Abstract: A rotary piston compressor with parallel internal axes has a driven external rotor (12) with a compression chamber (17) in which an internal rotor (20) is rotatably mounted. The internal rotor (20) is hollow, made of light metal, and mounted on a shaft (21). Perfect balancing of the masses of the internal rotor (20) is achieved by having metal pins (42,43) which extend the full length of the internal rotor (20). The heavy metal pin (43) also prevents rotation of the internal rotor (20) about the shaft (21) when these parts are not monolithic.