We present an algorithm for accurate localization of block detectors in a positron emission tomography (PET) scanner. Accurate reconstruction of PET images requires precise knowledge of the physical position and orientation of the detectors. However, in some systems, block detector positioning and orientation can have relatively large tolerances, leading to implicit errors in the coincidence line-of-response (LOR) positioning. To compensate we utilize a rotating point source phantom where the rotational angle of the phantom is used to precisely determine the location of each scintillator crystal within a detector block. The aggregate block positions are then applied to the system model to determine the true location of each LOR. Images reconstructed with the more accurate LOR positioning demonstrate improved image fidelity.