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G Thread?

G Thread?

I have a block with a G1/4 thread in it.  One person tells me this is a British standard thread.  Another tells me it is a American standard.  Could someone tell me what this G thread is so that I can get the correct fittings.


RE: G Thread?

I think its British straight pipe thread.

RE: G Thread?

Yes, its British. It's what the Europeans call a  1/4" BSP thread. BSP threads are slightly different from NPT threads - the TPI is not always the same, depending on diameter, and the thread apex angle is 55 degrees instead of 60 degrees.

RE: G Thread?

G (supposedly from German/English "Gas") are the cylindrical pipe threads used on originally inch based pipes. The internal (conical) pipe thread is is described as "GCon".
The size designation (like 3/8") does not mean actual thread OD; it is the internal diameter of the pipe, on which it is used. To be more exact - the original internal diameter, because the pipes became thinner (better material, probably) but their OD's remain, for the exchangability purposes obviously.
This thread is internationally standardized. As EM mentioned, apex angle is 55 deg.

RE: G Thread?

If you want it to seal the fitting needs an o-ring on the face.

RE: G Thread?

Yes, that's correct, although there is such a thing as a BSPT thread. I'm not sure what the european designation of those is. It was a little sloppy of me in an earlier post to roughly compare BSP's with NPT's, which are of course tapered.

RE: G Thread?

Please correct my previous posting:
The internal G thread is cylindrical; the external can be conical ("GCon")

RE: G Thread?

BSP threads can be straight or tapered. BSPT is tapered as far as I know.

They will seal without an "O"ring if PTFE tape is used or if a thread sealant like Locktite 515 is used.

Some sizes (1/2" for example)can be used on NTP fittings if you are a bit rough about quality. The will often tighten up about 3 turns before binding. The mixed thread joints always leak unless you use a good skived PTFE tape or Locktite.

It's not good practice, but it will get you home.


Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.

RE: G Thread?

G threads are identical with BSP. The "G" is from the German "Gewinde" meaning "Thread". They can be tapered or parallel, the parallel threads are sealed with an o ring, gasket or some times with a lock nut and sealant. Like all pipe threads the nominal dimension is the bore of the pipe, while the OD of the pipe is larger. eg 1/2" is about 22mm OD, 1" is 34mm etc. BSP threads mosly have a different pitch to NPT, a frustrating 1 tpi or even 1/2tpi. If necessary they can be mated successfully with the use of loctite 577 or similar.

RE: G Thread?

Well, It has always been my understanding that if you just say BSP, technically that is a straight thread, as is a G thread if it has no other designation but the size. If you want to specify a BSP tapered thread, then you should call it out as a BSPT. But I stand ready to be corrected.

RE: G Thread?

English Muffin:

I agree.  The "G" designation is used to denote a BSPP (British Standard Pipe Parallel) thread.  This is described in the Machinery's Handbook.  The tapered version is rather uncommon in Europe, and when used, is shown as a BSPT thread.  If a BSP thread is shown, it is generally understood to be the parallel thread.

Sealing is accomplished with a bonded washer.  A well-known trade name is the "Dowdy" washer.


RE: G Thread?

schlebb: I'm glad my memory is holding up. But actually, I think it's "Dowty" isn't it? (Aluminum with a rubber lip on the inside). But on Parker fittings I have also come across G thread (BSP) sealing which just used a standard "O" ring, and a plain metal ring outside the "O" ring to retain it, which amounts to much the same thing.

RE: G Thread?

The Gcon external thread combined with the G internal thread is used for water piping (zinc plated) in central Europe, as a sealant is used the tow (I hope the term is correct, I found it in a dictionary). For natural gas the steel pipes are used, the teflon tape is the sealant.
The most pipe thread cutting machines cut the Gcon thread automatically (I worked with them for a while).

RE: G Thread?

Well, it would seem we mostly agree that, to all intents and purposes, G = BSP (or BSPP), Gcon = BSPT. Correct ?

RE: G Thread?

         Just to show the weird places threads may lead - 'tow' is the stringy stuff associated with coaltar, used for rendering watertight the spaces between wooden planks in a boat's hull. Water-pipe joints are often rendered watertight with the same fibre, cut with a much longer fibre length, known as 'lead wool' or 'plumber's wool' Both tow and lead wool are products of that immensly versatile and useful plant, hemp.

RE: G Thread?

Thanks, yates!
Now I know what I used to seal the pipes. We also used some kind of plumbers paste with it.
English is my n-th language, what means that I do not know any language properly. But I see that we are talking about the same "stuff".
Thanks again, happy New year to you and to anybody who will read this words!

RE: G Thread?

   According to a recent Sutton Tools chart, 'G' is the ISO designation for BSPF (British Standard Pipe Fastening), which used to be BSPParallel. The tapered version, BSPT, is now internationally known to everyone (?!) as 'Rc'. Thank God for Standards.
   Is it certain that Gas threads are exactly the same as BSP? I haven't seen taps or dies labeled 'gas' for donkeys years, but I remember that they were in a seperate tin from the BSP stuff...

