As all divers know, you can never have enough O-Rings in your save a dive kit. Don't ever let another dive be ruined by an o-ring by keeping these high quality cylinder tank o-rings to hand. 

The BS112 o-ring fits standard DIN inserts (both front and back) as used in almost all modern day cylinders. This is an N90 o-ring, which is much more durable than the N50 or N70 o-rings, which are sometimes used, but have a much higher risk of bursting.