Abstract: A one-piece, circular, power end seal for use in sealing gear boxes of heavy-duty reciprocating pumps. The power end seal generally includes a u-shaped, circular seal body having an open channel, a plurality of ribs tangentially mounted within the channel and a ring-shaped, fiber-filled dynamic seal being formed within an inner diameter wear surface of the seal body. The ribs generally provide the seal body with improved flexibility to maintain a constant and effective seal around connecting rods within the gear box, while at the same time retaining sufficient resilience to maintain static interference in a packing gland within the gear box. The inner diameter dynamic seal can be filled with PTFE, bronze filled PTFE, carbon filled PTFE or aramid fiber filled HNBR, which reduces heat build up and seal wear, while minimizing the radial squeeze of the seal in contact with the circumference of the connecting rod.