Foundation support system shaft coupler fabrication and assembly

A modular foundation support system comprising:

• at least one shaft with opposing distal ends and a coupler extending on at least one of the opposing distal ends;
• wherein the coupler comprises a hollow body having a non-uniform wall thickness and a round inner surface;
• wherein the round inner surface includes:
• an open-ended first section having a first axial length, a first wall thickness, and a first uniform inner diameter extending along the first axial length, the first uniform inner diameter being sized to receive a distal end of a second shaft and the first uniform inner diameter including at least one fastener hole for connecting the distal end of the second shaft to the coupler;
• and
• a second section having a second axial length, the second section being formed with a seating surface for the distal end of the second shaft when the second shaft is received in the open-ended first section, the second section being formed with a second wall thickness that is different from the first wall thickness, the second section including a second uniform inner diameter that is smaller than the first unform inner diameter, and the second section having an axial length that is less than the first axial length.