Flat vs pointed-top tapered threads

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JHZR2

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Im replacing the shower arm that was ca. 1975 when the bathroom was redone with all USA-made Kohler stuff. The shower arm is just too low and Im too tall, plus our shower head sprung a leak.

(Im putting this in the general forum because Im primarily curious about threads and sealing, not the shower aspect of it - mods, feel free to move)

The old one is chromed brass, as is the new one.

The old one had a tapered thread that the outer ends of the threads were pointy. Here is the old one:

The new one has flat tops on the threads. It IS tapered (unless an optical illusion that tricked me, though it does seem the taper isn't the same), maybe not exactly the same as the original... Here is the new one:


I got the old one out using the "channel lock arm in the hole" trick I read about on here. I cleaned the female threads, they looked good (through the tiny hole in the wall and tile, which is only barely larger than the OD of the shower arm), vacuumed any dirt or dust, and then prepped the new one. I used dope then tape than dope on the arm going into the wall. Tested for leaks, no indication, but as mentioned, the hole in the wall is tiny, so I tested by removing the covers for the controls and watched for water dripping down the riser.

So... What got me wondering is that what started all this was the need for a new show head. I got a new Kohler single setting shower head and like it very much. I screwed it onto the old shower arm (with the pointy threads), only finger tight and without even changing the old teflon tape. Turned on the shower and it worked perfect! Not a drip from the threads. Install it on the new arm with new teflon tape, and it was dripping...

The shower arm concerned me as it says "IPS" as the thread. It was the only type of thread that Lowes (yeah, Im not a fan but we were there and they were only 3.5% sales tax) offered for a shower arm...

I wasn't sure if this was lost in translation or what.

It took me using a heavier set of teflon tape layers and a few more turns of the shower head with a wrench to seal up (for now, I hope). But given the leakiness, and difficulty of seeing any leaks in the wall, I wanted to get smart on thread types and if there is an issue...

Thanks for your advice.
 
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CountryBumkin

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JHZR2

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Well, Ive got the Kohler end sealed and since its out of the wall, in the shower, Im not terribly concerned with it...

Its the general depiction and differences/compatibility of these threads that Im interested in. Is it acceptable to replace an item with threads as are shown for the old arm, with a piece that is threaded like the new one in the image... That is, replace pointy top threads with flat top threads? Are they compatible?

Im more concerned because I don't know what kind of threads are on the female pipe end IN the wall, and what the long-term ramifications are. Like I mentioned, I cleaned it all well, taped and doped it, screwed it in tight, and checked for leaks down the riser, which I didn't see after running the shower many minutes...

But it doesn't mean its right, and understanding the threads' compatibility would be useful.

Thanks!
 

CountryBumkin

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If you put a straight edge along the pipe length you can see the thread taper. Pipe threads seal from mechanical contact and it is the thread taper that causes the pipe to tighten (have more mechanical thread contact) the more it is threaded in. With straight threads it will not get tighter the more it is screwed in, it will just bottom out in the female socket/end or run out of threads (which may feel tight).

Since your shower spout/nipple has no pressure on it (one end is always open) wrapping the straight threads with Teflon tape and screwing it in may have be good enough to not have any leaks. However, if you were to cap the end and test again, it would probably leak.
 

Kreemoweet

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It would be the rare showerhead that was intended to seal by a tapered pipe thread fit, like threaded plumbing pipe does. They
almost invariably have an elastomeric ( "rubber") seal ring in them that is intended to seal via pressure from the male end of
the shower arm. Whether any given shower arm will actually screw in far enough to exert sufficient sealing pressure with a given shower head is pretty much a trial-and-error thing if you're dealing with the cheap Chinese crap that most mass retailers offer. Fittings made by
reputable brands, bought from reputable plumbing-supply outlets, rarely give problems. "IPS" and "NPT" markings on the cheap
stuff cannot be relied on. Many's the time I've had to deal with client-supplied fixtures from "overseas" whose threading conformed
to no discernable threading standard whatever.

Some showerheads have an integral shutoff feature, and using it will put full water system pressure on the shower riser and arm
joints. It would therefore be advisable to test the installation under those conditions. There are screw-on pipe-caps with rubber seals
that can be screwed onto the head end of the shower arm for exactly that purpose.
 
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