An analytical model is developed to describe the torsional responses of the chain drive and crankshaft. The model is used to analyze system equilibrium, free vibration and forced responses. The equations of motion are linearized about the equilibrium to determine natural frequencies and mode shapes of the torsional modes. The forced responses of the system are investigated by including the excitations of valvetrain torque, gas combustion forces and inertia torques induced by the reciprocating motions of the piston and connecting rod.For an example engine considered, the free vibration analyses show the natural frequencies and the associated mode shapes. The forced responses display the resonant amplitudes of the system excited by the torques/forces. Experimental measurements from a test engine are used to confirm the theoretical predictions. The experimental measurements of the system's natural frequencies and the forced amplitudes are in good agreement with the theoretical predictions.