A seal system for closing the running clearance between two relatively rotating structural members, wherein an elastomeric annular packing ring has a first annular portion pressed into an undersized annular locking channel, formed by and within one structural member, so as to retain the ring immovably therein, and thus to distend the remaining second annular portion of the ring, which is now dynamically and internally biased, across an open working chamber into impingement upon the smooth annular sealing surface of the opposing structural member to achieve an activated seal which is operable over large relative excursions of the structural members.
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