The 125 S was powered by Gioacchino Colombo's 1.5 L (1497 cc/91 in³) 60° V12 with a bore/stroke of 55 x 52.5 mm .
Enzo Ferrari wanted the 125 S to use a five-speed gearbox as it matched the high revving V12 better than that of a traditional four-speed gearbox.