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"Gas" Threads

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Carl Wilson 411/01/2017 12:08:46
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I was having a discussion yesterday about so called gas threads. They are covered in a book I was shown. I've vaguely heard of them before and had always assumed they are Bsp. They are apparently found on some old British bikes, on fuel lines etc.

So. Are they the same as BSP or something slightly different? The data in the book lead me to think the same. Often used on brass fittings etc I'm told.
JasonB11/01/2017 12:18:45
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The table that I usually use for looking up odd threads has a "GAS(brass pipe) thread" form as well as BSP, though looking at it the tpi is about the same as NPT

Edited By JasonB on 11/01/2017 12:25:41

Carl Wilson 411/01/2017 12:41:26
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I was talking about this with my brother last night over whatsapp. He does motorbike restoration. He has a book of thread data from 1955. I asked him if the gas threads were anything like NPT as that was the first thing that occurred to me. Thanks for that.

Ajohnw11/01/2017 12:51:11
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Thanks Jason. Useful. I have a book with loads in but it's falling apart and a pc reference is useful anyway.

There will be a pdf version here for a month maybe. One of the wonders of Linux. Can be done straight from any browser.

**LINK**

I don't think there is any problem doing this providing the credits are left on the end. Not sure that freeserve exists any more either. The original doesn't.

John

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Clive Foster11/01/2017 13:12:37
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Careful. You can get in a right mess with oddball, but officially listed sizes for this sort of pipe fitting thing.

British Brass Thread (not British Standard Brass as there is no official standard) is 26 TPI on all sizes. Overall diameter is nominal size and tapped holes should be to 90% thread depth.

Gas threads go on British Brass Pipe sizes and are always little bit over nominal diameter. For example 1/4 has an actual OD of 0.260! 27 TPI on all nominal sizes from no 4, 0.246" actual diameter to 1" nominal, 1.006 actual diameter. 32 TPI on nominal sizes 0.148 and 0.196 which are also actual diameters, just to confuse folk.

Then there is copper tube thread covering nominal diameters from 1/8" to 4". 28 TPI on 1/8", 20 TPI from 1/4" to 1 1/2" in 1/8" intervals except for no 1 1/8" or 1 3/8" sizes. 16 TPI thereafter in 1/4" intervals.

All are Whitworth form but there are detail differences in fits and tolerances. Probably not enough for us to worry about except maybe the oversize on Gas threads.

I know of no book or reference which has them all in proper detail I had to scramble through three books although two would have done for this.

Whilst we are at it one more little ID nightmare courtesy of the inexpensive Asian import folk. I've run into a couple of air fittings using Admiralty thread sizes but using UN/Metric 60° form instead of Whitworth 55° to make life that bit harder.

Clive.

Harry Wilkes11/01/2017 14:40:26
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Posted by JasonB on 11/01/2017 12:18:45:

The table that I usually use for looking up odd threads has a "GAS(brass pipe) thread" form as well as BSP, though looking at it the tpi is about the same as NPT

Edited By JasonB on 11/01/2017 12:25:41

Good table Jason, I have a copy encapsulated in the workshop wouldn't be without it.

H

Carl Wilson 411/01/2017 14:51:15
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I don't need to know for practical purposes. I'm very familiar with Bsp, bspt and npt from hydraulic systems.

The gas thread seemed to share some similarities with npt being 27tpi...so what is the thread angle? I suspect 55 degrees or is it 60 degrees?
Georgineer11/01/2017 15:42:19
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Posted by Harry Wilkes on 11/01/2017 14:40:26:
Posted by JasonB on 11/01/2017 12:18:45:

The table that I usually use for looking up odd threads has a "GAS(brass pipe) thread" form as well as BSP, though looking at it the tpi is about the same as NPT

Edited By JasonB on 11/01/2017 12:25:41

Good table Jason, I have a copy encapsulated in the workshop wouldn't be without it.

H

That table is interesting, but needs to be handled with caution. It lacks the very important flank angles of the different thread series. Both history and personal experience say that threads with the same diameter and pitch but different flank angles cannot be predictably, or sometimes safely, mixed.

