Fisher Model 1400, Searching for 3/8 COMP x 3/8 FIP faucet supply hose (24" long)

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mliu

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Here is a good illustration of what happens when you try to thread a nipple with male NPS threads into a fitting with female NPT threads:

https://www.pbase.com/image/91533235

With tapered threads, as you tighten the joint, ALL the male threads within the fitting compress circumferentially against the female threads they are in contact with. Not only does this result in a minimal leak path between the roots and crests of the threads (which is easily sealed with pipe dope), but it also fixes the male pipe firmly in place within the female fitting.

However, threading male NPS threads into female NPT threads causes three problems:

1. Only a few threads are able to engage.

2. A tight connection only occurs at the first (leading) thread; the remaining threads are a loose fit in the taper. This means the leak path is not just in the tiny gap between thread crest and thread root, but along the entire flanks of the threads.

3. Because all but one of the threads are loose, the nipple/pipe is not firmly located in the fitting. In other words, any pressure exerted perpendicular to the nipple/pipe can cause it to move within the fitting, breaking any dope seal and opening up a new leak path.
 

mliu

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I would think that would mean you have the option to run a 3/8" NPT die onto the 3/8" NPS threads and taper the end yourself (assuming the wall thickness suffices).
Technically, I could. But there are some issues:

1. My Ridgid 11-R pipe threading set does not include a 3/8" NPT die. So that means it'll cost me ~$120 to buy that die.
2. It will be virtually impossible for me to clamp the riser for threading without damaging the chrome plating.
3. Why do I have to fix Fisher's mistake (either poor design or manufacturing defect)?
 

wwhitney

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I certainly agree with you about the manufacturer's apparent failings. Just trying to put out options that might be good enough and might let you get it working with the least hassle, assuming the manufacturer can't point you to a 3/8" NPS supply line and can't/won't swap the faucet stem for one with NPT threads at the end.

My current bet is on a manufacturing defect, that Fisher's current supplier skipped the step of tapering the threads at the end of the faucet, and Fisher missed it.

Cheers, Wayne
 

breplum

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Fisher is a lame company. T&S is also not much better, unfortunately. They should supply an adapter with a washer.
Of course the threads are straight running threads, and that is how the nut can spin up to tighten the faucet into place.
I'd first bench test a 3/8" F comp hose.
 
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