A throttle body or actuator for an engine, comprising a main body which defines at least one supply duct having a through lumen for a gas mixture, a choke valve placed inside said supply duct so as to modify said through lumen. The choke valve is provided with an occluding body connected to a control shaft, at least partially housed in a seat in the main body. The control shaft, inside the seat and on the side of the occluding body, is provided with a first gasket adapted to prevent the penetration of dirt and gas from the supply duct. Downstream of the first gasket, on the side of the control shaft, at least one vent is provided, adapted to allow the expulsion, outside the seat, of any gas which has leaked through the first gasket.