Just for starters there are series with 47.5°, 53°8', 55° and 60° flank angles and varying root and crest profiles in the table - I haven't looked for others.

That said, I have copied it into a spreadsheet and will edit it for my own purposes.

George

JasonB11/01/2017 15:48:32
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This is another site I keep bookmarked that has the actual angle etc detail of the various thread forms, some quite obscure ones in there.

JA11/01/2017 16:54:47
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Posted by Georgineer on 11/01/2017 15:42:19:
Posted by Harry Wilkes on 11/01/2017 14:40:26:
Posted by JasonB on 11/01/2017 12:18:45:

The table that I usually use for looking up odd threads has a "GAS(brass pipe) thread" form as well as BSP, though looking at it the tpi is about the same as NPT

Edited By JasonB on 11/01/2017 12:25:41

Good table Jason, I have a copy encapsulated in the workshop wouldn't be without it.

H

That table is interesting, but needs to be handled with caution. It lacks the very important flank angles of the different thread series. Both history and personal experience say that threads with the same diameter and pitch but different flank angles cannot be predictably, or sometimes safely, mixed.

Just for starters there are series with 47.5°, 53°8', 55° and 60° flank angles and varying root and crest profiles in the table - I haven't looked for others.

That said, I have copied it into a spreadsheet and will edit it for my own purposes.

George

The great "example" of mixing threads is 1/4"BSF and 1/4"Cycle. They are both 26tpi and can be mixed. However when mixed and subjected to vibration, as on a bike, they will loosen.

Also there is a Whitworth form brass thread which is 26tpi for quite a few diameters. It is not unknown for unscrupulous merchants to sell such taps and dies as Cycle.

JA

Clive Foster11/01/2017 17:39:44
864 forum posts
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Bad though the 1/4" BSF - Cycle mix is the 0 BA male in 1/4" BSF female is possibly worse. Such a pairing of decent quality components will tighten up to a seemingly adequate torque but the actual residual load carrying ability is pretty small. I've seen the fail demonstrated as part of an introductory How (Not) To Kill Yourself With Lifting Gear course devoted mostly to the sort of dangerous mistakes the unthinking and inexperienced can make. If you do the maths the load carrying ability of properly fitted bolts and similar threaded components is scary large to normal folk. Apparently you can hang grand piano of a single 1/4" bolt!

For the sake of completeness its worth mentioning that Cycle threads are 60° flank angle derived in the same manner as 55° Whitworth threads. Preferably made by rolling rather than cutting. The resulting thread form is different to 60° American so even though a US thread may mate, depending on tolerances and fit classes they may not actually go together sensibly despite being same TPI, the pair will not have the specified load carrying or tightening torque capability. If you include spoke sizes there are over 40 defined cycle threads, some rather application specific and obscure.

Clive.

Carl Wilson 411/01/2017 18:09:07
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I worked in an aircraft environment where 0Ba and 1/4 bsf kit was used on the same aircraft. In that world, measures are taken to avoid the human factors errors that would lead to a mistaken pairing.

47.5 degree into 55 degree doesn't bear thinking about.

So back to the original post. Nobody has yet answered what exactly is the gas thread and what is the thread angle?

Clive Foster11/01/2017 18:28:03
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Posted by Clive Foster on 11/01/2017 13:12:37:

Gas threads go on British Brass Pipe sizes and are always little bit over nominal diameter. For example 1/4 has an actual OD of 0.260! 27 TPI on all nominal sizes from no 4, 0.246" actual diameter to 1" nominal, 1.006 actual diameter. 32 TPI on nominal sizes 0.148 and 0.196 which are also actual diameters, just to confuse folk.

All are Whitworth form but there are detail differences in fits and tolerances.

Clive.

Source Newnes Engineers Handbook, second version, first (only?) edition, second printing 1964.

Brian Oldford11/01/2017 18:30:08
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I'm not able to give any further clarity to what is and what isn't a gas thread but have found the "3/8" x 26tpi" thread found at the root of brass bayonet lighting fittings has been changed by some manufacturers to 10mm x 1mm. Such threads often being a bit slopply will often engage . . . . . . . .but how well?

