I keep getting asked what size O-ring is for such an such regulator and will this fit so and so etc etc...
What they really mean of cause is "which size O-Ring will fit my cylinder valve?".
There is a quick an easy way to size your O-Ring for your cylinder valve or Din fitting???
Take this 'O-Ring'
to measure this O-Ring take a ruler and measure across the gap in the middle from the inner wall to the oposite inner wall
(<--?-->) like this
This will give you a figure x?
Standard dive cylinder O-Rings come in two sizes BS111 and BS112
these are 10mm(BS111) and 12mm(BS112) respectively.
This is a rough guide so you can quickly measure which O-Ring you require.
Dive cylinder O-Rings are standardised to the valves and din fittings and only come in the two sizes.