A shaft assembly for a turbine engine, wherein the turbine engine defines an axial direction and a radial direction, wherein the turbine engine includes a fan or propeller assembly and an engine core, and further wherein the fan or propeller assembly includes a gearbox, the shaft assembly comprising: a coupling shaft configured to be connected, at a first end, to the engine core and further configured to be coupled, at a second end, to the gearbox, wherein the coupling shaft defines an annular surface extended along the axial direction, and wherein the surface defines a groove extended in a circumferential direction; a spacer, wherein the spacer defines a first portion extended inward in the radial direction and a second portion extended in the axial direction, and wherein the first portion is disposed in the groove of the coupling shaft; wherein a first portion of the spacer is defined towards the first end of the shaft assembly; a sleeve, wherein the sleeve defines a threaded portion comprising a plurality of sleeve threads defined outwardly in the radial direction, wherein the threaded portion extends along the axial direction, and wherein the sleeve is disposed outward of the spacer in the radial direction; wherein a portion of the sleeve abuts the first portion of the spacer in the axial direction; and a nut, wherein the nut defines a plurality of nut threads defined inwardly in the radial direction, and wherein the plurality of nut threads of the nut is configured to mate with the plurality of sleeve threads of the sleeve, and further wherein the nut defines a radial portion extended inward in the radial direction and towards the second end of the shaft assembly, and wherein the radial portion is adjacent to at least a portion of the sleeve and at least a portion of the spacer in the axial direction, and wherein the radial portion abuts at least a portion of the spacer in the axial direction in an assembled state; wherein the coupling shaft, the spacer, the sleeve, and the nut are in adjacent arrangement along the radial direction. a coupling shaft configured to be connected, at a first end, to the engine core and further configured to be coupled, at a second end, to the gearbox, wherein the coupling shaft defines an annular surface extended along the axial direction, and wherein the surface defines a groove extended in a circumferential direction; a spacer, wherein the spacer defines a first portion extended inward in the radial direction and a second portion extended in the axial direction, and wherein the first portion is disposed in the groove of the coupling shaft; wherein a first portion of the spacer is defined towards the first end of the shaft assembly; a sleeve, wherein the sleeve defines a threaded portion comprising a plurality of sleeve threads defined outwardly in the radial direction, wherein the threaded portion extends along the axial direction, and wherein the sleeve is disposed outward of the spacer in the radial direction; wherein a portion of the sleeve abuts the first portion of the spacer in the axial direction; and a nut, wherein the nut defines a plurality of nut threads defined inwardly in the radial direction, and wherein the plurality of nut threads of the nut is configured to mate with the plurality of sleeve threads of the sleeve, and further wherein the nut defines a radial portion extended inward in the radial direction and towards the second end of the shaft assembly, and wherein the radial portion is adjacent to at least a portion of the sleeve and at least a portion of the spacer in the axial direction, and wherein the radial portion abuts at least a portion of the spacer in the axial direction in an assembled state; wherein the coupling shaft, the spacer, the sleeve, and the nut are in adjacent arrangement along the radial direction.