A precision fuel injector having an inwardly opening pressure operated valve positioned in a bore forming a valve chamber and guided for reciprocal movement to open and close the valve wherein a threaded stepped cylindrical valve guide sleeve at the end of the valve chamber opposite the valve tip mounts the valve and cooperates with a mating portion of the bore to form an annulus filled with an initially flowable settable sealant. The sealant is pressurized during assembly of the guide sleeve and is forced into the clearance between the threads of the guide and the threads of the bore on both sides of the annulus to increase the length of the seal and to lock the guide sleeve in place. The blind end of the guide sleeve bottoms against a shoulder in the bore to establish a predetermined minimum cross-section of the annulus to minimize the possibility of fracture of the sealant ring therein due to differential expansion, and a pilot extending from the end of the guide sleeve and closely fitted within the surrounding bore protects the sealant against corrosion by the repetitive high impact hydraulic forces within the valve chamber.