Extendable bellows and compensators with internal metallic reinforcements, usable at low and high temperatures (from -160°C up to 1400°C). Flexible, extendable and impermeable. Mainly used in the coating of pneumatic pistons in steel mills, in the guide of machine tools, in the curtains of furnaces and machines with hot processes and in the fumes air ducts. Custom made, with round or square sections, they can be manufactured using different types of materials according to the application and the operating temperature of the structure. They include a fixing system.