Modular fuel injector having interchangeable armature assemblies, and method of assembling the same

A fuel injector (100) for use with an internal combustion engine. The fuel injector (100) comprises a valve group subassembly (200) and a coil group subassembly (300). The valve group subassembly (200) includes a tube assembly having a longitudinal axis that extends between a first end (200A) and a second end (200B); a seat (250) that is secured at the second end (200B) of the tube assembly and that defines an opening; an armature assembly (260) that is disposed within the tube assembly; a spring (270) that biases the armature assembly (260) toward the seat (250); an adjusting tube (281) that is disposed in the tube assembly and that engages the spring (270) for adjusting a biasing force of the spring (270); a filter (284A) that is located at least within the tube assembly and may include an integral retaining portion (283). The coil group subassembly (300) includes a solenoid coil (310) that is operable to displace the armature assembly (260) with respect to the seat (250). The armature assembly (260) includes a sealing portion (264), a magnetic portion (262) and optionally an armature tube (266) in between.