According to an aspect, a system for a gas turbine engine includes a reduction gear train operable to drive rotation of a starter gear train that interfaces to an accessory gearbox of the gas turbine engine. The reduction gear train includes a starter interface gear that engages the starter gear train, a core-turning clutch operably connected to the starter interface gear, and a plurality of stacked planetary gear systems operably connected to the core-turning clutch and a core-turning input. The system also includes a mounting pad including an interface to couple a core-turning motor to the core-turning input of the reduction gear train.