Abstract: The invention concerns pneumatic actuators for valves comprising a body closed by head flanges, two pistons moving in said body in opposite directions, an output shaft that turns actuated by the pistons, and a stop device on each head flange to limit the stroke of the pistons when they move away from each other towards the head flanges. On at least one side of the actuator body an adjusting device (22) is provided which includes stop means (26, 27) at varying distances from each other in a direction parallel to the movement of the pistons and arranged in a position to intercept and limit the stroke of the pistons when the latter move towards each other.