Abstract: An engine exhaust gas system includes a conduit for transferring exhaust gas, and a butterfly valve within the conduit for controlling communication of exhaust gas through the conduit. The butterfly valve is mounted on a shaft for pivoting relative to the conduit. The shaft is supported on a bushing in a transverse bore. Leakage through the clearance between the bushing and the shaft acts upon the transverse face of a washer which is freely slidable on the shaft, urging the latter into engagement with a retaining ring carried on the housing to effect sealing of the clearance. As the pressure of exhaust gases increases, the washer is urged more forcefully against the retaining ring, thereby improving the effectiveness of the seal.