In Storia Della Lancia, Rosani has the changeover in 1939. Drawings for the 438 were begun in late 1938.
In the larger displacement engine, bore and stroke were both revised, from 72x83mm to 74.61 x 85 (bore and stroke). Conrod bearings increased in diameter.
In order to fit the larger pistons, the cylinder centerline moved a bit further out in the block and the bore angle slightly reduced. In the crankshaft, the crankpin offset angle was unchanged, but conrod bearing was increased in size.