The injector is mechanically retained in the molded composite socket member by an annular cap that is threaded onto the end of the socket member containing the end of the through-bore into which the injector was inserted. The sidewall of the cap contains an internal helical thread that threads to an external helical thread on the exterior of the socket member. The cap stiffens the socket wall at the thread to strengthen the socket wall against circumferential expansion caused by the pressure of fuel in an annular space that surrounds the injector interior of the socket through-bore. A method for making the cap and joining it to the socket member is also disclosed.