 A rectangular radiator immediately behind the two-bladed propeller provided cooling, replacing the external radiators of the transport.There was a serious dispute between the State and Latécoère about the engine choice; briefly, the former wished all BN4 machines to use more powerful Renault engines but failed to have them available, whereas Latécoère preferred the Lorraines on the grounds of a better power-to-weight ratio.The outer engines of the bomber were similarly mounted to those of the Latécoère 4 with bracing to the wing roots and fuselage, but in deeper and flatter sided cowlings which extended downward almost to the wing.