pvc plumbing threads

beaslbob · Jun 22, 2017

I don't know if it's just me but I constantly have fittings from home depot/lowe's and so on where the threads only go in a thread then jam. Not anywhere near completely seated into the female fitting. I seem to notice that most with male reducing bushings into female reducing bushings. Which I use as bulkheads. I have also observed the same issues in normal male to female fittings as well.

I seem to get it working but was wondering if this is normal. The fittings are all the same schedule and the common white fittings.

TIA

Fishfinder · Jun 22, 2017

They can be tough to thread together by hand sometimes. just make sure you are not cross-threading them

beaslbob · Jun 23, 2017

They're not cross threaded.

JAMSOURY · Jun 23, 2017

Same here. Mine doesn't completely go in. I'm just test fitting but does the pvc glue or teflon tape make it easier to fully put on. Here's a picture of where one of mine stops and is hard to fully put in

Sleepydoc · Jun 23, 2017

NPT/MPT/FPT (National Pipe Thread/Male Pipe Thread/Female Pipe Thread) fittings are tapered - the are broader at the base than they are at the end/opening, this means they will be progressively harder to turn the farther in you thread them. This combined with the natural friction between the PVC surfaces (and manufacturing imperfections) means you get a fair amount of resistance pretty quickly. It also means that if you over tighten a joint, the male fitting puts too much outward pressure on the female fitting and can crack/split it. I don't know if I've ever seen a threaded PVC connection completely 'bottomed out' on the threads.

The general rule for PVC fittings is finger tight plus 1-2 turns. You should also use a non-hardening thread sealant (on the male threads) to help seal the joint. Many people recommend teflon tape, but the reputable sources I've seen recommend the thread sealant. In my experience this works better than tape anyway.

JAMSOURY · Jun 23, 2017

Sleepydoc said:
NPT/MPT/FPT (National Pipe Thread/Male Pipe Thread/Female Pipe Thread) fittings are tapered - the are broader at the base than they are at the end/opening, this means they will be progressively harder to turn the farther in you thread them. This combined with the natural friction between the PVC surfaces (and manufacturing imperfections) means you get a fair amount of resistance pretty quickly. It also means that if you over tighten a joint, the male fitting puts too much outward pressure on the female fitting and can crack/split it. I don't know if I've ever seen a threaded PVC connection completely 'bottomed out' on the threads.

The general rule for PVC fittings is finger tight plus 1-2 turns. You should also use a non-hardening thread sealant (on the male threads) to help seal the joint. Many people recommend teflon tape, but the reputable sources I've seen recommend the thread sealant. In my experience this works better than tape anyway.

Thanks makes a lot of sense and I see it now when I look at it closer. I always thought they would sit flush. Thanks!

cracker · Jun 23, 2017

If You making bulkheads where the threads need to pinch a washer. try the grey electrical fittings. they are the same size & the threads are not tapered. they will screw all the way down.

Idoc · Jun 25, 2017

I've been digging through all my past links, but can't find what I am looking for! But, the bottom line was that a particular aquarium plumbing site or article was discussing the differences in the threads on PVC. The site didn't recommend attaching two different kinds of plastic piping together using threads because bulkheads are made from ABS plastic and the pipes are either schedule 40 or 80 PVC...the threads are different in not only how they are threaded (slight difference), but different in how they contract and expand due to the different plastic types. The article actually showed the thread designs and the slight differences in shapes...I'm a visual person, so that helped to clarify in my mind what they were talking about. The site basically said to use only slip fittings and solvent sealants. Also, it stated that Teflon tape is designed for metal threads and not PVC threads...thus thread sealant is the way to go when connecting two of the same PVC schedules together with threads.

I'll keep looking for the supporting documentation...I'm sure I have it saved on one of my computers!!!

Sleepydoc · Jun 25, 2017

The outer threads on a bulkhead are not NPT threads; they are intended only for the nut that comes with the bulkhead. NPT is a standard, so if there are differences they should be simple manufacturing variations, not actual design differences. There are other pipe thread standards (like BSPT - British standard pipe thread) that are different. Is that perhaps what they were referring to?

The other, much more common problem with threaded bulkheads is that people tighten them too much. The relatively weaker ABS plastic will split under the force of an over-tightened male PVC fitting.

Regardless, the advice to use solvent-welded slip fittings is sound. Solvent welded fittings are much more reliable than threaded fittings. If you will need to disassemble in the future, a union is a much more secure option.

