Tapping for a tapered thread.

[DaveL485]

I need to tap an alloy manifold to take an injector. The injector has a thread on its collar but it's a tapered thread...as in it gets wider on the way up the thread. Pic should show this.



Googling seems to return results for tapered taps (see below)



This is the style I have now for normal holes....the tap tapers but the thread it creates does not, it's kind of a nose-in to the hole.

Does anyone know 1)The proper name for the tapered thread (Unless its just tapered thread) and 2)Do I need a special type of tap to thread the hole in the manifold? I can't see it going in to a standard straight thread properly, tbh. Also I dont want to chemical metal them in as it's under pressure (Turbo) and I might want them out again at some point.

Thanks in advance.

[skene]

It is just simply called a tapered thread AFAIK, and yes you will need tapered taps to be able to cut the thread, not sure on where to get them though

[Patrick Bateman]

You'd need to know what kind of thread it was.

NPT, BSP, BSF etc.

[DaveL485]

Patrick Bateman said:

You'd need to know what kind of thread it was.

NPT, BSP, BSF etc.

Yes, its definitely not Metric, but I can find out the thread with a quick visit to Pirtek or similar.

[MX7]

That almost looks like a stepped thread. Could you not tap it out slightly larger afterwards?

[thinfourth2]

1/8 NPT

Maybe 1/4 NPT

[Patrick Bateman]

The tapered threads can be extremely similar so I'd get a proper check on it to make sure.

[sfaulds]

Try here

[cahami]

BSTP taps are tapered for pipe fitting,to identify the size maybee take it into a decent plumbers merchant and try it in a few fittings, You may even find they have a bush with tapered internal thread and parallel external thread. If you want to purchase tapered tap try here
http://www.threadtools.com/Departments/Taps/BSP--B...

[BliarOut]

Those taps in your first pic are sometimes called first second and third as that's the order you use them to make a normal blind tapped hole by hand.

[Bricol]

You actually want a parallel thread to screw that fitting into. Pipe fittings taper male threads seal on the thread form when screwed into a female parallel thread - the addition of suitable thread sealant is a good idea though. Sealant also helps prevent the thread galling. If you insist on using PTFE tape, don't cover the first few threads - then when you screw it in and cut the tape to ribbons, there's less chance of the bits dropping into whatever you are screwing it into - and a fuel or oil system is not where you wants bits floating around in.

Parallel male threads would screw into a female tapered thread - BSP and NPT taps are available to do this - care to be taken as if you go too far, the thread form is too big. Again the sealing is between thread forms.

Parallel male pipe threads screwed into parallel female threads will use something else to seal - either bottoming on a seal in the base of the female thread, or have a flange on the male threaded fitting to seal against an o-ring or dowty washer.

Taper male into taper female doesn't work - you can't turn it properly to screw it in from where it lands when you drop it in.

If you want to get really technical, there are different classes of thread form fit and tolerance to exactly specify what you want - but then you will get supplies making your parts ringing up to ask about it - whereas simply putting "tap XY BSP" on a drawing gets you what you want close enough most of the time. Only once in 15 yrs have I had a supplier ring up and ask me what fit/tolerance I wanted on the thread.

Here endeth the lesson . . . for now ;-)

Bri

[Patrick Bateman]

How on earth would a parallel thread fit a tapered one?

[wobert]

Patrick Bateman said:

How on earth would a parallel thread fit a tapered one?

Quite easily is the answer. if the thread size, pitch are the same then the threads will match until the taper is taken up creating a mechanical seal.

In engineering it is most usual to screw a tapered plug into a standard parallel threaded hole!

Regarding standard threads, it could be BSPT, NPT or NPTF.

HTH

Robert

Professional Engineer

[aww999]

I assume the deformation of the male (or female) thread in that instance is what makes the seal. Whilst not wishing to argue with the chap above (who seems to know his stuff), I have had success in the past using tapered taps to suit tapered threads. A lot of automotive stuff tends to be NPT, either 1/8th for the small parts like sensors, or 1/4 for the larger ones.

[Patrick Bateman]

wobert said:

Quite easily is the answer. if the thread size, pitch are the same then the threads will match until the taper is taken up creating a mechanical seal.

In engineering it is most usual to screw a tapered plug into a standard parallel threaded hole!

Regarding standard threads, it could be BSPT, NPT or NPTF.

HTH

Robert

Professional Engineer

I was under the impression it was a totally different thread which got me wondering what the crack was there.

Saying that, I work for an oil company and the majority of our fittings are Male NPT to Female NPT, haven't heard of, never mind seen, a parallel thread going into a tapered thread.

[wobert]

aww999 said:

I assume the deformation of the male (or female) thread in that instance is what makes the seal. Whilst not wishing to argue with the chap above (who seems to know his stuff), I have had success in the past using tapered taps to suit tapered threads. A lot of automotive stuff tends to be NPT, either 1/8th for the small parts like sensors, or 1/4 for the larger ones.

Correct, once the taper engages the micro deformation of the thread effects the seal.

[DaveL485]

Thanks everyone- very educational.

Whats posted above has led me here- http://en.wikipedia.org/wiki/National_pipe_thread

I'm pretty sure what I have is a 1/8th NPT thread, which in turn has shown up a few taps on eBay and the like....although the point about using a straight thread is interesting. Maybe this will be more appropriate in a soft material like the alloy manifold i'll be fitting to.

I'll have a ring around the various companies now I can sound like I half a clue what im on about and see what they recommend (Straight or tapered hole). In case you haven't guessed already, it's a Nitrous Oxide fogger nozzle, one of which will land in each of 4 inlet branches

[BliarOut]

1/8 npt is also a brake thread IIRC.

[wobert]

DaveL485 said:

Thanks everyone- very educational.

Whats posted above has led me here- http://en.wikipedia.org/wiki/National_pipe_thread

I'm pretty sure what I have is a 1/8th NPT thread, which in turn has shown up a few taps on eBay and the like....although the point about using a straight thread is interesting. Maybe this will be more appropriate in a soft material like the alloy manifold i'll be fitting to.

I'll have a ring around the various companies now I can sound like I half a clue what im on about and see what they recommend (Straight or tapered hole). In case you haven't guessed already, it's a Nitrous Oxide fogger nozzle, one of which will land in each of 4 inlet branches

Yep, steel adapter into aluminium manifold should give a nice mechanical seal!

The anodised adapter looks like JIC tapered on the left hand side to NPT

[AdeTuono]

Ignore the bks about screwing a taper fitting into a parallel thread. It will fit, but it's not correct, at best, and lethal at worst.

What you have is probably NPT, and more than likely 1/8", but with nothing to compare it with....

You need a tap like this...



notice the taper......

We work in the oil industry, and use NPT all the time. For high pressure applications, DO NOT use in a parallel thread. It won't seal, and will more likely than not blow out. Used properly, an NPT thread is good for 10,000psi.

Note that you can also screw a BSPT thread into NPT, but there is a slight difference in thread pitch, and you really shouldn't.