A combination shaft for transmitting power comprising: a first shaft including an axial protrusion including a first circumferential surface defining a first outer diameter (OD1) and a second circumferential surface defining a second outer diameter (OD2), configured to attach to and drive a fuel side piston; a second shaft having a cavity therein configured to receive at least a portion of the first shaft and being configured to drive an air valve , having internal threads , and defining at least a first inner radial surface having a first inner diameter (ID1) and a second inner radial surface having second inner diameter (ID2), the internal threads being engagable with external threads of the first shaft to prevent axial movement between the first shaft and the second shaft and the first inner radial surface being engagable with the first circumferential surface and the second inner radial surface being engagable with the second circumferential surface to provide radial centering of the first shaft to the second shaft; wherein the first shaft is concentrically aligned within the second shaft ; wherein the protrusion of the first shaft includes a first section defining the first outer diameter and a second concentric section defining the second outer diameter configured to fit within the cavity of the second shaft; wherein the cavity of the second shaft includes a first section defining the first inner diameter and a second section defining the second inner diameter; wherein the second inner diameter of the second shaft is smaller than the first inner diameter of the second shaft; and wherein the first outer diameter of the first shaft is greater than the second outer diameter of the first shaft, characterized in that the combination shaft further comprises a locking insert located between the first outer diameter (OD1) of the first shaft and the first inner diameter (ID1) of the second shaft . a first shaft including an axial protrusion including a first circumferential surface defining a first outer diameter (OD1) and a second circumferential surface defining a second outer diameter (OD2), configured to attach to and drive a fuel side piston; a second shaft having a cavity therein configured to receive at least a portion of the first shaft and being configured to drive an air valve , having internal threads , and defining at least a first inner radial surface having a first inner diameter (ID1) and a second inner radial surface having second inner diameter (ID2), the internal threads being engagable with external threads of the first shaft to prevent axial movement between the first shaft and the second shaft and the first inner radial surface being engagable with the first circumferential surface and the second inner radial surface being engagable with the second circumferential surface to provide radial centering of the first shaft to the second shaft; wherein the first shaft is concentrically aligned within the second shaft ; wherein the protrusion of the first shaft includes a first section defining the first outer diameter and a second concentric section defining the second outer diameter configured to fit within the cavity of the second shaft; wherein the cavity of the second shaft includes a first section defining the first inner diameter and a second section defining the second inner diameter; wherein the second inner diameter of the second shaft is smaller than the first inner diameter of the second shaft; and wherein the first outer diameter of the first shaft is greater than the second outer diameter of the first shaft, characterized in that the combination shaft further comprises a locking insert located between the first outer diameter (OD1) of the first shaft and the first inner diameter (ID1) of the second shaft . <EOS>