A method of making an open-head power tong assembly includes the steps of providing a power tong assembly that has a first case assembly and a transmission assemble. A second case assembly is provided that includes a casing having a sidewall with a plurality of supporting ribs, a base wall, and a top plate that define an interior region that is adapted to receive a pinion gear, a conversion shaft having first and second axial end portions, and an open-head tong assembly. The first case assembly is removed from the transmission assembly of the power tong assembly to expose a transmission shaft. The transmission shaft is removed to expose a transmission shaft opening. The second case is mounted to the transmission assembly such that the second axial end of the conversion shaft is engaged with the transmission shaft opening.