We present a study of the solid-fluid phase equilibrium for flexible hard sphere site united atom models of n-alkanes using Monte Carlo computer simulation. We have considered models with different torsional potentials to examine the effect this has on the phase diagram. Extensive calculations of the fluid and solid phase equations of state have been made and solid phase free energies have also been determined. The initial solid phase structure used for each system tvas that which allows the chains to reach the highest density at close packing. The data for hard core chain models have been used as a reference system in a generalized van der Waals or mean field calculation of the n-alkane phase diagrams. This theory reproduces trends in the triple point temperature seen in experimental data. These trends are interpreted in terms of the changes in the close packed densities of the solids with chain length and the effect of the torsional energy on the relative stability of the fluid and solid phases.