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Delrin, brass, or bronze for a pipe fitting with a hole for an interference fit?

Randalthor

Cast Iron
Joined
Mar 7, 2018
Location
Kansas City
I am about to make a specialized pipe fitting for someone, and unsure of an adequate, yet cost effective material to use. It's just one custom fitting, that I almost assuredly will never make again.

This fitting is for a hot tub. The fitting needed is a solid mip 2" fitting (i.e like a 2" npt plug) to start with. Inside this plug I will need to bore and tap a 1" fip hole. But the hole needs to be offset, not centered. In other words, it's not a standard 2" to 1" npt bushing. The 1" threaded hole will be closer to one edge than the other.

The reason for this is that the special fitting has to have room for not only a 1" fip hole (which a 1" male npt heating element will screw into) but also have room for an additional hole to press fit a SS .378" tube, which holds a thermostat. The custom fitting has to accommodate both holes, hence reason to offset of both holes. Obviously both holes will be through holes.

I have thought of simply buying a 2" bronze or red brass plumbers plug, machining off the square drive, boring and threading the hole for the 1" heating element and boring a hole for the .378 tube. This would minimize time and cost of material. I'd make a little spanner wrench which would take advantage of the two holes to tighten the custom fitting into the existing plumbing of the hot tub. The existing plumbing is 2" abs, so the custom made fitting will screw into a 2" abs female npt X glue adapter..

I plan to use a V drill (.377") for the press fit of the thermostat tube. I could also bore the hole for the thermostat tube larger than .378 and silver soldier the SS tube into the bronze or brass custom fitting. I plan to bore a 1-5/32" hole and thread it with a 1" pipe tap for the heating element.

I have also considered making the custom fitting out of a delrin round. Turn the round down, thread the outside to 2" mip, and thread the offset 1" fip hole and bore a hole for the .378 tube (although I am unsure of the correct bore size needed for an interference fit in delrin). This would obviously be a lot more time involved.

Advice please?
 
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It sounds like you are relying on the PF to be leakproof. I think that is a mistake.

Everything else is pvc. So use pvc. Make a solid threaded pvc plug. Tap the two holes npt. Use a 3/8 hose barb with 3/8 ID vinyl tubing to the tube, secured with hose clamps.
 
id think a marine grade stainless steel might be better ..........Brass or bronze will turn green from all the urine in the water.
Ha ha. Disgusting, and funny. I guess if kids use a hot tub, they may pee in the water. I hadn't thought of that.

I had considered stainless steel, but thought it might be pretty hard to tap with a pipe tape (given that I hadn't planned to use a pipe tap reamer). I could simply buy a pipe tap reamer, but it adds a lot of cost to the project, if I don't have to do it.
 
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It sounds like you are relying on the PF to be leakproof. I think that is a mistake.

Everything else is pvc. So use pvc. Make a solid threaded pvc plug. Tap the two holes npt. Use a 3/8 hose barb with 3/8 ID vinyl tubing to the tube, secured with hose clamps.
Some very good thoughts. Thank you.

The reason I haven't considered making it out of ABS is that I'm unaware of a solid 2" plug for ABS. All the ABS plugs I've seen are cored/hollow, which wouldn't be conducive to structural integrity, given the amount of material to be removed from the plug. I tried to find some abs solid 2.5" rounds online but haven't located them. Would they be better than delrin rounds I have on hand, if I could find ABS rounds?

Based on your comment, it sounds like soldering is the way to go if I go with a red metal. Thanks.
 
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McMaster has a wide variety of materials and fittings.

The delrin is fine. Not sure, but I think ABS is a pain to machine. My hot tub uses no abs. Only pvc.
 
You might use a short piece of stainless pipe or a plug. Weld in a one inch coupling and another one of the correct bore for the temperature well. Then weld up any gaps to make it water tight. You may have to redo the threads after all that welding distorts things.
Look at how a sanitary well seal is made for ideas.
BilL D
 
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You might use a short piece of stainless pipe or a plug. Weld in a one inch coupling and another one of the correct bore for the temperature well. Then weld up any gaps to make it water tight. You may have to redo the threads after all that welding distorts things.
Look at how a sanitary well seal is made for ideas.
BilL D
That's my go to approach for fittings. Find what is close to what I need in stainless, and drill for some extra weld bungs or couplers or elbows or whatever.
 
I have always heard for household plumbing that a metallic male fitting into PVC female thread will eventually crack the PVC, and that such a union should always be PVC male into metal female. ABS might perform differently from PVC, but rapid thermal cycling is to be expected in a hot tub. That's one argument for a plastic plug.
 
You sure you want a metal male fitting being threaded into a plastic female fitting??? I have seen too many failures of the female side in this situation, both hot and cold environments…

Murf
 
You might use a short piece of stainless pipe or a plug. Weld in a one inch coupling and another one of the correct bore for the temperature well. Then weld up any gaps to make it water tight. You may have to redo the threads after all that welding distorts things.
Look at how a sanitary well seal is made for ideas.
BilL D
That sounds like the easiest solution I've read. No threading, almost no nothing. I can easily weld stainless. Drilling it is nothing. Thanks. I gravitate strongly toward your solution.
 
"You sure you want a metal male fitting being threaded into a plastic female fitting??? I have seen too many failures of the female side in this situation, both hot and cold environments…"

That's a valid issue. I am aware plastics and chlorinated plastics have a radically different expansion coefficient than metals. But I've used lots of metal fittings inside various plastics in my life. I've not seen them bust a female fitting so far. In my experience, most of the time the female fitting breaks because the male fitting is cranked in too tight. But we don't have the super cold temps that are normal in more northern states.
 
I'd make it from PVC. Under $20 from McMaster for a foot of material:

 
Is chlorine used in this hot tub? I believe Delrin/acetal is susceptible to attack by chlorine. Some stainless will also develop pitting if the filler metal doesn't make up for segregation of the elements (chromium carbides?) in the HAZ/weld.
 
I believe Delrin/acetal is susceptible to attack by chlorine. Some stainless will also develop pitting if the filler metal doesn't make up for segregation of the elements (chromium carbides?) in the HAZ/weld.
Thank you. I hadn't considered chlorine attacking ABS, or attacking a SS weld. I had planned to use 309L for the filler material, which I have on hand. I'm not able to find anything on the web whether or not it's susceptible to degradation from chlorine. Do you happen to know?

I'd make it from PVC. Under $20 from McMaster for a foot of material:

One of my concerns about PVC is that it doesn't have a lot of heat resistance. I doubt a hot tub gets hot enough to degrade PVC, but I wonder if the heating element would get hot enough to degrade the fitting, if made from PVC? From the picture I've seen, it looks like the hot tub is plumbed with ABS where the heating element goes, but then there is some PVC farther away from the heating element.

I did find ABS rounds (which has more heat resistance than PVC) on McMaster. So thanks for the link.
 
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316L is usually used in water service. I believe the recommended filler is also 316L. Try to minimize heat input from welding and cool rapidly once the part is below 1700F to minimize the formation of chromium carbides although 316L goes a long way in reducing that.
 








 
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