A compound shaft coupling having a flexible disk shaft, with two flexible disks or diaphragms, and a tie bolt shaft connecting two rigid or stiff shafts. One flexible disk diaphragm of the flexible disk shaft is coupled with an interference fit to the first stiff shaft, while the other flexible disk diaphragm of the flexible disk shaft is coupled with an interference fit to the tie bolt shaft which removably mounts the second stiff shaft. A quill shaft connects the two flexible disk diaphragms of the flexible disk shaft. The first stiff shaft can be a hollow sleeve with a magnet mounted therein and the second stiff shaft or power head shaft may include a compressor wheel, a bearing rotor, and a turbine wheel removably mounted on the tie bolt shaft.