A nozzle housing for a flapper servo valve, the nozzle housing comprising: a nozzle cavity for housing a nozzle, the nozzle cavity comprising a first open end and an opposing, second open end; and a flapper cavity for housing a flapper, wherein the first open end opens to the exterior of the nozzle housing and the second open end opens into the flapper cavity, and the nozzle cavity further comprises: a first cylindrical portion for receiving a nozzle inserted into the nozzle cavity from the first open end; a second cylindrical portion configured to hold a nozzle in its operating position; a third cylindrical portion positioned between the first and second cylindrical portions (12; 16) for centering a nozzle prior to positioning in the second cylindrical portion, wherein the first, second and third cylindrical portions (12, 14, 16) are coaxial, and a diameter of the third cylindrical portion is smaller than a diameter of the first cylindrical portion but larger than a diameter of the second cylindrical portion; a first transition portion connecting the first cylindrical portion to the third cylindrical portion; and a second transition portion connecting the third cylindrical portion to the second cylindrical portion; characterised in that the diameter of the third cylindrical portion is 3-7% smaller than the diameter of the first cylindrical portion and 3-7% larger than the diameter of the second cylindrical portion. a nozzle cavity for housing a nozzle, the nozzle cavity comprising a first open end and an opposing, second open end; and a flapper cavity for housing a flapper, wherein the first open end opens to the exterior of the nozzle housing and the second open end opens into the flapper cavity, and the nozzle cavity further comprises: a first cylindrical portion for receiving a nozzle inserted into the nozzle cavity from the first open end; a second cylindrical portion configured to hold a nozzle in its operating position; a third cylindrical portion positioned between the first and second cylindrical portions (12; 16) for centering a nozzle prior to positioning in the second cylindrical portion, wherein the first, second and third cylindrical portions (12, 14, 16) are coaxial, and a diameter of the third cylindrical portion is smaller than a diameter of the first cylindrical portion but larger than a diameter of the second cylindrical portion; a first transition portion connecting the first cylindrical portion to the third cylindrical portion; and a second transition portion connecting the third cylindrical portion to the second cylindrical portion; characterised in that a first cylindrical portion for receiving a nozzle inserted into the nozzle cavity from the first open end; a second cylindrical portion configured to hold a nozzle in its operating position; a third cylindrical portion positioned between the first and second cylindrical portions (12; 16) for centering a nozzle prior to positioning in the second cylindrical portion, wherein the first, second and third cylindrical portions (12, 14, 16) are coaxial, and a diameter of the third cylindrical portion is smaller than a diameter of the first cylindrical portion but larger than a diameter of the second cylindrical portion; a first transition portion connecting the first cylindrical portion to the third cylindrical portion; and a second transition portion connecting the third cylindrical portion to the second cylindrical portion; characterised in that the diameter of the third cylindrical portion is 3-7% smaller than the diameter of the first cylindrical portion and 3-7% larger than the diameter of the second cylindrical portion.