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 are coaxial, and a diameter (D3) of the third cylindrical portion is smaller than a diameter (D1) of the first cylindrical portion but larger than a diameter (D2) 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 (D3) of the third cylindrical portion is 3-7% smaller than the diameter (D1) of the first cylindrical portion and 3-7% larger than the diameter (D2) 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 are coaxial, and a diameter (D3) of the third cylindrical portion is smaller than a diameter (D1) of the first cylindrical portion but larger than a diameter (D2) 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 are coaxial, and a diameter (D3) of the third cylindrical portion is smaller than a diameter (D1) of the first cylindrical portion but larger than a diameter (D2) 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 (D3) of the third cylindrical portion is 3-7% smaller than the diameter (D1) of the first cylindrical portion and 3-7% larger than the diameter (D2) of the second cylindrical portion . <EOS>