Installation forces play a central role in the design and performance of helical piles, especially since the installation torque is often used as an indicator of the pile’s ultimate capacity. This paper presents an analytical model for predicting the installation torque for single-helix piles in clay. As an extension of a recent study by the authors, the proposed model considers not only the forces occurring on the helical plates but also the shear stresses generated along the shaft, both of which impact the installation forces. The model yields a straightforward expression that relates installation torque to the undrained shear strength of the soil, embedment depth, helix diameter and pitch, shaft diameter, crowd (axial) force, and adhesion coefficient along the shaft. The influence of these factors on the installation torque, as well as the “capacity-to-torque ratio” used to infer capacity from the installation, is assessed through a sensitivity analysis. Some level of validation is provided through a comparison with empirical capacity-to-torque ratios, and the sensitivity analysis reveals factors that are neglected in empirical models but nevertheless have a significant influence.