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

Reply to
Mungo
Loading thread data ...

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.

Reply to
Andy Hall

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.

Reply to
Autolycus

Go to local Plumbers Merchant and get a suitably sized fibre washer to fit over the 3/4" thread and slide up to the flange. Then fit. Do not use 'O' rings as probably made of EDPM rubber and may deterioate.

Failing that, make a washer out of Hemp & use Boss White. I have been a C & G Plumber for 50 years and aint seen a hemp/ Boss White fail yet through the Hemp/Boss White.

Reply to
Bookworm

Taper threads taper and parallel threads remain parallel! Taper threads, when used with suitable sealers (usually ptfe these days) allow sealing to take place over a range of angles between one part of the fitting and another (ie you can move the joint while it remains sealed). They also don't require a mating face at all and are ideally suited to iron fittings (usually cast) that then don't have to be machined. The most likely place to find them today is the stubs for radiator valve fittings, which exit the radiator with no mating face, having a tapered thread and fitting into a (sometimes recessed) thread in the radiator. IMHO the most effective way to seal them is to wrap about 4 layers of ptfe tape on the thread and assemble, tightening a little. This will drive ptfe into the threads. I then disassemble, add another few layers and do up properly.

Parallel threaded fittings require two mating faces and a washer. Once tightness is achieved you are usually stuck with whatever angle it ends up at. They can be considered neater, however.

I had some problems withe the tails on my original shower pump, which seemed to require tapered fittngs and I was at a loss to undertand this. I believe the replacement pump was different (AFAICR).

Reply to
Bob Mannix

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.

Reply to
zymurgy

Hemp is not prohibited for use on water fittings by the Water Regulations.

Reply to
Aidan

Aidan wrote: rubbish

Should be

Reply to
Aidan

Aidan wrote: Hemp is prohibited for use on water fittings by the Water Regulations.

So is Boss White, which is only suitable for use on non-potable water systems (heating). For water fittings, including showers, you should use Boss Super Green, Fernox Water Hawk or similar.

Reply to
Aidan

Hi,

A few years ago, I posted a similar question about threads. Using Francis or fjk might pull up those threads. I came across some fascinating sites, one with a chart for mixing or not mixing the two types. If I find it again, I will post it.

Francis

Reply to
Francis

Nothing showed when I did an Advanced search on google groups using: thread (Francis OR fjk) group:uk.d-i-y

No great rush Francis: I'll keep looking.

Mungo

P.S. Thanks to all repliers.

Reply to
Mungo

Don't sweat it. I have 22mm isolators that my pump drags water through, then pumps out via a right angled 15mm chromed isolator in the shower pipework.

It has great pressure still, and it's not even set to max ;)

Cheers

Paul.

Reply to
zymurgy

What size pump Paul (are we comparing like with like)?

Mungo

Reply to
Mungo

Have you had a look at what BES do?:

cheers, Pete.

Reply to
Pete C

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.

Reply to
Doctor Drivel

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.

Reply to
CampinGazz

Gases are fluids, n*****ts.....

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.

Reply to
Andy Hall

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.)

Reply to
Matt

Matt, do not give advise on topics you know little.

Reply to
Doctor Drivel

Lord Hall, and?

Reply to
Doctor Drivel

HomeOwnersHub website is not affiliated with any of the manufacturers or service providers discussed here. All logos and trade names are the property of their respective owners.