RE: G Thread?

I am pretty sure that in "BSPF" the "F" stands for Fuel. If you have an old SMC Pneumatics catalogue there is an excelent comparison chart near the front. I will happily email a scan of it to any one who wants it.

RE: G Thread?

Thanks everyone for your responses.  I now have the correct fittings.  I really appreciate your help!

RE: G Thread?

Can someone tell me how I can remove my original post?  I have the answers I need and it seems like it is a waste for others to keep posting answers.  Thanks for the help.

RE: G Thread?

Hasn't it occurred to you that others might be interested in the thread, either now or in the future? Someone else has just asked the same question :
Now all one has to do is refer them to your thread (which I did). If your thread were erased, that's what would waste people's time, since it would all have to be repeated!

RE: G Thread?

These threads have many names, depending upon the country of origin but most of them agree:

BSPP = ISO(G) = cylindrical pipe thread with 55°apex angle

BSPT = ISO(R) = conical pipe thread with 55° apex angle


RE: G Thread?

Hay, I'm from Belgium(Europe, maybe Brussels or Antwerp, says more to the most of you)
This is aal about pipe threads:
As far as I can check:
for example:
G 1/2 (for the female)
G 1/2 A (for the male thread add a tolerance class A(medium) or B(coarse))
DIN ISO 228 Part 1: cilindrical male & female with nominal dimensions
This parallel thread is not used with a seal on the thread(use a sealing washer & flange)
or with other words:
apex angle= 55³
or Pitch p=1,814mm
Thread Depth h=1,162mm
Pitch Dia d2=19,793mm
Root Dia d1=18,631mm
R 1/2 (for the male thread)
Rp 1/2 (for the female thread)
DIN 2999 DIN 3858
WhitWorth pipe thread with parallel female(Rp) & tapered male(R) threads
This parallel thread is not used with a seal on the thread(use a sealing washer & flange)
or with other words:
apex angle= 55³
or Pitch p=1,814mm
Thread Depth h=1,162mm
Pitch Dia d2=19,793mm
Root Dia d1=18,631mm
BSW-British Standard Whitworth   for example 1/2"BSW
BSF-British Standard Fine   for example 1/2"BSF
BSP-British Standard Pipe   for example 1/2"BSP
(if BSP-British Standard Pipe Parallel is used it is in fact the same as: BSPP  for example 1/2"BSPP)
BSPT-British Standard Pipe Tapered  for example 1/2"BSPT
are all British Standards
and are not defined in the DIN or ISO norms as named above.
And I think I do not have to say that:
NPSM-National Pipe Straight Mechanical
NPSF-National Pipe Straight Fuel
NPT-National Pipe Taper
NPTF-National Pipe Taper Fuel
threads are all American standards.
So, I hope I helped with this.
If some one knows more let us know.
By the way:In Europe we use steel pipes and copper pipes as well for water as for gas,
depending on the kind of connections....
and , yes we use "hennep" and a kind of paste, but also "teflon"...

Greetings from Belgium (Europe).

RE: G Thread?

Hay, again, i took some time to try to clear out this item.
It's quit complex.
Let's look at the "G" and "R" used in these thread designation.
By all means we are talking about a Whitworth shaped thread.
apex angle=55³
And it's about tubes or pipes, that's also clear.
Where comes this "R" from?
I think it comes from the German word for tube(or pipe): Rohr.
Where comes this "G" from?
I think it comes from the word Gas or it may come from the German word for thread: Gewinde

Designation "R" is used in:
DIN 259
Whitworth pipe threads: parallel internal and parallel external threads, basic sizes
and is linked (in this norm) to the earlier named: ISO 228
(in which the designation "G" is used)

Designation "R" is also used in:
DIN 2999
Pipe threads of Whitworth form: parallel internal and taper external threads

DIN 3858 is linked with DIN 2999 and is also about
Pipe threads of Whitworth form: parallel internal and taper external threads

I also found an old sheet in which a BSP was also designated with that "R" and
also with KR(BSPT):
It seems that "R" as well as "Rc" is used for BSPT in BS 21 (British Standard)

Designation "G" is used in:
BS 2779 for BSP (BSPF)

In this forum I noticed a link:
in which "G" is used for parallel Whitworth form threads: BSPP and BSPF
and where "R" is used for taper Whitworth form threads: BSPT

Look at this, different again(angle 60³ I think that this is a mistake):

Our question is not new, see this forum link:

And this is what i found also:
In March 2003 the British Standards Institute withdrew BS-2779 and replaced it
with ISO-228 (technically: BS-EN-ISO-228). The statement is that all requirements
for BS-2779 will now be filled with ISO-228.
So in the end, it's still not clear to me what is what.
The "G" en "R" are than used here, than used there, it's a kind of mixed up.

Greetings from Belgium (Europe).

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