The invention discloses a single crank used for an internal combustion engine or a compressor crank shaft, which comprises a single crank main journal and a crank pin, wherein the crank pin is vertical to the single crank main journal and extends out, the axial line of the crank pin is offset relative to the axial line of the single crank main journal, the front end of the crank pin is tapered, at least one first positioning half hole is provided on the circumference of the end face of the tapered end, and the first positioning half hole can match with a second positioning half hole on a crank to form a positioning pin hole. The invention discloses the crank used for the internal combustion engine or the compressor crank shaft at the same time, which comprises a crank main journal provided with a taper hole matched with the tapered end of the crank pin, wherein the taper hole is offset relative to the axial line of the crank main journal; the inner diameter surface of the taper hole is provided with a second positioning half hole corresponding to a first positioning half hole which is formed on the single crank and is matched with the second positioning half hole, and the second positioning half hole can form a complete positioning pin hole after the single crank is assembled with the crank. The crank and the single crank are assembled together to obtain an assembled crank shaft which can be applied to a crank round slider type internal combustion engine or an air compressor.