Cylinder threads?

[Zodiacdiverdave]

If it is a tapered thread then it is most likely a British Standard Pipe Tapered BSPT thread which has a thread pitch of 55 deg. vs NPT which has a pitch of 60 deg. You should be able to pick up fittings for your hydro test rig at any Parker Fittings Store.
ZDD

 

[iain/hsm]

Gentlemen.

I’m guessing better late than never. The three major taper versions of pillar valve used in Europe for diving cylinders.

1. As stated earlier the AGA (Interspiro) thread was a 19.8 mm taper but originally it was to DIN 477 That being a 18TPI on a 3 in 25 taper. Common in Germany.

2. We also had two other taper versions the most common for diving was also used in fire fighting cylinders. That being a .715 taper 14TPI and a 1 in 8 taper. To BS 341

3. The other taper thread was known as a Siebe thread from Siebe Gorman the original father of diving who had a 0.6 taper 14TPI and 1 in 8 taper.

I also recall a 1 in 5.625 taper

Being different to the American NPT thread we always use BSP (British Standard Pipe) thread. It’s a Whitworth form thread at 55 degrees generated at 90 degrees to the cone (not the axis as is with NPT) The “rounded” root and crest of the Whitworth thread is therefore also less sharp than the US NPT thread. The problem was all these threads were so close it was impossible to work out which was what without expensive tapered ring and plug gauges dive shop were reluctant to purchase.

On Parallel threads:
Our original parallel thread was also a Whitworth thread ¾” BSPP (British Standard Pipe Parallel) Then when into metric with the EU metric a M25x2 thread

Again the hard bit was working out “on sight” the difference between an American US ¾ inch NPSM thread and a ¾ BSPP or a M25x2 thread. Again the interesting bit with all these threads is they are impossible to tell on sight. However not for the first time has a dive shop dented there ceiling at sonic velocity when "Mix and Matching" threads LOL

To add to the confusion we have also used countersunk cylinder necks as well as the similar to US counter bored cylinders. But a fatter 0-ring gave a clue to the difference So confusion reigns supreme LOL Iain Middlebrook

 

[iain/hsm]

I guess I should also dispell this American misconception of NPT (national pipe thread) being used in older American diving cylinders. It’s not strictly a true NPT thread but in fact a National Gas Thread. NGT and it was the ½ inch NGT taper thread that was machined in the older steel divers cylinders from the likes of Pressed steel tank on the US divers, AMF Voit and Heathways cylinders that we used to see in Europe 30 years ago.

The NGT thread is much longer than the NPT thread that is typically used in pipe and tube fittings. The extra length on the thread in NGT gives it a "hand tight" capability before torque is applied.

Frankly to resolve all this confusion all these standards and threads are all wrong as they all should in fact be called Briggs Threads. In honour of Robert Briggs 1820 of the Pascal Iron Works who was I suppose the lead thread. Iain Middlebrook