Fuel injectors are subjected to extremely rapid movements due to the necessity of spraying the fuel in the combustion chamber in less than a millisecond. A multibody model has been developed using RecurDyn to study the high-speed dynamical behavior of the injector during a typical working cycle. The model contains both rigid and flexible bodies: the pin is modelled using the proprietary Full Flex technology which includes a finite element body in the dynamical simulation. This choice allows to evaluate the influence of the flexibility on the body motion and to calculate the stress on the component. The spring is modelled using beam elements to correctly represent the inertial properties and the contact between the coils. All these elements create a reliable numerical model which can be used to evaluate the position of the bodies and the contact forces between the pin and the housing in order to predict the performances of the injector.