Abstract: A valve having a housing from which projects a stem rotatable about an axis has an actuating assembly with an abutment ring fixed on the housing, formed with an annular slot, and having an abutment projecting radially into the slot. A knob mounted on and rotatable with the stem adjacent the ring carries a single-arm abutment lever pivoted on the knob and having an end engaged in the slot and displaceable radially between a radial outer position angularly engageable with the abutment and a radial inner position so that when the end is in the outer position rotation of the knob is blocked by engagement of the lever end and the abutment. A spring braced between the knob and lever urges the end into the outer position. A radially displaceable button on the knob engageable with the lever is radially inwardly displaceable to move the lever into its inner position.