If you use BOC oxygen cylinders with your O2 Dive kit please check that you regulator fits the cylinder correctly. BOC fit a cap/seal to the full cylinder which holds a cap or seal in place.

Some O2 regulators have a grip which overhangs the treads on the bullnose fitting.

This grip will not allow the bullnose fitting tread completely into the O2 cylinder.

You will need to remove the grey ring from the cylinder neck to achieve a proper connection between the regulator and the cylinder. Be sure to check the regulator fits the cylinder every time you replace the cylinder. This ring is not easy to remove in an emergency. Check the seal and contents regularly to insure the dive boats O2 supply is available when needed.