Gas injection element for a system for distributing a gas inside an enclosure of a fluid catalytic cracking unit, said injection element comprising a through passage passing all the way through it, characterized in that it comprises: - an internal element of ceramic material, an inner surface of which defines said through passage over its entire length, said internal element having an outer surface opposite to said inner surface, - a hollow metal sheath having an inner surface, inside which is housed at least part of the internal element, the inner surface of the sheath being opposite the outer surface of the internal element, the inner surface of the sheath and the outer surface of the internal element having complementary shapes allowing displacement of the internal element relative to the sheath in a direction parallel to an axis of the through passage, said inner surface of the sheath and outer surface of the internal element being provided with fasteners (26, 36) cooperating to reversibly fasten the sheath and the internal element, wherein the outer surface of the internal element and the inner surface of the sheath are surfaces of revolution, in particular cylindrical, having an axis of revolution coincident with or parallel to the axis of the through passage, and the fastening elements comprise lugs or notches cooperating respectively with grooves or ribs of a suitable shape to allow the displacement of the sheath and of the internal element between a state of fastening in which these elements are joined together and a released state of these elements in which they can be separated from one another following translational movements in a direction parallel to the axis of revolution and rotational movements about the axis of revolution, the grooves are in the shape of an L, part of which extends parallel to the X axis from one end of the internal element, allowing insertion of the internal element into the sheath following a translation of axis X, the end of the groove bifurcating at right angles to allow rotation of the internal element relative to the sheath about the X axis and locking thereof. characterized in that it comprises: - an internal element of ceramic material, an inner surface of which defines said through passage over its entire length, said internal element having an outer surface opposite to said inner surface, - a hollow metal sheath having an inner surface, inside which is housed at least part of the internal element, the inner surface of the sheath being opposite the outer surface of the internal element, - an internal element of ceramic material, an inner surface of which defines said through passage over its entire length, said internal element having an outer surface opposite to said inner surface, - a hollow metal sheath having an inner surface, inside which is housed at least part of the internal element, the inner surface of the sheath being opposite the outer surface of the internal element, the inner surface of the sheath and the outer surface of the internal element having complementary shapes allowing displacement of the internal element relative to the sheath in a direction parallel to an axis of the through passage, said inner surface of the sheath and outer surface of the internal element being provided with fasteners (26, 36) cooperating to reversibly fasten the sheath and the internal element, wherein the outer surface of the internal element and the inner surface of the sheath are surfaces of revolution, in particular cylindrical, having an axis of revolution coincident with or parallel to the axis of the through passage, and the fastening elements comprise lugs or notches cooperating respectively with grooves or ribs of a suitable shape to allow the displacement of the sheath and of the internal element between a state of fastening in which these elements are joined together and a released state of these elements in which they can be separated from one another following translational movements in a direction parallel to the axis of revolution and rotational movements about the axis of revolution, the grooves are in the shape of an L, part of which extends parallel to the X axis from one end of the internal element, allowing insertion of the internal element into the sheath following a translation of axis X, the end of the groove bifurcating at right angles to allow rotation of the internal element relative to the sheath about the X axis and locking thereof.