Sizing skin fittings

[Avocet]

Can anyone give me some pointers on how to specify skin fittings please? The usual "mushroom" type, with a male thread that pokes through the hull and engages with some female threaded thing on the inside. Ideally, I'd like a way of relating the diameter across the threads to an ordering size. For example, would a 3/8" one mean that it was 3/8" across the male thread? (presumably that's too easy)! Is it the bore?

 

[scottie]

See earlier thread on bsp thread sizes but yes it's bore size (almost)

 

[nigelmercier]

See earlier thread on bsp thread sizes but yes it's bore size (almost)

Why "almost"?

 

[prv]

Why "almost"?

Because it's actually based on the inner diameter of a Victorian malleable-iron gas pipe, on which the thread was cut on (I think) the outside. So in fact, due to the wall thickness of said pipe, the 1", 3/4", etc BSP size doesn't match any dimension now found on the fitting. I wouldn't even say "almost". The size is just an identifier; google BSP thread sizes to find one of various tables listing the actual dimensions for each size.

Pete

 

[VicS]

Can anyone give me some pointers on how to specify skin fittings please? The usual "mushroom" type, with a male thread that pokes through the hull and engages with some female threaded thing on the inside. Ideally, I'd like a way of relating the diameter across the threads to an ordering size. For example, would a 3/8" one mean that it was 3/8" across the male thread? (presumably that's too easy)! Is it the bore?

The BSP thread size will be the nominal bore of the fitting.

Because the wall thickness of the fitting may well be less than the wall thickness of the pipe on whose bore the BSP size was based the fittings will probably be a little larger in internal diameter than the BSP thread size.

 

[nigelmercier]

Because it's actually based on the inner diameter of a Victorian malleable-iron gas pipe, on which the thread was cut on (I think) the outside. So in fact, due to the wall thickness of said pipe, the 1", 3/4", etc BSP size doesn't match any dimension now found on the fitting...

OK, but it is still the bore size, albeit nominal.

 

[cryan]

The bsp size is the size of the thread not the bore of the pipe. There are several types of bsp threads. It can be straight, tapered etc. If you purchase a set of Zeus tables they will give you all the information you require.
http://www.amazon.co.uk/gp/aw/d/B0000CLZUO

 

[oldbilbo]

Beware the masked variations..... 'British Standard Pipe - Parallel' does not mate comfortably with 'British Standard Pipe - Tapered'. See recent thread on this ( unintentional pun! )

 

[vyv_cox]

One of the example DZR ball valves shown on my website has its bore cast into the body, as DN20, i.e. 20 mm. It was bought as a 3/4 inch valve.

 

[VicS]

The bsp size is the size of the thread not the bore of the pipe. There are several types of bsp threads. It can be straight, tapered etc. If you purchase a set of Zeus tables they will give you all the information you require.
http://www.amazon.co.uk/gp/aw/d/B0000CLZUO

If the bsp thread size is not historically the nominal bore of the pipe what is it?

Skin fittings which the OP is likely to buy will have a parallel thread.

Zeus gives basic details of the thread dimensions but it does not give information regarding the bore of fittings. The manufacturers' and suppliers' data should include this information.

Beware the masked variations..... 'British Standard Pipe - Parallel' does not mate comfortably with 'British Standard Pipe - Tapered'. See recent thread on this ( unintentional pun! )

Male threads may be tapered. I can't say I have ever seen a fitting with a tapered female bsp thread. I think you may well find that any valves etc which the OP buys to fit on to his skin fittings will have parallel threads. Lock nuts are useful to lock valves in the required orientation on the skin fittings

 

[PCUK]

Anyway, back to the question. If you order the bore size you want, to match whatever it is you are draining through the fitting, basin for instance, it will be near enough for your needs although as said it may not be a perfect match but it will do the job you want it to do.

 

[cryan]

If the bsp thread size is not historically the nominal bore of the pipe what is it?

Skin fittings which the OP is likely to buy will have a parallel thread.

