally Posted By: heynow
Does it thread in where the old one did? If so, use it.
Yes. Doped and taped and it doesn't leak. The odd thing was that I started this by replacing the shower head with a new Kohler unit. I just hand tightened the new head on the old arm, and it worked perfect, no leaks. Replaced the arm (for more height) and the shower head leaked at the fitting on the new (and freshly taped) arm. That's what made me note the difference in threads. The hole through the tile where the arm goes is too small to observe the fitting, but the riser can be observed by removing the control cover or through an access panel in the linen closet in the bathroom. Checking both in cafes that it is not leaking at the female fitting in the wall. Additional tape and another turn stopped it leaking at the shower head. So it's really more academic regarding thread shape.
 
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It's just a different cutter used to make the threads. Obviously use pipe tape and slap it in there. It will be fine. When I threaded tons of pipes my threads had sharp threads. It was a rigid pipe threader I used.
 
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If the threads are only flattened at the shower head end, it would be because a lot of shower heads seal on to the arm with a rubber gasket, so the flats do not matter. The flattened thread will not seal properly on to the threaded elbow inside the wall. The pressure is reduced and is only present while the shower valve is on, so even the flattened threads will sort of work at the elbow. I sure would not trust it to seal under full continous pressure, especially in a concealed location.
 

JHZR2

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Originally Posted By: walterjay
The threads are flat because it is a plastic shower arm. If you have a problem with leakage, get a metal shower arm.
No it's chrome plated brass.
 
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It's simply a slightly different style of cut on the top of the thread. There's no difference in functionality as long as the taper is the same. NPT is a US standard for tapered threads used on threaded pipes and fittings, as opposed to the straight threads on a bolt and nut. The taper will pull tighter and help make a liquid tight seal. NPT typically only needs to seal on threads which is why you put tape on them. Straight threads require something like a locknut and packing material to create a seal. IPS stands for Iron Pipe Size, as indicated on your packaging. It's a fairly old standard but still used in plumbing because of all of the vintage piping still around. The difference is not because it's made of metal or plastic, nor is it a by-product of the pre-plating process. It's simply a different design of the cutter.
 
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ASME A112.18.1 dictates that
Quote:
4.4.4 Shower heads for installation on standard shower arms shall be capable of being connected to a 1/2 NPT male thread.
And that threads must comply with ASME B1.20.1 which are the standards for NPT. IF they comply by the standards they claim on the package then it is compatable with NPT. IPS just means the pipe is made to Iron Pipe Size. That becomes an issue with dealing with plastic pipes and tubing.
 

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Thanks. I walked through HD today, and stopped to look at their stock. Glacier bay branded, and not from the same Chinese factory. Interestingly, on any one thread set, half the threads were pointy and half of them were flat, with varying degrees of flatness. The threads also had some black residue on them. At least the thing from Lowes was clean, bright and consistent. The original one still seems the best to me, and the pic I shared was the section in the wall, scratched from layers of tile and cement board. The other side is a little pitted but fared well. But it's too low for me, so when the shower head went bad, it was time to replace. Thanks for the info!!
 
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Originally Posted By: JHZR2
Thanks. I walked through HD today, and stopped to look at their stock. Glacier bay branded, and not from the same Chinese factory. Interestingly, on any one thread set, half the threads were pointy and half of them were flat, with varying degrees of flatness.!
This reinforces what I pointed out earlier. The flattened thread ends seal with a gasket, so the flats don't matter.
 

JHZR2

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Both ends were like this, both on the item at HD and the one I installed. Yes, a shower head does have a gasket inside of it, so that is a plausible seal... But the female threaded connection on the riser off the shower controls does NOT have a gasket, and relies on NPT threads. I doped and taped and it does not leak. The shower arm that I installed did not have a defined shower head end, and is symmetric, so it could be installed either way.
 
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Its all academic at this point. The mating flats of the threads meet 1/2" NPT. My guess is that if you measure with calipers, you will find that your old USA arm is made to the old IPT standards regarding outside diameter. The newer Chinese arms have reduced outside diameter to shave pennies off production cost. Its not a different thread die shape. There simply is no "meat" (metal) to be left to form a pointed "crest" on the thread. On the later ones you saw at HD with some flat, some sharp crests, I imagine the sharp ones are more near the tapered end(?). You can't have a pointed "crest" if there is no metal to start with due to smaller outside diameter:
 
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