An improved transition coupling for a helical soil pile assembly transfers a compression load between two coupled shaft segments with little or no compression loading on the bolts that fasten the parts together. The coupling body has a shaft-receiving socket that extends axially into the body from one end to a socket bottom that axially abuts the end of one of the shafts. The body also has at least one shoulder between its ends that extends laterally outward and faces toward the end remote from the socket. A cylindrical portion of the body, which fits closely within the hollow end of the other shaft, extends axially toward the socket end up to the shoulder, which is adapted to abut the end of that shaft. At least one pair of aligned transverse holes in the body is adapted to receive a fastener.