A turbine and variable-vane assembly for a turbocharger, comprising: a turbine assembly comprising a turbine housing and a turbine wheel mounted in the turbine housing and connected to a rotatable shaft for rotation therewith, the turbine housing defining a chamber surrounding the turbine wheel for receiving exhaust gas from an engine and for supplying the exhaust gas to the turbine wheel, the turbine assembly defining a nozzle leading from the chamber generally radially inwardly to the turbine wheel, the turbine housing further defining an axially extending bore through which exhaust gas is discharged after passing through the turbine wheel; a generally annular nozzle ring having opposite first and second faces, the first face forming one wall of the nozzle, and an insert having a tubular portion sealingly received into the bore of the turbine housing and having a nozzle portion extending generally radially out from one end of the tubular portion, the nozzle portion being axially spaced from the nozzle ring and forming an opposite wall of the nozzle, the first face of the nozzle ring defining a radially outwardly facing step of axial height h at a radially outer perimeter of the nozzle ring; an array of vanes circumferentially spaced about the nozzle ring and disposed radially inwardly of the step, exhaust gas flowing between the vanes to the turbine wheel, each vane being rotatably mounted to the nozzle ring and connected to a rotatable actuator ring such that rotation of the actuator ring rotates the vanes for regulating exhaust gas flow to the turbine wheel; and a generally annular retainer ring disposed radially outward of the nozzle ring and extending generally radially inwardly, a radially inner edge of the retainer ring engaging the first face of the nozzle ring radially outward of the step, characterised by the radially inner edge of the retainer ring having an axial thickness t that is less than h such that a remaining portion of the radially outwardly facing step is presented to the exhaust gas flowing through the nozzle. a turbine assembly comprising a turbine housing and a turbine wheel mounted in the turbine housing and connected to a rotatable shaft for rotation therewith, the turbine housing defining a chamber surrounding the turbine wheel for receiving exhaust gas from an engine and for supplying the exhaust gas to the turbine wheel, the turbine assembly defining a nozzle leading from the chamber generally radially inwardly to the turbine wheel, the turbine housing further defining an axially extending bore through which exhaust gas is discharged after passing through the turbine wheel; a generally annular nozzle ring having opposite first and second faces, the first face forming one wall of the nozzle, and an insert having a tubular portion sealingly received into the bore of the turbine housing and having a nozzle portion extending generally radially out from one end of the tubular portion, the nozzle portion being axially spaced from the nozzle ring and forming an opposite wall of the nozzle, the first face of the nozzle ring defining a radially outwardly facing step of axial height h at a radially outer perimeter of the nozzle ring; an array of vanes circumferentially spaced about the nozzle ring and disposed radially inwardly of the step, exhaust gas flowing between the vanes to the turbine wheel, each vane being rotatably mounted to the nozzle ring and connected to a rotatable actuator ring such that rotation of the actuator ring rotates the vanes for regulating exhaust gas flow to the turbine wheel; and a generally annular retainer ring disposed radially outward of the nozzle ring and extending generally radially inwardly, a radially inner edge of the retainer ring engaging the first face of the nozzle ring radially outward of the step, characterised by the radially inner edge of the retainer ring having an axial thickness t that is less than h such that a remaining portion of the radially outwardly facing step is presented to the exhaust gas flowing through the nozzle.