A variable geometry turbocharger assembly, comprising: a turbine housing defining an inlet for exhaust gas and an outlet; a turbine wheel within the turbine housing and attached to a shaft, the turbine wheel being configured for radial entry of exhaust gas thereinto; a nozzle defining a nozzle passage for directing exhaust gas radially inwardly to the turbine wheel; a plurality of vanes disposed within the nozzle passage, each vane mounted on a bearing pin and each vane configured to pivot about an axis defined by the respective bearing pin; a center housing connected to the turbine housing and comprising an outer pilot surface defining an outer radius, a circumference of the outer pilot surface extending about a longitudinal axis defined by the shaft; a unison ring connected to the vanes and rotatable substantially about said longitudinal axis defined by the shaft for pivoting the vanes about the respective axes, the unison ring defining a radially inner surface positioned radially outward of and opposing the outer pilot surface of the center housing; and a plurality of guide pins for guiding rotation of the unison ring, the guide pins being secured to the center housing adjacent to the outer pilot surface; characterised in that the guide pins are positioned around and circumferentially spaced apart about only a portion of the circumference of the outer pilot surface, each guide pin defining a wear surface that extends to a radius which is larger than the outer radius of the outer pilot surface proximate each guide pin, wherein the wear surface of each guide pin defines a circumferential radius of curvature substantially equal to a circumferential radius of curvature of the radially inner surface of the unison ring; wherein the radially inner surface of the unison ring makes sliding contact with the wear surface of each guide pin and with the outer pilot surface of the center housing at a position opposite of the guide pins as the unison ring rotates substantially about the longitudinal axis, each guide pin and the outer pilot surface restraining radial movement of the unison ring while allowing for rotational movement of the unison ring. a turbine housing defining an inlet for exhaust gas and an outlet; a turbine wheel within the turbine housing and attached to a shaft, the turbine wheel being configured for radial entry of exhaust gas thereinto; a nozzle defining a nozzle passage for directing exhaust gas radially inwardly to the turbine wheel; a plurality of vanes disposed within the nozzle passage, each vane mounted on a bearing pin and each vane configured to pivot about an axis defined by the respective bearing pin; a center housing connected to the turbine housing and comprising an outer pilot surface defining an outer radius, a circumference of the outer pilot surface extending about a longitudinal axis defined by the shaft; a unison ring connected to the vanes and rotatable substantially about said longitudinal axis defined by the shaft for pivoting the vanes about the respective axes, the unison ring defining a radially inner surface positioned radially outward of and opposing the outer pilot surface of the center housing; and a plurality of guide pins for guiding rotation of the unison ring, the guide pins being secured to the center housing adjacent to the outer pilot surface; characterised in that the guide pins are positioned around and circumferentially spaced apart about only a portion of the circumference of the outer pilot surface, each guide pin defining a wear surface that extends to a radius which is larger than the outer radius of the outer pilot surface proximate each guide pin, wherein the wear surface of each guide pin defines a circumferential radius of curvature substantially equal to a circumferential radius of curvature of the radially inner surface of the unison ring; wherein the radially inner surface of the unison ring makes sliding contact with the wear surface of each guide pin and with the outer pilot surface of the center housing at a position opposite of the guide pins as the unison ring rotates substantially about the longitudinal axis, each guide pin and the outer pilot surface restraining radial movement of the unison ring while allowing for rotational movement of the unison ring.