The present invention relates to a sealing head for machines for heat treating threads, comprising a frame fixed to a pressurized chamber through which a conveyor belt passes, a pair of horizontal rollers placed on top of each other and pressed against the opposite faces of the conveyor belt and of seals to form a sealed closure between the rollers and the frame, at least one of the horizontal rollers and the seals being connected to the corresponding movement actuators. Sealing head characterized in that the actuators of the horizontal roller or rollers and the seals between the rollers and the frame are in the form of pneumatic actuators without pistons, namely of the balloon or bellows type.