Abstract: A flow shut-off valve shuts off natural gas if there is a break in a gas line, for example, the line to a water heater toppled by an earthquake. The shut-off valve is fitted into a standard pipe nipple that is installed in a vertical position. The shut-off valve includes a plastic float having a tapered nose with a O-ring in a groove at the base of the nose, a flow-restricting orifice region, and guide fins at the lower end of the float to guide it along the bore of a plastic body having a bore and shoulders at either end to contain the float. The body includes an conical aperture at the upper end into which the float nose and the O-ring are inserted when a break occurs and the downstream pressure drops, forcing the float upward as the gas flow surges. No spring is used; the balance of gravity and gas=flow forces controls the operation of the shut-off valve. The body is spin-welded from two pieces and crimped into position in the nipple.