Injector having structure capable of reducing required insertion force and assembly method thereof

A method of making an injector system, the method comprising:

• providing a first injector and a second injector which are to be mounted to a single intake port having insertion holes, and inserted into respective injector insertion holes, wherein an O-ring and a back-up ring are mounted to each of the first and second injectors, and wherein an axial length of the back-up ring mounted to the first injector is greater than an axial length of the back-up ring mounted to the second injector;
• simultaneously inserting the first injector and the second injector into the respective injector insertion holes such that:
• at a first assembly operation time point, the O-ring mounted to the first injector starts to be compressed,
• at a second assembly operation time point, the compression of the O-ring mounted to the first injector is completed and the O-ring mounted to the second injector starts to be compressed, and
• at a third assembly operation time point, the compression of the O-ring mounted to the second injector is completed.