The pressure regulator is threadable onto the neck of a tank of gas under pressure that has a plug mounted in the neck and a rupturable disk blocking fluid flow through a plug bore. The main body includes a central reduced diameter portion extendable into the plug bore without rupturing the disk when the regulator is threaded to the neck, but when further threaded, ruptures the disk. The combination of the main body and a cap have a bore extending therethrough which opens to the tank interior when the disk is ruptured and to the regulator outlet. A piston is mounted in the body and cap bore that blocks fluid flow through the last mentioned bore when the pressure at the outlet is at or above a preselected level but permits flow therethrough when the outlet pressure is below the preselected level.