DIN Scuba Fittings
This page was developed for the uk.rec.scuba website here but I will keep this copy current so I can fine tune it. (O-rings) (Threads)
Nigel's home page.
Picture from Kevin at FISHinthecity Most scuba divers are familiar with the A-clamp or International fitting. Originally designed for lower pressures this has been strengthened for up to 232bar and is still widely used. It has certain disadvantages and is progressively being superseded by DIN especially in technical diving circles.
300 bar DIN reg This is a 300bar DIN regulator. Notice that on the A-clamp the O-ring was on the cylinder and is often visible when the regulator is assembled to the tank valve here it is not only on the regulator but it ends up buried inside the system and is hence captive and protected in its working position. The thread is actually a 5/8" BSP as the DIN standard is based on what was used and most dive gear was BSP even my Russian stuff.
232bar DIN valve 232bar DIN valve 232 bar DIN - 5 thread
Notice 1 inch (25mm) front to back outside dimension and the dimple for the A-clamp's screw. When I bought these they were sold as A-clamps by being supplied with the slug (see below) factory installed. I used them as A-clamps for my first couple of years diving.
If you screw a 300bar DIN regulator in here it winds down the 5 threads and then compresses the O-ring and is ready to dive. Also notice the quite wide but shallow hole at the bottom of the threaded section.
300bar DIN valve 300bar DIN valve 300 bar DIN - 7 thread
Notice 1.4" (35mm) depth and no dimple so an A-clamp frame just will not fit (Yes I know the bull bars are in the way too).
If you screw a 232bar regulator in here it is too deep for the O-ring to tighten against the bottom and it will not seal so there is no danger of blowing your 232bar regs up on 300bar pressure. Also notice that the hole at the bottom of the threaded section is narrower than the 232 bar DIN - what is hard to see in a picture is that it is deeper.
232bar DIN 300bar DIN valve More differences. On the left a 232bar end, notice the diameter of the central piece is larger then the 300bar end on the right. The 300 will fit either. These are regulator inputs coded to prevent a 232bar only fitting being placed on a 300bar tank.
300bar DIN whip end And you remember I pointed out the hole at the back of the DIN pillar valve? Well this is a 300bar DIN filling adapter. It has a tube sticking out that goes into that hole but will not fit the 232bar pillar valve. This is done to try and stop you overfilling a tank way beyond its rated pressure.
By using long or short, wide or narrow ends and setting the number of threads on the fitting you can include or exclude any combination of fittings so the two sizes can be used together with safety. eg: Most regs are usable on 232 or 300 bar so they have a 7 turn body and a short, narrow core and fit both. If it was a 232 only reg it would only have a 5 thread body.
Slub Adapter The 232bar DIN slug to screw in and convert DIN to A-clamp. It has a captive DIN O-ring at the other end and this end has one to seat the A-clamp.
If you screw it into a 300bar DIN fitting it just keeps on going down before it seals down and is lost in a hole where the A-clamp cannot reach.
The DIN to A adapter with 1.1" (28mm) max opening, this one's by Apeks.
A-DIN adapter Um... We don't talk about this one. It's a 300bar DIN to A-clamp converter and I use it for filling at places where I can only get on an A-clamp pump. eg: my club who pump clean air but only 232bar.
It would allow you to put an A-clamp reg on a 300bar tank but if the extra stress broke it we would all say "What a fool. We do hope he's alright" as they drag you off to A+E.
DIN nitrox DIN Nitrox This was the wonderful Euro Nitrox valve that was supposed to take over from DIN for Nitrox mixes. I only ever saw it in Germany on Draeger equipment but who knows... Personally I didn't like the external thread. It just looked too vulnerable.
However in the couple of years since I first put this page together (04/2003) I only ever came across one of them and that was being swapped out. Sadly the EU came up with the M26 body which looks like a standard DIN but is slightly bigger thread as the new Nitrox standard. This is only a recomendation but some places are trying to force it on us. I'm not impressed as I see it as fixing a problem that does not exist.
Inline valve Thing Finally two specials. On the left:
Hands up the guy who said you could probably convert anything. It is a 232 bar tank but a 7 turn 300bar fitting. Put a slug in that and fit an A-clamp.

On the right....
Well... What do you think that is for?



Threads and O-rings
I wanted to list the thread names we use on Scuba gear as I can never remember them.
O-rings are identified by their 'dash number' and ID/CS dimensions. (Internal Diameter/Cross Section)
Notice that the DIN and A-clamp O-rings are listed as different but a DIN ring seems to work in every A-clamp reg I see. This is not true the other way round. They seem to seal OK but fall out at every possible opportunity. I think everybody uses the DIN size and thinks they are the same.

What I call itWhat it is  O-ring
Standard low pressure port3/8” UNF-011 .301” .070”
The Apeks bigger version1/2” UNF-013 .426” .070”
Standard high pressure port  7/16” UNF-012 .364” .070”
Standard 'DIN' thread5/8” 14TPI BSP-112 .487” .103”
New funny EU nitrox thingM26x2Apparantly as DIN
A clamp-014 .489” .070”
HP swivels-003 .056” .060”
2nd stage connection9/16” UNF-010 .239” .070”
Compressed air pipe fittings
G: Series British Standard Pipe - Parallel (Straight) BSP or BSPF
Also referred to as British Gas, British Pipe Parallel or Parallel Fastening thread.

Rc: Series British Standard Pipe - Taper- BSPT
Also referred to as British Standard Taper Pipe or Pipe Taper, or Conical Thread.
Taper is 3/4” taper per foot (1 in 16 on the diameter)

Of course the size refers to the o/d of pipe so the fittings are much bigger.
The pictures are of a 1/4” thread. The major and minor diameters are close enough to get an identification - well - provided you're sure it can't be the American NTP which is horribly similar.
Inside 1/4” thread Outside 1/4” thread
Nominal Size (inches)T P IMajor Diameter (inches)Minor Diameter (inches)
1/16”280.3040.2583
1/8”280.3830.3372
1/4”190.5180.4546
3/8”190.6560.5886
1/2”140.8250.7336
5/8”  (DIN)140.9020.8106
3/4”141.0410.9496
7/8”141.1891.0976
1”111.3091.1926

Spotting the taper, to know if it's BSP or BSPT, can be problematic but the calipers can help.
1/4” taper thread

Comments/complaints/pointing out obvious errors to nigelh@nigelhewitt co uk

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by Nigel Hewitt