Zeus gives basic details of the thread dimensions but it does not give information regarding the bore of fittings. The manufacturers' and suppliers' data should include this information.


Male threads may be tapered. I can't say I have ever seen a fitting with a tapered female bsp thread. I think you may well find that any valves etc which the OP buys to fit on to his skin fittings will have parallel threads. Lock nuts are useful to lock valves in the required orientation on the skin fittings

BSP is just the name given to the thread type and the size of it. It does not relate to the bore of the pipe as that could be any diameter smaller than the outside diameter of the pipe (O/D) You could have a 1/2" BSP fitting with a 2mm orifice through it or it could have 1/4" bore. You do get BSPT female fittings in fact if you try putting a straight thread into a tapered thread or vice versa you won't have a very strong connection at all as you are simply jamming the straight fitting into the taper and it will only be holding on a couple of threads. This is quite a common mistake and a common cause of failures under pressure.
The Zeus will give you the outside diameter of the thread which is what you need to know to make sure it fits in the hole.

 

[NormanS]

BSP is just the name given to the thread type and the size of it. It does not relate to the bore of the pipe as that could be any diameter smaller than the outside diameter of the pipe (O/D) You could have a 1/2" BSP fitting with a 2mm orifice through it or it could have 1/4" bore. You do get BSPT female fittings in fact if you try putting a straight thread into a tapered thread or vice versa you won't have a very strong connection at all as you are simply jamming the straight fitting into the taper and it will only be holding on a couple of threads. This is quite a common mistake and a common cause of failures under pressure.
The Zeus will give you the outside diameter of the thread which is what you need to know to make sure it fits in the hole.

That seems to be a totally unnecessary complication. BSP threads are indeed based on the Nominal Bore. Obviously the actual bore of the thing with the thread on it can be anything, but historically, and practically, that's how the thread is termed.

 

[VicS]

BSP is just the name given to the thread type and the size of it. It does not relate to the bore of the pipe as that could be any diameter smaller than the outside diameter of the pipe (O/D) You could have a 1/2" BSP fitting with a 2mm orifice through it or it could have 1/4" bore. You do get BSPT female fittings in fact if you try putting a straight thread into a tapered thread or vice versa you won't have a very strong connection at all as you are simply jamming the straight fitting into the taper and it will only be holding on a couple of threads. This is quite a common mistake and a common cause of failures under pressure.
The Zeus will give you the outside diameter of the thread which is what you need to know to make sure it fits in the hole.

I am afraid you are wrong.

The BSP thread size is, as others have already said, the nominal bore of the gas and water pipes it was historically used with.

See the headings in the tables below! ~~~ "Nominal bore of tube"

FWIW the entry in Wikipedia reads

Two types of threads are distinguished:
Parallel ('straight') threads, British Standard Pipe Parallel thread (BSPP; originally also known as[1] British Standard Pipe Fitting thread/BSPF and British Standard Pipe Mechanical thread/BSPM), which have a constant diameter; denoted by the letter G.
Taper threads, British Standard Pipe Taper thread (BSPT), whose diameter increases or decreases along the length of the thread; denoted by the letter R.
These can be combined into two types of joints:
Jointing threads: These are pipe threads where pressure-tightness is made through the mating of two threads together. They always use a taper male thread, but can have either parallel or taper female threads. (International standards require all female threads to be parallel.)
Longscrew threads: These are parallel pipe threads used where a pressure-tight joint is achieved by the compression of a soft material (such as an o-ring seal or a washer) between the end face of the male thread and a socket or nipple face, with the tightening of a backnut.

 

[nigelmercier]

BSP is just the name given to the thread type and the size of it. It does not relate to the bore of the pipe ....

The bsp size is the size of the thread not the bore of the pipe...

These statements are not correct.

A pipe with a given BSP thread will have a fixed thread size, but this measurement will NOT be the diameter of the thread, it is based on the bore of an iron pipe with about 1/8" walls. So in general, if you measure the diameter of the thread it will be about 1/4" more than the BSP value.