Nigel McBurney 111/01/2017 19:35:56
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British standard pipe threads and gas threads are essentially the same,both are whit form, in the above comments mention is made of NPT with 27 pitch X i/4 inch pipe, this has nothing to do with any English pipe threads,it is part of the American pipe thread system pitches and o/d are not the same as the BSP/ Gas thread system, I had to adapt a grease gun to pump oil and had to make a new nozzle ,I found the thread was 27 x 1/4 in NPT the gun which was very old probably came over here during the war or from the toolkit of an Americam built tractor, I have a number of NPT taps as I restore stationary engines ,the American built ones use NPT though as some have now been over here for over a 100 years the threads have sometimes been butchered with BSP taps to suit english pipework.Mention was also made about copper pipe thread,in my time I have come across it once , my neighbour had a problem with some pipe work on a TK bedford lorry and after a bit of head scratching and searching through Machineries Screwthread book I identified it was coper pipe thread.

Carl Wilson 411/01/2017 19:43:30
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I know very well that NPT has nothing to do with British Standard thread forms. I also have a number of NPT taps as well as BSPT and BSP. I work with the three systems every day. What no-one has yet been able to tell me is if gas threads are of Whitworth form and nothing to do with NPT then why does the data state that they are 27tpi?

JasonB11/01/2017 19:47:04
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Nigel, take a look down that first table I linked to and you will see 1/4" GAS, 1/2" Gas, 3/4" Gas etc all with 27tpi. As Clive also said.

These are nowhere near the same diameters as the BSP threads that have 27tpi on about two sizes

Edited By JasonB on 11/01/2017 19:47:32

Carl Wilson 411/01/2017 20:05:54
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According to the book I'm consulting Gas Threads (Brass Pipe Sizes) everything from nominal size 3/8 to nominl 1" is 27tpi.

Georgineer12/01/2017 01:05:42
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Posted by Carl Wilson 4 on 11/01/2017 20:05:54:

According to the book I'm consulting Gas Threads (Brass Pipe Sizes) everything from nominal size 3/8 to nominl 1" is 27tpi.

Carl,

Is your book of US or UK origin? I have just consulted my copy of Machinery's Encyclopedia (Published in New York in 1917) and it states:

Gas Fixture Threads. - Thin brass tubing is threaded with 27 threads per inch, irrespective of diameter. The so-called "ornament brass sizes" have 32 threads per inch. The standard sizes of the thread are 0.196 inch (large ornament brass size) and 0.148 inch (small ornament brass size). There then follows a table of "Gas Fixture Threads (Brass Pipe Sizes)" giving nominal diameters, actual diameters, and pitch. There are three nominal diameters given below 3/8", starting with No.4 (0.246" diameter) followed by 1/4" and 5/16".

I presume that these threads were used by the American manufacturers of gas fittings (lamps and the like). I also presume that the equivalent British manufacturers used the British Brass Pipe thread of 26 TPI.

The British Standard Pipe series is also listed, as is the American Briggs Standard for taper threads, ASME standard for parallel pipe threads, hose coupling threads...

It is interesting to note that the contents of the table JasonB linked to are very similar to the thread section of Machinery's Encyclopedia, including the obscure watchmakers' threads and the reference to CEI threads (which became the British Standard Cycle series at least as early as 1950). I suspect that the compiler of the table taddled most of it from the Encyclopedia or its close relative.

George

Edited By Georgineer on 12/01/2017 01:09:20

David Standing 112/01/2017 02:53:28
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Posted by Ajohnw on 11/01/2017 12:51:11:

Thanks Jason. Useful. I have a book with loads in but it's falling apart and a pc reference is useful anyway.

There will be a pdf version here for a month maybe. One of the wonders of Linux. Can be done straight from any browser.

**LINK**

I don't think there is any problem doing this providing the credits are left on the end. Not sure that freeserve exists any more either. The original doesn't.

John

-

John

Freeserve > Wanadoo > Orange > EE

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