A turbocharger includes a thin-walled insert installed in the bore of the turbine housing. The insert is generally ring-shaped and is radially resilient. The insert can be made of sheet metal formed into a ring shape, and in some embodiments is a split ring. A piston is axially slidable in the bore for varying the size of the turbine nozzle. The outer surface of the piston slides against the inner surface of the insert, and in preferred embodiments a piston ring that encircles the piston engages the inner surface of the insert to substantially seal the interface between the insert and the piston.
FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT
 This invention was made with United States Government support under DOE Contract No. DE-FC26-06NT42873 awarded by U.S. Department of Energy. The United States Government has certain rights in the invention.