A coil assembly includes an electric coiling included in an overmold, the coiling being suitable to drive the control valve of a fuel injector. The coil assembly is provided with a longitudinal bore suitable for receiving a spring and with an adjusting member for the setting of the spring which can be moved from outside the injector, the adjusting member forming a transfer of movement such that the movement of the adjusting member from outside the injector is transformed into a compression of the spring.