A novel type of photonic crystal nanocavity nanocavity tailored to sensitively measure torques is theoretically investigated. Suspended low-mass elements (< pg) in the nanomechanical resonator are sensitive to environmental stimuli, such as a magnetic field from external sources or from embedded nanomagnetic systems. The torsional mechanical motion of these elements directly influences the optical field concentrated inside the optical nanocavity, resulting in a strong cavity optomechanical coupling rate up to 90 GHz/nm. The actuation of the mechanical resonator is readout with high sensitivity using evanescent coupling between the photonic crystal nanocavity and an optical fiber taper. A sub-100nm physical air gap in the middle of the nanobeam cavity allows torsional mechanical degrees of freedom as well as strong optical field confinement in a small mode volume. Numerical simulations show that high-Q ~ 10<sup>6</sup> optical cavities with a gap are possible. Potential applications incorporating these devices include sensitive magnetometry and probing the quantum properties of nanomagnetic systems.