Taper versus Parallel thread

Hi Folks, Okay, showing one aspect of my vast collection of ignorance here: beyond "spelling", what's the difference between a "taper thread" and a "parallel thread"?
The word "taper" implies a gradual reduction to my mind, but this doesn't square with the whole purpose of a thread.
Reason behind this is my researching an alternative set of tails for a bar mixer for a power shower. I've bought a 3 bar pump and the book tells me to run the output in 22mm as far as practical. We picked up all the fittings for our new en-suite in the sales in January, and it's only now that I've got round to taking a closer look at the plumbing. My plan is to run a pair of 22mm copper pipes down a stud wall cavity and then turn these out ninety degrees into the shower bay and connect to the mixer bar. The mixer bar comes with a pair of dog-leg fittings - three quarters bsp male on one end, half inch bsp male on the other. When you look inside the dog-leg the aperture is (roughly) ten millimetres. From a 22mm pipe down to 10mm feels wasteful, hence this quest to find a better fitting.
On my desk is a 22mm female to three-quarters male straight BSP fitting courtesy of Screwfix. The BSP thread isn't deep enough to attach the mixer bar (the mixer bar nut grounds out before it tightens: no use). Ecstasy would be an elbow joint taking 22mm copper pipe on one end and having (say) a two inch long 3/4" BSP male thread (so I could cut it down if necessary) on the other end. I could even find a nut for the 3/4" thread and attach the fitting to some marine plywood as part of the install (someone here posted a URL of some pictures on how they did this: can't seem to find it - would the owner reply please with the url?) I suppose a 22mm female elbow connecting to a 3/4" bsp female could be of use, especially with some "barrel nipples" (found via another thread here on uk.d-i-y; all the jokes on this phrase also covered too).
Any help appreciated; thanks in advance
Mungo
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Taper fittings are often used for fittings involving gases and either PTFE tape or a sealant are used with them.
An alternative sometimes used with compressed air for example, are parallel thread fittings with a flange at the top where two flat surfaces meet and an O-ring to seal.
Some people do try to use taper male threaded fittings in parallel female fittings again with PTFE tape, but it is not best practice for some applications.
For water, parallel threaded fittings are used.

I think that you may well have to go for the barrel option. I presume that the reason for looking at taper fittings was that some don't have the shoulder on the male part of the fitting and would screw in further. This *might* work, but I think a better solution would be to use a barrel.
Possibly BES part no. 6577 would be long enough.
--

.andy


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wrote:

What bollocks. Taper threads are as described. The thread tapers away from deep pitch to narrow pitch. They are for permanent joints which will not swivel. The taper locks the pipe into the fitting although maybe not making a permanent seal of the fluid or gas. Iron pipe has taper threads. The taper is usually on the male thread. The male is usually parallel, although not always. Overtightening taper threads can burst a fitting.
Parallel, are usually where the joint is demountable, or undoable. A union will have a parallel thread, as would the threads on a compression nut on a compression fitting, which is there only to pull the nut and body together and not make a seal. If there is a union and washer the thread will be parallel, with the washer making the seal, not the threads. NEVER, tighten up parallel male and female thread unless a washer or O ring makes the seal. PTFE and the likes is not sufficient.
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On Thu, 16 Mar 2006 19:59:48 -0000, "Doctor Drivel"

Gases are fluids, numbnuts.....

So which is it? Taper on the male thread, or parallel?
Make up your mind....
You need to read your Conex catalogue more carefully. Ask the rep. for a new one.
--

.andy


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wrote:

Matt, do not give advise on topics you know little.
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On Fri, 17 Mar 2006 00:53:05 -0000, "Doctor Drivel"

I leave that to you.
--

.andy


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wrote:

Matt, just do not give advise on topics you know little.
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On Thu, 16 Mar 2006 19:59:48 -0000, "Doctor Drivel"
What bollocks, there is no such thing as "deep pitch" and "narrow pitch"
Machinery's Handbook 26th Edition
Taper Thread: A taper thread is a screw thread projecting from a conical surface.
Pitch: The pitch of a thread having uniform spacing is the distance measured parallel with its axis between corresponding points on adjacent thread forms in the same axial plane and on the same side of the axis. Pitch is equal to the lead divided by the number of thread starts.
Pitch Cone: The pitch cone is an imaginary cone of such apex angle and location of its vertex and axis that its surface would pass through a taper thread in such a manner as to make the widths of the thread ridge and the thread groove equal. It is, therefore, located equidistantly between the sharp major and minor cones of a given thread form. On a theoretically perfect taper thread, these widths are equal to one-half the basic pitch. (See also Axis of Thread and Pitch Diameter.)
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wrote:

