Pipe thread size

[Jack james]

Why is it the pipe fitting do not seal. It's as tho there are metric and american sizes.

[John P Ryke]

what are you trying to seal from .....gas or water

[KhunBENQ]

Are you referring to PVC pipes/threads?

For those you always need Teflon tape to seal.

[Jack james]

I have used pvc in us for many years, they were never this loose fitting, nevever could you screw them all the way.use maybe three wraps tape. Here you put 10 wraps and still bottom out.

[Fruit Trader]

There are fittings with tapered male but most plastic are straight threads with poor tolerance. Half a roll or more tape on a 2 inch plastic fitting is the norm.

 

A smear of silicone gasket sealant after applying layers of tape can help prevent the annoying drip.

[impulse]

I've never used a threaded connection on PVC piping, just glued connections.  But then, I'm not in the biz...  Just small projects around the house.   The advantage is that glued connections can be cut to exact length onsite without threading equipment.  The downside is that glued fittings can't be undone.

 

Given the low elastic modulus of PVC (about 500,000 PSI vs 30,000,000 PSI for steel), I can't imagine threads won't deform if you tighten them enough for a leakproof connection.  And, in fact, they are designed to be used with a sealant- tape or putty.

 

https://www.pvcfittingsonline.com/resource-center/pipe-fittings-sizes-guide/

[Jack james]

Yes  but when you connect pvc to something there are always threads involved, there lyes the problem.

[Kwasaki]

15 hours ago, Jack james said:

Here you put 10 wraps and still bottom out.

Put 20 and use metal thread in plastic connector not plastic.

[eyecatcher]

1 hour ago, Kwasaki said:

Put 20 and use metal thread in plastic connector not plastic.

I was waiting to jump in with that one.

 

But as I am here....never screw metalthread into plastic, always other way around.

The steel will easily strip the plastic thread especially if you dont set if correctly.

 

Also the teflon, to me its PTFE tape and its available in different guages and different widths so dont always need to use a mile of it.

Edited December 1, 2018 by eyecatcher

[grollies]

34 minutes ago, eyecatcher said:

never screw metalthread into plastic, always other way around.

Eh? So it's ok to screw a plastic thread into a steel thread? What's the difference?

[johng]

36 minutes ago, grollies said:

What's the difference?

no difference if it goes in cross threaded whatever is plastic will lose.

 

Lots of PTFE tape is needed for Thai plumbing !!!!

[eyecatcher]

1 hour ago, grollies said:

Eh? So it's ok to screw a plastic thread into a steel thread? What's the difference?

Ok, I didnt make myself clear.

what I should have said was dont fix a male metal thread i to a female plastic thread.

Its very easy to overtighten the metal male and either deform the plastic fitting or even break the neck where its glued on.

 

But of course its an around bad idea to mix fittings anyway.

[Fruit Trader]

????

 

[grollies]

4 minutes ago, Fruit Trader said:

????

 

 

51 minutes ago, eyecatcher said:

Ok, I didnt make myself clear.

what I should have said was dont fix a male metal thread i to a female plastic thread.

Its very easy to overtighten the metal male and either deform the plastic fitting or even break the neck where its glued on.

 

But of course its an around bad idea to mix fittings anyway.

But the PVC fittings with threaded metal inserts - simples.

[grollies]

1 hour ago, johng said:

no difference if it goes in cross threaded whatever is plastic will lose.

 

Lots of PTFE tape is needed for Thai plumbing !!!!

PTFE - Plastic Tape For Engineers.....

[johng]

PTFE - Plastic Tape For Engineers.....

I've seen some "Thai plumbers" use a Big C plastic bag instead.

[Fruit Trader]

24 minutes ago, grollies said:

But the PVC fittings with threaded metal inserts - simples.

And then you have the ham-fisted guy who separates or fractures the metal insert creating a leak that cant be easily fixed. Plastic to plastic or metal to plastic is all about knowing when to back off before the threads cross or excess tape splits the fitting.  

Edited December 1, 2018 by Fruit Trader

[grollies]

3 minutes ago, Fruit Trader said:

And then you have the ham-fisted guy who separates or fractures the metal insert creating a leak that cant be fixed. Plastic to plastic or metal to plastic is all about knowing when to back off before the threads cross or excess tape splits the fitting.  

Ah, but I'm an 'expert', I spent 25-years screwing threaded fittings together.......BTW, never ever use stainless steel threaded fittings, if you don't know what you're doing you'll always knacker the thread.

 

TBH I don't have a problem with PVC threaded fittings and rarely use the metal insert type.

[Fruit Trader]

Just now, grollies said:

Ah, but I'm an 'expert', I spent 25-years screwing threaded fittings together.......BTW, never ever use stainless steel threaded fittings, if you don't know what you're doing you'll always knacker the thread.

 

TBH I don't have a problem with PVC threaded fittings and rarely use the metal insert type.

Yes stainless has a nasty habit of picking up to the point where unscrewing a thread is not possible.

[eyecatcher]

2 hours ago, grollies said:

PTFE - Plastic Tape For Engineers.....

nah....

Plumbers tape for efficiency

[eyecatcher]

But if you really want to know why its shortened.

Poly Tetra Fluoro Ethylene

[Jack james]

but just out of curiosity why is everything so poor fitting? whether its metal or plastic its bad.

[sometimewoodworker]

3 hours ago, Jack james said:

but just out of curiosity why is everything so poor fitting? whether its metal or plastic its bad.

If you buy the cheap fittings then that is what you get. I use PP-R that has metal inserts and with good quality taps and stopcocks, they fit nicely.

[Jack james]

what is pp-r?

[sometimewoodworker]

2 hours ago, Jack james said:

what is pp-r?

Polypropylene random copolymer pipe and fittings.

Edited December 2, 2018 by sometimewoodworker

[impulse]

On 12/1/2018 at 3:48 AM, johng said:

I've seen some "Thai plumbers" use a Big C plastic bag instead.

 

I've done it, too in a pinch.  Though I use a cut-up LDPE bag (Baggies) as opposed to the HDPE used in BigC bags.   The biggest difference between PTFE and LDPE is melting temperature.  For low temp applications, that's not important.

 

They have similar coefficients of friction (in the same ballpark, anyway), most adhesives won't stick to either of them, and they're both resistant to most solvents.  I've never had a connection fail when I used LDPE.  That said, I've never used it for a critical application, either.