Explain the importance of 'Groove Parallelism' in high-thrust retaining ring applications.

Groove parallelism refers to the alignment of the two walls of the retaining ring groove. In high-thrust applications, if the groove walls are not parallel (e.g., if the groove is 'V-shaped' due to a worn cutting tool), the ring will not be supported uniformly. This creates a 'toggling' effect where the ring tries to twist out of the groove under load. This concentration of force on one edge of the ring leads to localized yielding of the groove material and eventual failure. Aerospace standards often specify a parallelism tolerance of $0.001$ inches per inch of diameter to ensure the ring remains flat and fully seated against the load-bearing wall.