A turbine rotor assembly comprising: a rotor disc having a male mounting member comprising: a cylindrical radially inward facing surface; a cylindrical radially outward facing mounting surface; a plurality of radially outward extending mounting pins spaced about the circumference of said mounting surface; and a pair of apertures defined by said mounting member, each aperture being adjacent one of said mounting pins; an auxiliary annular wheel having a female mounting member comprising: a cylindrical radially inward facing mounting surface; and a plurality of mounting slots defined by said mounting member and being spaced about the circumference of said mounting member, each of said mounting slots having an open axially extending portion and a closed circumferentially extending portion, said auxiliary annular wheel being carried by said rotor disc in a predetermined axial and radial alignment wherein said radially inward facing mounting surface of said wheel frictionally engages said radially outward facing mounting surface of said rotor disc and each of said mounting pins is positioned adjacent a closed end of a circumferentially extending portion of one of said mounting slots; and a retaining collar comprising: a ring-shaped body having a pair of end portions separated by a gap, and an arcuate radially outward facing surface extending between said end portions; and a pair of retention pins, each pin extending radially outward from one of the circumferential end portions, said retaining collar being positioned so that said radially outward facing surface frictionally engages said radially inward facing surface of said male mounting member and each of said retention pins extends radially outward from said body through one of said apertures and one of said mounting slots. a rotor disc having a male mounting member comprising: a cylindrical radially inward facing surface; a cylindrical radially outward facing mounting surface; a plurality of radially outward extending mounting pins spaced about the circumference of said mounting surface; and a pair of apertures defined by said mounting member, each aperture being adjacent one of said mounting pins; a cylindrical radially inward facing surface; a cylindrical radially outward facing mounting surface; a plurality of radially outward extending mounting pins spaced about the circumference of said mounting surface; and a pair of apertures defined by said mounting member, each aperture being adjacent one of said mounting pins; an auxiliary annular wheel having a female mounting member comprising: a cylindrical radially inward facing mounting surface; and a plurality of mounting slots defined by said mounting member and being spaced about the circumference of said mounting member, each of said mounting slots having an open axially extending portion and a closed circumferentially extending portion, a cylindrical radially inward facing mounting surface; and a plurality of mounting slots defined by said mounting member and being spaced about the circumference of said mounting member, each of said mounting slots having an open axially extending portion and a closed circumferentially extending portion, said auxiliary annular wheel being carried by said rotor disc in a predetermined axial and radial alignment wherein said radially inward facing mounting surface of said wheel frictionally engages said radially outward facing mounting surface of said rotor disc and each of said mounting pins is positioned adjacent a closed end of a circumferentially extending portion of one of said mounting slots; and a retaining collar comprising: a ring-shaped body having a pair of end portions separated by a gap, and an arcuate radially outward facing surface extending between said end portions; and a pair of retention pins, each pin extending radially outward from one of the circumferential end portions, a ring-shaped body having a pair of end portions separated by a gap, and an arcuate radially outward facing surface extending between said end portions; and a pair of retention pins, each pin extending radially outward from one of the circumferential end portions, said retaining collar being positioned so that said radially outward facing surface frictionally engages said radially inward facing surface of said male mounting member and each of said retention pins extends radially outward from said body through one of said apertures and one of said mounting slots.