Valve Threads

[NeilB]

I have these two 10l steel cylinders. They are years old but now I have a wing the mind obviously moves onto making a twin set. I am not sure what size valve I would need so I removed the valve from one cylinder. Taking a valve that had 3/4bsp stamped on it I screwed it in.....no problem. Then I took a new valve that I know is M25 and that also screwed all the way in!

Now I am confused....as far as I know a 3/4 bsp valve should not screw in where an M25 came from and vice versa. So what have I got? Is my 3/4 bsp valve incorrectly labled?

The cylinders are about 15 years old and marked Spiro......could they really be M25?

Your thoughts appreciated.

[JStop]

Originally Posted by NeilB

I have these two 10l steel cylinders. They are years old but now I have a wing the mind obviously moves onto making a twin set. I am not sure what size valve I would need so I removed the valve from one cylinder. Taking a valve that had 3/4bsp stamped on it I screwed it in.....no problem. Then I took a new valve that I know is M25 and that also screwed all the way in!

Now I am confused....as far as I know a 3/4 bsp valve should not screw in where an M25 came from and vice versa. So what have I got? Is my 3/4 bsp valve incorrectly labled?

The cylinders are about 15 years old and marked Spiro......could they really be M25?

Your thoughts appreciated.

At 15 yr old they are unlikely to be M25. If both fit I suggest you bin them, they will fail the next test on the Go/Nogo thread test. Unless of course you are using a dodgy test station.

What you have are dangerous cylinders

[Scuby]

If the thread isn't stamped on the cylinder (they usually are?), then I wouldn't use a different valve until you've checked it's the right fit. Don't think having a valve pop out the top of the cylinder as its being filled would be a fun experience! If you've got a friendly local cylinder testing place (ideally not a dive shop who will try to charge you £20 just to look at it) then they'll probably have a look for you and be able to tell you straight away what threads they are... and if they're still safe threads!

David

[NeilB]

Good idea Scuby....The club have just had 20 cylinders tested so we are on good terms with the testing centre. I will pop my cylinder down there.

The cylinder that I tested passed a hydrostatic test not 3 months ago so I am assuming the threads are ok. What I may have is a valve that someone has stamped with 3/4bsp when it is not, If I hold the two vales next to each other then the threads match. According to a tame service tech they should be completely different and not screw into the same cylinder.

I have had a look on the cylinder and cannot see anywhere where the thread size is stamped....is this a new idea?

Neil

[Prometheus]

Any half decent engineering shop will have thread guages to tell you what thread they are and wether they are worn.

[TeddyDiver]

Originally Posted by NeilB

What I may have is a valve that someone has stamped with 3/4bsp when it is not, If I hold the two vales next to each other then the threads match. According to a tame service tech they should be completely different and not screw into the same cylinder.
I have had a look on the cylinder and cannot see anywhere where the thread size is stamped....is this a new idea?
Neil

Take a M25 sylinder and try to fit your 3/4 valves to it. You propably find out they don't match bcs of a tiny difference in diameter. Opposite way they seem to fit but may shoot out under pressure..

[Padowan]

A 3/4 BSP nut (which is effectively what the cylinder thread is) is apparently 26.44mm at the thread troughs, and 24.66mm at the thread peaks - it also has 14 threads per inch (TPI) or a thread pitch of 1.814mm.

An M25 bolt is nominally 25mm across the peaks and has a 1.5mm thread pitch.

Therefore the M25 "bolt" will fit into the 3/4" BSP nut by diameter, but you would expect it to not thread easily as the pitches are about 0.3mm out. But if both components are slightly worn and used, it may well be possible to thread an M25 valve into a 3/4 BSP cylinder most of the way in before the thread interferrence stops it. But by this point you now have a valve that's threaded in, but only being held in by the tips of the threads.

Remember that valves are generally brass, and the cylinders are steel, so if anything's gonna wear it's likely the valves, with ally cylinders it'll likely be the cylinder wearing.

I heard a story from a friend who had soome work done on an ally stage cylinder and they noticed in the car, on the drive from the shop a hissing from the cylinder after it had been filled. They thought it was the 1st stage, but on inspection found that the gas was coming from the cylinder neck. They went back to the shop and only then was it discovered what had happened - it was exactly this problem.

I agree with Prometheus - take them to an engineering shop, and get them to measure everything. There are "Go/No-go" guages that allow engineers to determine thread sizes and whether they are within the acceptable tolerances - this will both answer your questions as to what size they are, and whether they are worn out.

Padowan

[NeilB]

Thanks Padowan.....I will take my vernier to the threads and pop them down to the testing station.....we don't have 'good' engineering shops out here!

[NeilB]

All the mystery is solved now......it was me being stupid

My cylinders are 3/4bsp. The valve I have that is stamped 3/4bsp is. The valve I thought was M25 isn't. I was miss-sold by the local shop.

I had my sons pony apart yesterday and that definately is M25 as it says so on the valve and cylinder neck. My supposed M25 valve would not fit - it was too big. His M25 valve fitted in my cylinder until it was 3/4 of the way in then the miss match of the threads stopped it going in further.

So now I am all sorted out......all I need is a manifolded pair of 3/4bsp valves.....if anyone knows of a supplier my wife will be in the UK in a couple of weeks and can pick them up for me.

[sgri300]

Hi Neil,
Beaver sports supply the valves your looking for. Drop me a pm and I'll send you what your after.

SG