Lord Hall, and?
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wrote:

No they don't. The pitch is constant and so is the thread depth. It's the base diameter that tapers.
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wrote:
|wrote: | |> The thread tapers away from deep pitch to narrow pitch. | |No they don't. The pitch is constant and so is the thread depth. It's |the base diameter that tapers.
When I learned to cut tapered threads <mumble> years ago, the thread profile was constant and both the base diameter, and the thread top diameters tapered.
--
Dave Fawthrop <dave hyphenologist co uk>
Freedom of Speech, Expression, Religion, and Democracy are
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Look at the dies and cut one and it is obvious.
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On Fri, 17 Mar 2006 22:22:54 -0000, "Doctor Drivel"

THERE IS NO SUCH THING AS DEEP PITCH OR NARROW PITCH
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<cut much Lord Hallism>
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Like Andy says: taper threads seal on the threads, with the help of hemp, gunge, ptfe, or various combinations thereof, so you have to feel the thing tighten gradually, not stop suddenly when metal meets metal. Parallel threads seal elsewhere: an olive or a washer, commonly.

You've got plenty of pressure from the pump, the 22mm pipe will not drop much pressure, and you're about to put the water through assorted narrow passageways in the mixer before sending both hot and cold down an 11mm hose. I wouldn't worry about an inch or two of 10mm constriction.
Given that all this plumbing is likely to be a tad inaccessible once it's done: KISS.
Don't even consider using any iron fittings in this setup.
--
Kevin Poole
**Use current month and year to reply (e.g. snipped-for-privacy@mainbeam.co.uk)***
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Autolycus wrote:

I've just piped my garage for compressed air, so have recently become very acquainted with taper threads and iron fittings
The taper doesn't taper down ad infinitum, it's more the shape of the tread profile for the 3/4" or so that you cut onto the end of the pipe.
It also allows a high degree of tolerance to overtightening that a parallel thread and shoulder wouldn't. I've given several joints a good 1/4 to 1/2 turn over what would normally be done to line up fittings. I assume the fitting expands slightly to compensate.
I only wish i'd found gas tape before using standard ptfe on the joints. Take a hell of a lot of standard ptfe to seal at 8 bar, (100 psi or so in old money) so you soon know when your joint is dodgy :)
Paul.
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i've got my garage piped for air, but i went the simple route, that pe-x type semi rigid pipe and push fit fittings, not the expensive stuff sold by some tool companies, just bog standard pe-x hose as used for diesel lines on my van,
used 12mm pipe, got a few T pieces, 90 degree angles, few elbows with wall ancors for the air line connectors to the tools and so on,
took about half an hour to pipe up, and i've modified the lay out a couple of times, takes minutes, no leaks what so ever first time (as long as the pipe is cut square at the ends it'll not leak) and i run my compressor at 150 psi, been in place for 3 years now, and it'll hold air for days, the place i rekon it leaks is the connector between the compressor and the pipework which is a standard pcl air-line fitting to a rubber hose, with a 12mm push fit connector on the other end, those pcl connectors never give a 100% air tight joint, but loosing 30 psi over 3 days or so isn't too bad i rekon,
and total cost for the pipes and fittings was around 40 quid.
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CampinGazz wrote:

Nice, I looked at this route, but it all looked a bit complicated and expensive from the various compressed air firms. They too have gone over to 'specialised' pushfit for compressed air.
I took the low tech route and went with galvanised steel tubes, iron fittings and munson rings to the wall. I already had a BSP thread cutter and tube clamp I picked up at a local car boot.
Cheers,
Paul.
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DON'T mess about with compressed air doing improvisations. It can easily kill. It only take a few bar to propel metal about the place. Use the proper fittings or don't bother at all.
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On Fri, 17 Mar 2006 11:37:32 -0000, "Doctor Drivel"

He did use one of the correct approaches. Steel tubing, iron fittings are the traditional way.
I used another of the correct approaches which is Speedfit blue tube and the black push fit fittings that go with it. It has the advantage of being easy to change and to add additional components when required.
However, I would agree with you that use of unsuitable materials is a really bad idea with compressed air.
--

.andy


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