Small LPG Fittings

I'm investigating various small LPG fittings, as used on things like blow lamps (and camping stoves etc) 'powered' from disposable gas canisters.
As far as I can tell, the thread used on most (all?) small 'tin' canisters with a threaded top is a 7/16 UNSF thread, with 28 TPI. (Taps for this thread are difficult to find or expensive, die are as rare as rocking horse poo.)
I assume the fine thread is used to secure a gas tight seal, perhaps through the use of the harder fitting and the softer canister threads. After all, the latter has a limited (planned) service life.
However, reading up on other fittings (BSP threads), which are widely used the thread pitch is far more coarse. Plus, the fittings tend to have a similar hardness. While various compounds etc. can be used to 'seal' the threads, I'm not convinced that accounts for the difference- especially if UNSF threads are used else where in gas systems ie besides canister connection.
(I appreciate that in compression fittings, it is the 'olive' which seals but I am not referring to those. )
Can any one shed some light on the topic, please?
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Brian Reay wrote:

I've not studied these gas cylinders and fittings but would have expected the sealing to be unrelated to the thread with the sealing done by a rubber seal of some sort and the thread to provide mechanical compression of that seal. Threads designed for sealing (generally at lower pressure) are tapered and jam with the aid of ptfe tape or similar.
Not knowing your application, I would be tempted to cannibalise an old appliance to get the correct fitting and then solder/braze onto an adaptor to a more readily available thread.
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Thank you Bob. I was thinking that, should I need to adapt things, using existing parts would be the best way. However, I was more curious than anything. Over the years I've had several torches and at least two seem to be useless due to the cylinders being either hard or difficult to get. My current main torch runs from a large bottle and I can simply change the fitting, should the one I currently use go obsolete (which I would think is unlikely).
I have just ordered a small, pencil, torch which is refillable. I see on YouTube some one has cut one in half and, using a plastic pipe!, runs it from one of those cheap canisters with a notched ring collar. He cut down something to use the connector and needle valve.
I like the concept put the plastic pipe seems a bit risky. Firstly it is vulnerable but there has been a lot of coverage in the motor home / caravan world about plasticisers leaching from pipes. I'm not sure how plastic reacts to LPG.
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On 28/10/15 22:12, Brian Reay wrote:

For fuel gases, some disposable canisters, the types used by plumbers, usually propane/mapgas rather than butane or butane/propane, also use 1" x 20tpi threads.
Both the 7/16 x 28tpi and 1" x 20 tpi threads do not seal at the threads, which only hold the "prong" in position. The "prong" in the adapter seals in a hole in the top of the thread, there is an O-ring inside the top of the can.
There are also the "puncture" type tins, I don't know which thread they use. They seal with an O-ring which is in the fitting rather than in the can.
There is also the type of disposable canister used for welding gases like argon or Ar/CO2 mix, with m10 x1 threads. Then there are the disposable oxygen canisters, which mostly have inbdividual manufacturer's choice of threads ..
-Peter Fairbrother

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Thank you Peter.
I was aware of the other cylinder types but not the threads they use. I've examined a few types I have here plus read a bit on the net.
The various types, at least for essentially the same role, are something of a mess. Rather like the various fittings found on the larger LPG cylinders used in motor homes and caravans across Europe. I ended up buying (well renting technically) a French cylinder and getting an adaptor to our POL system so I can get Propane in France if I need to. I chose one which is widely available in supermarkets in France.
On a different tack, I wonder if you have ever looked a domestic gas torches? I ask because I seem to recall that at school ( in my pupil days) the torches used for brazing used mains gas and air from a compressor.
I'm pretty sure it was mains gas for two reasons. One something a teacher said about the change to 'North Sea Gas' and the pipe work for the gas poker used to light the Coke fuelled furnace was common.
Of course, it was over 40 years ago so I could be totally wrong.
I assume the air was essential to compensate for the lower calorific content of domestic gas compared to propane. (I know you can need assistance with propane for larger work pieces.)
I don't envisage getting a domestic gas torch, I'm just curious. I only braze small items from time to time plus do the odd bit of heat treatment, with tempering in the oven as required.
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Brian
You are right about mains gas and compressed air being used for brazing. Coal gas and compressed air was used at the Technical High School I went to 55+ years ago.
I did investigate the use of mains gas and compressed air a few years ago and came to the conclusion it wasn't worth pursuing. Suitable torches didn't appear to be readily available and the possibility of annoying the Gas Board all made it a no go.
John H
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Thank you John, at least I know my memory still works :-)
I can imagine the 'gas people' getting all hot and bothered about such things in these days of health and safety gone made. Having said that, I don't do enough work with torches to warrant going to the lengths of such a system, I only do the odd bit of silver soldering and heat treatment.
I'm not sure of the relative calorific value of town gas vs that of natural gas, I expect it is on the internet somewhere. I recall when the switch was made the Gas Board visiting homes etc to modify appliances. We never had gas when I lived with my parents but I believe it was all modified for free. I assume the change was simple, just a 'jet' change, as they do when people go from using propane from large tanks to piped gas.
I think town gas was largely hydrogen and made by spraying water onto hot coal. The coal turn to 'Coke', which could be used in furnaces and as a 'smokeless' fuel. There was a large Coke works near the town I lived in as a youngster. The flame to burn off the gas was an impressive sight.
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