Abstract: A rotary water blast nozzle assembly includes an inner body member and a mandrel that supports a spray head having an internal bushing that rotates on the mandrel. Seals on the of the bushing are spaced at selected distance points from radial flow ports in its center. The resultant of operating pressure forces acting inward on the outside of the bushing between the seals, and gradually diminishing pressures acting outwardly in the clearance from the center toward the ends of the bushing, together with frusto-conically shaped sections of the bushing, cause uniform inward deflection that reduces the clearance in proportion to the operating pressure to inhibit leakby.