A turbine or turbine system includes a compressor, a combustor, a turbine section, a diffuser with a parting line between two components, and a seal along the parting line. The seal includes a protrusion, a recess, and a flexible portion between the protrusion and the recess. The flexible portion has a bent cross-section perpendicular to the parting line. An outer surface of the bent cross-section contacts an inner surface of the recess. An inner surface of the bent cross-section contacts an outer surface of the protrusion. Sealing a parting line between two parts of a diffuser in a gas turbine system includes disposing a flexible portion between a protrusion and a recess of the two parts, contacting an outer surface of the bent cross-section with an inner surface of the recess, contacting an inner surface of the bent cross-section with an outer surface of the protrusion, and pressurizing the diffuser.