Valve outlets threads for flammable gases are usually left-hand threaded, i.e. turned anti-clockwise to close. Valve outlets threads for non-flammable gases are usually right-hand threaded, i.e. turned clockwise to close.
This convention helps to limit the inappropriate interchange of fittings between cylinders containing flammable gases and non-flammable gases.
- non-flammable gases such as oxygen, nitrogen, argon and air generally have right-hand valve outlet threads
- flammable gases such as acetylene, hydrogen, propane and mixtures containing fuel gas generally have left-hand valve outlet threads
This guide applies generally, but has some exceptions such as for some special cylinders of dissolved acetylene, specified for purposes other than welding and cutting, and some propane cylinders used on fork lift trucks, which may have right-hand thread valve outlets despite being in nominally flammable service.
The convention means that oxygen and fuel gas pressure regulators are usually not interchangeable. Spindle keys are interchangeable.