 

[prv]

BSP is just the name given to the thread type and the size of it. It does not relate to the bore of the pipe

The bsp size is the size of the thread not the bore of the pipe.

These statements are not correct.

I think you're misunderstanding each other. You're both right.

What Cryan is saying is that the size number refers to the thread, not the hole (if any - it could be a blanking plug!) in whatever the thread is on. This is correct. A 1" BSP nipple has a 1" BSP thread on it, regardless of the size of any holes. What he's not saying, I don't think, is that the thread will be one inch in diameter - you've put those words in his mouth. Yes, in most cases a 1" fitting will have a bore of about one inch - if it was right for Victorian gas pipes it's probably right for lots of other things too. But the official standard, which is metric, says nothing about pipe bores or inches - to a German plumber, the symbol 1" BSP simply means a thread with a certain form and a major diameter of 33.249mm.

Pete

 

[cryan]

I think you're misunderstanding each other. You're both right.

What Cryan is saying is that the size number refers to the thread, not the hole (if any - it could be a blanking plug!) in whatever the thread is on. This is correct. A 1" BSP nipple has a 1" BSP thread on it, regardless of the size of any holes. What he's not saying, I don't think, is that the thread will be one inch in diameter - you've put those words in his mouth. Yes, in most cases a 1" fitting will have a bore of about one inch - if it was right for Victorian gas pipes it's probably right for lots of other things too. But the official standard, which is metric, says nothing about pipe bores or inches - to a German plumber, the symbol 1" BSP simply means a thread with a certain form and a major diameter of 33.249mm.

Pete

Thanks,yes were confusing each other. what I'm talking about when I say, "Bore," I'm talking about the hole through the pipe which is what I thought the OP was asking.
ie, The hole through the middle of the pipe or fitting through which the fluid flows has no relation to the BSP size.

 

[prv]

what I'm talking about when I say, "Bore," I'm talking about the hole through the pipe which is what I thought the OP was asking.

Hmm. I don't think anybody was confused over that part. We all know what the bore of a pipe is.

And despite the fact that BSP is defined by the thread rather than the bore, people are quite right to point out that in practice, for normal skin fittings, the diameter of the bore in inches will be pretty close to the BSP size of the thread. I hadn't really thought about it before, but of course it's obviously the case because the standard was originally (though not any more) based on the bore of a pipe with a male thread, which is exactly what the business end of a skin fitting is.

Pete

 

[nigelmercier]

What Cryan is ... not saying, I don't think, is that the thread will be one inch in diameter - you've put those words in his mouth...

I don't think I am:

The bsp size is the size of the thread not the bore of the pipe...

The OP asked:

I'd like a way of relating the diameter across the threads to an ordering size.

 

[prv]

I don't think I am:

The bsp size is the size of the thread not the bore of the pipe.

Right, and he's not wrong in saying that. It may not be especially useful in the context of the OP's question, but that's thread drift for you.

The disconnect here is that "BSP size" does not mean "dimension in inches". When he says 'the BSP size [of a 1" BSP part] is the size of the thread', he is not saying 'the thread is 1 inch in diameter'. He's saying 'the thread is 33.249mm in diameter', expressed in a more convenient way that also captures the standardised form of the thread.

Unless I'm reading it wrong and he really does think that the thread on a 1" BSP part is an inch in diameter, in which case he's a moron.

The OP asked:

Ideally, I'd like a way of relating the diameter across the threads to an ordering size.

Right, and the answer to that specific question is "look up a table of BSP sizes, go down the 'major diameter' column until you find the value you measured, then run across to find out what BSP size that is". Or, assuming he can measure the bore instead, rely on the historical basis of the system and the relatively large gaps between sizes and say "it's the one closest to the bore size in inches". That'll work fine for skin fittings, but it's not official, and he shouldn't expect the measurement to be exact.

Pete