JAMSOURY · Jun 25, 2017

Idoc said:
I've been digging through all my past links, but can't find what I am looking for! But, the bottom line was that a particular aquarium plumbing site or article was discussing the differences in the threads on PVC. The site didn't recommend attaching two different kinds of plastic piping together using threads because bulkheads are made from ABS plastic and the pipes are either schedule 40 or 80 PVC...the threads are different in not only how they are threaded (slight difference), but different in how they contract and expand due to the different plastic types. The article actually showed the thread designs and the slight differences in shapes...I'm a visual person, so that helped to clarify in my mind what they were talking about. The site basically said to use only slip fittings and solvent sealants. Also, it stated that Teflon tape is designed for metal threads and not PVC threads...thus thread sealant is the way to go when connecting two of the same PVC schedules together with threads.

I'll keep looking for the supporting documentation...I'm sure I have it saved on one of my computers!!!

Interesting. Didn't even know that. The bulkreefsupply bulkheads are schedule 80 thread x thread, so I think the inner threads on the bulkhead will match pvc threads?

Idoc · Jun 25, 2017

JAMSOURY said:
Interesting. Didn't even know that. The bulkreefsupply bulkheads are schedule 80 thread x thread, so I think the inner threads on the bulkhead will match pvc threads?

They should match up easily... but, BRS sells both ABS and Schedule 80 bulkheads...ABS are cheaper, I think.

Sleepydoc · Jun 25, 2017

ABS bulkheads are available in all combinations and significantly cheaper ($6 vs $15 for a 1" bulkhead) Schedule 80 bulkheads also require a bigger hole, requiring more safety clearance & theoretically weakening the glass more. If you have a tank that's already drilled, odds are it's drilled for an ABS bulkhead.

Also, even though they are 'beefier,' you can still split or crack a schedule 80 bulkhead if you over tighten the threads.

Idoc · Jun 26, 2017

@Sleepydoc take a look at this link, http://www.lascofittings.com/threads , ...this is very informative concerning PVC threads and their strengths. Ultimately, the strengths don't really have a huge impact on our smaller pressure systems, though. But, it gives excellent explanations (and pics) on why NOT to use teflon tape with PVC threads! Also...good explanations with data showing failure rates of when you tighten a PVC thread beyond the 1-2 turns after hand-tight. Just for thought...

http://www.lascofittings.com/threads

chipmunkofdoom2 · Jun 26, 2017

I'll agree that using teflon or thread-sealing paste is probably better than using teflon tape. However, at the National Aquarium in Baltimore we use tape on PVC threads everywhere. I've seen teflon tape on pressurized PVC connections up to 3". I just yesterday helped hook an external pump up to a several hundred gallon holding tank. The pump had threaded PVC connections and every single one of them got taped.

Sleepydoc · Jun 26, 2017

@Idoc - I've seen that page before (I may have even cited it.) It does a good job or explaining the rationale for using paste.

It's not that tape can't work; there are certainly plenty of connections around that use tape and not paste. My point is that paste is recommended by the manufacturers as a better and more reliable means of making a tight connection without unnecessary strain on the fittings. I have to say that in my experience, the thread sealant method is easier and more reliable than teflon tape.

Idoc · Jun 26, 2017

Sleepydoc said:
@Idoc - I've seen that page before (I may have even cited it.) It does a good job or explaining the rationale for using paste.

It's not that tape can't work; there are certainly plenty of connections around that use tape and not paste. My point is that paste is recommended by the manufacturers as a better and more reliable means of making a tight connection without unnecessary strain on the fittings. I have to say that in my experience, the thread sealant method is easier and more reliable than teflon tape.

Do you have a thread sealant type/brand that you use or recommend? I'm only going to have a coupe of threaded connections in my plumbing system, but got a little overwhelmed with all the options of sealants at Lowe's today, lol. The pic is of one I saw that stated it was safe for potable water.

Sleepydoc · Jun 26, 2017

That's what I've used in the past - it's safe for potable water and PVC. The Lasco site recommends against using products with teflon in them, but finding a non-hardening, teflon-free thread sealant is pretty difficult. I just smear it circumferentially on the male threads so any extra squeezes out rather that into the lumen of the pipe. The stuff never dries and doesn't wash off very well either, so I use paper towels to clean up the extra.

beaslbob · Jun 27, 2017

Interesting and thanks for all the replies.

In my experience I don't get 1-2 turns after finger tight. And further would require a wrench and probably crack the fittings as mentioned.

But now I got some ideas and appreciate al the input.

ps on my diy bulkheads I got some scrap 1/4" rubber sheet from a local gasket maker. So I was able to get it water tight.