Convert American tap thread

Forums › Home Workshop DIY › Convert American tap thread - NZ tap thread

steve98

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#153734 6-Oct-2014 16:10

Hi all

Bit of a long shot this one, but here goes.

We've got some pop-up sprinklers on three separate lines that until now have been going to a 3-way manifold and we manually turn the water on and switch between the lines. Then we found this which seemed to automate all of that very nicely and for a reasonable price, and we found it at an even more reasonable price at Amazon, so ordered it.

Went to install it on the weekend but ran into an unexpected speed-bump... seems that the US uses a different tap screw thread (NPT) to almost everywhere else in the world who use BSP.

So now I need to find a way to either:

A) Connect my existing 19mm poly irrigation tubing to the male NPT screw thread (tubing looks like and ends with the below):

or

B) Buy whatever Americans would buy to connect a NPT male thread to 19mm poly irrigation tubing and replace our existing BSP couplings with that.

It sounds like it should be simple enough but I am going round in circles on the web and can't seem to find anything suitable.

Like I said it's a long shot but does anyone have any ideas?

BruceHamilton

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#1148362 6-Oct-2014 16:39

I could be wrong, but NPT threads are tapered, and you appear to want a parallel thread - which would be NPS or BSP Parallel (BSPP), or something else. It's hard to tell from the photo - which shows the parallel female hose fitting best.

There are NPT to BSP ( also tapered ) converters available from some engineering suppliers, but most are brass or stainless steel. I would suggest carefully measuring the male thread and then seeking an adaptor from a plumbing or engineering supplier - they should have them for both parallel and taper threads, and mixtures.

If it is a parallel thread, you should not use PTFE tape on the thread. Worst case, an engineering shop with a lathe can make an adapter for about $20+, which is far cheaper than the Swagelok versions.

steve98

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#1148611 6-Oct-2014 20:45

You're right, it's parallel so NPS - all those acronyms are frying my brain!

I've never been anywhere near an engineering shop before... Don't suppose you know of any in Christchurch?

Thanks for your advice.

Niel

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#1148691 6-Oct-2014 22:06

Ha ha, start of the year I did exactly the same to the letter!

It gets worse, USA has a special non-standard thread for outdoor fittings so you can't mix them with indoor fittings. If I remember correctly it is called a GHT = Garden Hose Thread. Suggest you find someone in the US that sells the 19mm hose fitting and let me know as I'll also buy a few. At the time I could not find any on eBay that would ship international (except by DHL...).

I've added an extra washer to seal before the thread interferes. Had to dig through old sprayers to find the right size washer, it is a bit hit an miss.

Some sprayers actually have GHT (if that is what it is called) and an adapter to clip-on. I think it is the cheap orange ones from Bunnings that are bundled with garden hoses that are GHT. But it is the wrong gender so don't help in this case.

Might have to see if someone that has a friend in USA to send us some.

You can never have enough Volvos!

BruceHamilton

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#1148966 7-Oct-2014 10:35

Thanks Niel, I wasn't aware of the US Garden Hose Thread, which is incompatible with all other US threads. As a parallel thread, the critical seal is compressing a soft washer/gasket at the bottom face, so your solution may be the quickest, perhaps just make a flat washer from some old rubber sheet or disk.

I'd also check online industrial and garden hose suppliers in NZ to see if there is a cheap plastic GHT hosetail fitting, and use a short length of hose to connect the two items. You might be able source an adapter from China, which may be where most US adapters come from.

Sorry, I don't know any small engineering shops in Christchurch, but the yellow pages should offer a couple. Cutting a brass parallel GHT to hosetail adapter should not be too difficult, but Christchurch businesses may have plenty of other work.

ubergeeknz

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Vocus

#1148997 7-Oct-2014 10:58

Why not just go to M10 and get a standard hose end fitting (Nylex et al)? Should fit your 19mm hose no problem.

steve98

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#1149206 7-Oct-2014 15:12

I don't know that you'd get a completely water-tight seal compared to the normal barb ending on the actual irrigation couplings. Worth a try though perhaps.

freitasm

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#1149225 7-Oct-2014 15:31

A thread about threads. Meta.

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BruceHamilton

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#1149226 7-Oct-2014 15:35

The issue, if I understood it correctly, is that the imported unit has male GHT fittings, and they are incompatible with BSP female hose fittings used in NZ. They are not compatible, even though both are parallel thread and can be used with 3/4" hose.

The US GHT hose fitting has an OD of 26.99 mm, and a thread pitch of 2.21 mm, and the NZ BSP hose fitting has an OD of 26.44 mm and a thread pitch of 1.81 mm. Brute strength and ignorance will lead to failure, as they are parallel threads that rely on the rubber seal at the base, hence the suggested use of an additional gasket and hope that the few interlocked threads will hold.

steve98

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#1149303 7-Oct-2014 17:16

Niel: Ha ha, start of the year I did exactly the same to the letter!

It gets worse, USA has a special non-standard thread for outdoor fittings so you can't mix them with indoor fittings. If I remember correctly it is called a GHT = Garden Hose Thread. Suggest you find someone in the US that sells the 19mm hose fitting and let me know as I'll also buy a few. At the time I could not find any on eBay that would ship international (except by DHL...).

I've added an extra washer to seal before the thread interferes. Had to dig through old sprayers to find the right size washer, it is a bit hit an miss.

Some sprayers actually have GHT (if that is what it is called) and an adapter to clip-on. I think it is the cheap orange ones from Bunnings that are bundled with garden hoses that are GHT. But it is the wrong gender so don't help in this case.

Might have to see if someone that has a friend in USA to send us some.

Niel, thanks for this. I just got home from work and tried your workaround of an extra washer and it seems to be doing the job... for now. I will keep looking for a 'proper' part and let you know if I find one. Glad to know it's not just me that gets caught out with this kind of thing!

Niel

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#1149581 7-Oct-2014 23:16

At Bunnings (I think) I did see a GHT adapter, but it was for the other way round to use US fittings in NZ. Since I am an engineer, I did go down the path of searching through many, many catalogues and Chinese web site but was unable to find the right fitting. I do think the easiest is to try and get someone in USA to send you some, or use a US parcel forward service. I'll also see if I can get something sorted, but to be honest I have not had any issues with the double washers on 4 fittings (3x solenoids and 1x metal garden hose fitting) over all of Summer. Keep in mind the 19mm fittings never see full pressure, and only the garden hose see full pressure if you close the nozzle (or kink the hose), so it does not really need all of the thread in this application.

You can never have enough Volvos!

ubergeeknz

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Vocus

#1149583 7-Oct-2014 23:20

steve98: I don't know that you'd get a completely water-tight seal compared to the normal barb ending on the actual irrigation couplings. Worth a try though perhaps.

They tend to clamp tight around the hose, which I imagine is going to give a better seal than a simple barb. Just a cheap option to try before importing exotic adapters.